Over the years I have spoken many times about the importance of a quality plastering job. That importance has not waned, and I am unfortunately hearing more and more stories about failed plaster jobs around the world. A large percentage of the consulting work I do is helping clients deal with these plastering issues. There are two common themes, or dare I say causes for the failures. If you avoid these two approaches to plastering your home, your plaster should provide you with a very long life.

Failure #1: Mixing earthen and lime plasters on a wall surface. This is perhaps the most common mistake that I see over and over again. People choosing to use earthen plaster for the scratch and brown coats and a final, “durability coat” of lime. The problem here is that what you have is stronger plaster over weaker plaster when in reality, you want it the other way around: weaker plaster over stronger plaster.

If you consider all plaster work over the last say…thousand years, one thing holds true no matter what material you use. The second coat has more sand in it than the first coat and the third has more than the second. That makes the coats “weaker” as they move away from the wall. This is important because plaster moves, as do homes. If the weaker plaster beneath a strong lime finish coat can move more than the finish coat, you will ultimately get delamination between the two coats which will lead to eventual plaster failure. By laying weaker plaster over a stronger finish coat, it will always be able to move at least as much as the coat beneath it. This keeps the plasters well bonded and eliminates the high risks seen in the opposite application. (more…)

I recently received an email from a gentleman named Curtis in Kansas who has literally 10,000 gallons of lime putty he is looking to sell. He used the lime to make custom plasters in his company for the last few years; however, the slow economy has forced him to close his doors. Once whipped up with a paddle, the lime putty reaches a beautiful, creamy consistency as shown in the photo. The putty was all slaked with water and mixed in a 300 gallon stainless steel mixer and then placed in 55 gallon drums or 5 gallon buckets to age. The lime  has great plasticity and consistency, is high in calcium, and is sourced from the Mississippi Lime Company in St. Genevieve, Missouri. It actually looks a bit like tofu before it gets whipped up…delicious!

The putty is all shrink wrapped and on pallets for easy transportation. They have a forklift and can assist with loading at their facility; however, all shipping arrangements are up to the buyers and all orders must be picked up from the seller’s facility in the Wichita, Kansas area. The putty has been stored in a temperature controlled warehouse and has never been subjected to freezing temperatures. They are currently selling the putty at $2 per gallon which is a very good price for such yummy lime putty.

Keep in mind that if you plan to use lime putty in the US, you will have to make it yourself by slaking hydrated lime. This is a slow, laborious and often dangerous process so you are much better off purchasing the lime pre-slaked. Unfortunately, there are not many companies that offer pre-slaked lime in the US. THat makes this deal even better. What’s more, the longer the slake, the better the putty, and three years is a very good time line for quality lime putty. Obviously shipping will come into play in regards to price, but I think it is still worth the effort and a good value. If you know others in your area in need of lime, perhaps you can share the trucking costs. You can reach Curtis to find out more or to place an order at this email address.

Plastering is perhaps the hardest part of the entire process when building a straw bale house. Think about it, your framing, although difficult, is hidden within the walls nine times out of ten. As long as it is structurally sound, you will be fine. Furthermore, it is inspected (in many cases) so you end up with a “free” set of helpful eyes to make sure you are doing the work properly. The same is true for the other major systems of the house: plumbing, electrical, mechanical, and so on. As long as the systems are built properly and they meet or exceed codes, you are all set. What those systems actually look like is mostly irrelevant.

The same cannot be said about plaster. That’s a system that not only has to be structurally sound and function in a way that protects the bales, and ultimately your entire house, but it also has to look good. After all, when have you ever heard someone say “Wow, you really did a great job with the rough plumbing in this house. It sure is beautiful.”? Probably never. How about someone commenting on plaster? Now that’s one that you have likely heard or even uttered yourself. “Man, that plaster looks amazing!”

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TransMineral USA is hosting a plastering workshop this fall, October 11 and 12, 2012. This is a great opportunity to learn the material from some of the best. Keep in mind that TransMineral USA is the sole US importer of Natural Hydraulic Lime, so they know their stuff. I don’t know exactly who will be leading the workshop, but I imagine he or she will be top notch and I expect Michel, the owner of TransMineral USA will be there as well. I highly recommend you attend this workshop if you can make it.

Here’s what they have to say in their workshop announcement:

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This message is from the host of the Junction City, CA workshop. She has just finished plastering her scratch and brown coats and has some extra bags for sale at a great price (50% Off!). See below…
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I have 18 bags of 3.5 NHL. They have been stored in the house the entire time and are dry.

I paid $50.00 a bag for them and they are 55 pounds each.

I will sell all 18 for 450.00 ($25/bag) if some one wants to come get them in Junction City, CA.

Please contact me at mijanow@sbcglobal.net. My name is Jan.

Thanks!

I wanted to share a (relatively long) conversation with you all about lime plaster over an earthen plaster (clay) base that is happening now on a straw bale list serve. This conversation sums up what I have been seeing and hearing about quite a lot in recent years: failures of lime over clay plasters. I hope everyone considering this option reads this conversatrion and changes their mind. Here’s the letter from Chris, the man whose plaster has failed. My response is below. (more…)

In regards to plastering by hand or by spray application, there are several schools of thought. Some people believe that plastering by hand is better because the force of the application is higher when arm power is used to press the plaster into the bales, giving it a high level of “key” or adhesion. Others believe that the force applied by the spraying is actually better because it shoots the plaster deep into the webbing of the straw. I have discussed this question at length with several master plasterers over the years and have learned a lot about both processes. The fact of the matter is that both systems are absolutely acceptable as long as certain details are maintained.

The most important aspect of plastering, whether it be by hand or sprayer, is the mixing ratio. Just like many aspects of construction, there are many different recipes used by different builders; however, the most common mixing ratios are as follows. Scratch coat (first coat) 2:1 (sand:lime). The brown coat (second coat) would be 2.5:1 (sand:lime) and the third coat would be either 3:1 (sand:lime) or a ratio matching the scratch or brown coat ONLY if the lime used is a weaker lime. For example, a scratch coat of NHL 5 could be applied 2:1 followed by a brown coat of NHL 5 applied 2.5:1 and finished with a coat of NHL 3.5 at 2:1. This is because the 3.5 is a weaker lime than the 5. Plaster should ALWAYS get weaker as it “comes off the wall.” In other words, the coats further from the straw should be weaker than those beneath them. This is an industry standard.

I’ll discuss hand application first. Muscle is perhaps the most important part of hand application. If the plaster is not properly worked into the substrate (straw in this case) it will not anchor properly or achieve the right key/tooth. If the plaster is not properly anchored, then even if it hardens perfectly, it is hardening AWAY from the wall surface and thus is prone to full delimitation (where it peels away from the wall). Delamination can also be caused by the walls not being properly prepped. For example, if loose straw remains on the wall and no meshing system is in place, that loose straw can peel away from the wall with the plaster attached to it: thus delaminating. I should mention that this is true for both hand and spray applications.

The next important detail is often referred to as “put it on the wall and leave it alone.” What that means is that if the plaster is overworked with the trowel, it will start to separate. The creamier aspects of the mix, the lime, will be pulled from the aggregate, the sand, and left at the surface. When the plaster dries/cures, this material will dust off of the wall and the remaining sand will be weak because the binder (the lime) has dusted off, leaving the sand to bind itself, something it cannot do. The best hand application is one that is done with minimal “extra” working of the mud once it is placed on the wall.

www.StuccoSprayer.com

Spraying plaster, as mentioned above, is just as viable an option as hand application. The biggest mistake that people make when spraying plaster is they water it down under the assumption that it needs to be “lighter” in order to be sprayed. This is 100% incorrect and when done, has a very negative affect on the plaster. The more water there is in a plaster, the more room for voids in the curing process. Keep in mind that as plaster dries and cures, the water molecules (H2O) are driven off leaving voids behind. The lime crystals need to grow into those voids in order to create a strong plaster. If the voids are bigger than the crystals can fill during the curing period, the walls will end up weak. Plaster should therefore be sprayed at the same consistency as it would be hand applied. Exactly how much water needs to be added varies with each load as the sand for plaster is often supplied in bulk with varying amounts of water within it. Therefore it is crucial that the plasterers keep a watchful eye on the mix as it is created to ensure that just enough water is used in each batch.

As with hand applied plaster, the concept of overworking sprayed plaster should be managed as well. Spray it, press it into position with a trowel and then leave it alone. One MUST follow up sprayed plaster with a trowel immediately after it is sprayed to smooth out big irregularities of thickness on the surface and make sure the key with the straw is achieved.

A step that is often missed in lime plaster application is “pressing back.” This is when a trowel is held vertically against the wall and drawn up and/or down over the plaster with adequate force to push the plaster back into the wall. This is only done after the scratch coat and should be completed when the plaster reaches a “leathery” feel. Too soon and the plaster will simply squish around. Too late and the plaster will be too hard to compress. The purpose of this step is two fold. First, it compresses the plaster onto itself, tightening up any voids in the plaster as mentioned earlier. Secondly, it creates an even stronger key/tooth with the straw. Once the pressing back is complete, the scratching should take place immediately leaving marks roughly 1/4″ wide by 1/8″ – 1/4″ deep. Any deeper and the plaster will slump on itself. Any shallower and the brown coat will not have enough tooth to properly attach. The scratches should be made with a scarifier or other tool designed for this specific purpose. Make shift scratching devices often pull mud from the wall or create inadequate scratch patterns. The scratching should be run horizontally as much as possible.

After the scratch coat is complete, it is vital that the walls be kept damp, or misted, for at least the first 3 days. The curing time between coats is a minimum of 10 days and if misting can be done for the entire 10 days, even better. This slows the curing process and allows the plaster to grow into the voids as discussed earlier. Failure to wait the 10 days can cause a weak plaster. Before the application of the next coat it is vital to pre-wet the walls. The night before the second coat’s application, the walls should be completely dampened. The morning of application, they should be misted once again. Failure to fully saturate the walls the night prior will give you a “snap, crackle, pop” effect on the plaster. You will literally be able to hear the prior coat of plaster sucking the moisture out of the new coat. This rapid reduction in moisture ruins the fresh coat of plaster which will likely result in delimitation or chalking of the coat. This dampening MUST happen between both the first/second coat and the second/third coat for proper plaster application. The 10 day cure time and misting must happen after the scratch and brown coats, but not after the final coat as misting of the final coat can cause color to run. Furthermore, the final coat is applied very thin and as such will dry/cure fast enough that misting would have little effect on it.

www.GreenStrawBaling.com

The brown coat does not receive the pressing back like the scratch coat. Instead, it is burnished with a “devil float” or some other float in a circular form to 1) compress the plaster onto itself and 2) press the plaster into the tooth created by the scratching tool. The final result will be a rough, swirly coat. This is vital to the final coat’s ability to hang onto the scratch/brown below it. If the brown coat is smoothed out, the finish coat will fail and delaminate.

There are so many details to a quality plaster job and so many places to make mistakes. The real question may not be whether to spray or to apply by hand, but rather, what steps need to be taken, no matter which application method is chosen, such that the plaster job will be successful. I strongly recommend learning to plaster on a small project before you decide to tackle your own home. Consider taking a workshop with me to get your hands dirty, literally, or at very least, check out my instructional plastering DVD. Plastering is hard work and takes skill. It’s also what people notice the most about a home when it’s complete. Take the time to learn the process and you will have success.

Over the years I have heard several people ask about the American Clay line of plasters so I felt it was time to contact them to ask them some of these questions.  Here is the Q&A session:

1. How do you handle the fact that clay is more breathable (allows more vapor to pass through it) than typical exterior coatings such as lime and, in some cases, cement based stucco? How do you keep the bales from becoming saturated by moisture that can get into the walls through the clay, but cannot get back out through the exterior coating?

Even though the clay is more permeable, it is also hygroscopic, and holds the vapor in its pore structure.  It also has an affinity for water that is greater than that of the straw, thus helping the straw to dry.  The clay allows the water to dissipate through a larger area and then move out of the wall system to that of lower vapor as the humidity levels drop to the interior of a building space.

For the moisture that moves through the wall and collects on the exterior, the importance of continued permeability remains high.  A lime plaster, even though it is lower than a clay, is still highly permeable to vapor, and also has high dissipation of water, allowing it to move to the outside as the humidity drops to the outside, thus allowing the system to balance and not remain in the wall system.

The importance of having a climate that dries out enough to allow for the dissipation of the moisture is high. Without it, the balance will not be reached and therefore, climate must be taken into consideration when choosing your plaster both exterior and interior.

2. Isn’t the quality of the American Clay finish heavily dependent on the quality of the substrate? In this case, the adhesion and overall quality of the American Clay will be based on the quality of the earthen scratch and brown coats. Does the makeup of those coats effect the finish?

It is true that the American Clay finish is dependent on the quality of the substrate, but that is true of any finish plaster or stucco.  Typically the “makeup” of the coats will determine whether it will work or not as a sound base.  If the substrate is both sound and solid then the American Clay will attach well.  If the base is loose and not very sound, there would be issues.

3. How do you handle wet areas with American Clay such as bathrooms and kitchen walls near sinks? Do you have to treat the finished surface of the clay to waterproof it? Howe does that effect the overall performance of the product?

With the clay, the areas to seal and protect are areas that are going to be regularly exposed to items that will saturate, stain and degrade the material.  Behind a kitchen sink or a bathroom sink (or even around a toilet with young boys and even men that tend to miss often).  For the rest of the areas in the kitchen or bathroom, using the clay to help mitigate and disperse the moisture is an advantage.  Keeping in mind that good ventilation to allow the moisture to dissipate, after a hot shower for example, is still important.

The type of sealer determines how the permeability would be degraded.  A silicate would work great for water but does not block oil stains very well, yet it remains very permeable.  Similarly, a limewash works for water as well, but not oils.  Acrylics are another option but move away from the natural finishes.  They do protect against both water and oils; however, they can reduce moisture transmission out of the system.  Oil based sealers, like polymerized linseed oils are great for protection against all of the above but do not breath.  Wax is a good option, but limits additional coats over the area later.

4. I’ve heard that up to 4 coats of finish are required to get the product to look as beautiful as it does in your advertising. Is that true?

No, with a sound, even base coat, one or two coats is all that is necessary.  If it is taking more coats, the base was either very coarse or not very level. That said, the base may be two coats thick, so the overall plastering process from start to finish can be considered 4 coats.

5. Is there a wait time in between coats for the product application? In other words, does the subsurface need to dry first before it is re-wet for the plaster application?

The base does need to be dry prior to the applications of American Clay.  The need to pre wet your base is dependent on the site conditions (i.e. the difference between Seattle during the wet season, or Tucson in July.  I would not pre wet my base in Seattle, but I would in Tucson).

6. Are there adhesion problems if the base coats are not earthen? Can AC be applied over a lime scratch and brown coat? What issues might one encounter?

The base does not need to be earthen.  Our plasters go over a lime brown without issues, as long as it is a true brown coat with adequate key.  If it is a smooth lime finish, it would require additional prep prior to lay application of either a primer to provide tooth or a bonder to provide adhesion.  Cement brown coats will work in the same manner as lime.  Gypsum brown coats can be an issue, and require a sealer prior to the clay application.  For the mentioned exceptions above, the clay would begin to pull away from the brown coat upon application of the second coat if the detailing noted above was missed.

7. How do you wash the walls over time?

The required washing of the walls that we are used to with paint is not generally required with any natural finish.  A light brushing or vacuuming of the wall to remove cobwebs and dust attached to texture on the wall is typical.  For stains or marks, a very well wrung sponge used to wipe the surface will most always do the trick.  For deeper stains a 25% white vinegar solution sprayed on the surface will help.  Allow this to dry, and then wipe with the sponge mentioned prior.  If it is a stain that cannot be removed with washing, the plaster can be removed and patched with reserved material from the original project.

8. How do you change colors over time if you decide to change a room color? Can the walls be painted or color washed?

The are many ways to change the colors over time.  Painting with any number of materials is possible.  A color wash worked into the clay works well for subtle shifts, and if a complete change is desired, another coat of AC plaster can be applied.

(Note:  Straw Bale Innovations is in no way affiliated with American Clay and this post is purely for informational purposes only)

They say that necessity is the mother of invention, and in the case of Branislav Prvanovic, a Serbian innovator and businessman involved in the building trades, a gigantic straw bale building project turned out to be the mother load of necessities. Charged with the task of rendering 150,000 sqft of straw bale walls on his industrial complex, he knew he had to get innovative and quickly in order to get the job done. Here we enter the story of how a progressive couple, Branislav and Gorica (his wife), living in Serbia came to design and create what may become the new standard for mortar sprayers around the world.

Serbia is a very small country in the Balkan region of Europe that is on it’s way to joining the European Union. This is an exciting time for this tiny nation and Brainslav and Gorica are on the forefront of environmental progress in the building sector of their country. They are both passionate about building their industrial facilities as efficiently and cost effectively as possible. As such, in between 2007–2009, they brought together the best Serbian team of engineers and prepared for investing in the largest straw bale industrial facility in all of Europe (50,000sqft). The team chose straw bale technology because they love how sustainable, cost effective, healthy, and energy efficient it is. In the process they intend to prove that it doesn’t matter if you are dealing with residential, commercial, or industrial projects, straw bale technology can be a solution for each of those situations.

When presented with the mega task of rendering the 150,000sqft of wall surface, they had to devise a solution that would produce excellent quality, be cost effective, and be able to plaster much faster than anything else on the market. Branislav, being the innovator that he is, took to the drawing boards and began to conceptualize a mortar sprayer that would suit their needs. After many renditions, trials and errors, and modifications, they were able to fine tune the mortar sprayer to be a sleek, lightweight and highly-productive tool. At the time, they were calling this sprayer the “Revolutionar”, however, one day Gorica was looking at the sprayer and was struck by how it looked like a dragonfly. The sprayer has been called the DragOn PRO ever since.

The DragOn PRO mortar sprayer is a heavy duty pneumatic tool which delivers all kinds of plasters and rendering mortars to the walls surface quickly. One of the best details is the design ergonomics. It’s balanced well, so it’s possible to plaster for several hours without the strain that is typically associated with plastering work. Okay, it’s still hard work, don’t get me wrong, but it does take less physical strength to deliver the plaster to the wall than using a hawk and trowel. It’s also cool that the DragOn PRO can apply all density mixtures with fillers up to 6mm including mortar of all types, plasters, clay renders, papercrete, and micro concretes. It works in conjunction with a compressor test which generates the force to push the plaster through the sprayer and out of the nozzle. Make sure you have a big enough compressor so that it can keep up with the required air supply!

One of the qualities we love about the DragOn PRO mortar sprayer is that it has been designed to be super easy to learn to use (it takes just minutes and can be used by pretty much anyone). We always appreciate innovations that are geared not only towards professionals but also towards the do-it-yourselfer. It weighs less than a shovel when empty and is actually quite comfortable to use (especially when compared to the job of applying plaster by hand with a trowel all day!). It is pretty much indestructible so one can spray plaster with confidence. That said, if you don’t wash it after each use it will get heavier and heavier each time. I’ve seen that happen with other sprayers, and it makes the job more tiring and less efficient, so take the time to keep the tool clean.

It’s true that other mortar sprayers exist on the market today, however, Branislav and Gorica are confident that the DragOn PRO outperforms them all. They tout that their sprayer is significantly faster and more efficient than other sprayers because of the unique concept of their nozzle design and how it manages to spray the plaster. They have patented this solution in order to protect their innovation, so they obviously believe in it. The reception from buyers around Europe and the Southern Hemisphere has been tremendous. The cost is reasonable and pays itself off after several working days, making it an obvious fit for not only professionals but also for owner-builders with their projects. Branislav and Gorica are also very pleased with how well the sprayer applies plaster in vertical, horizontal, slanted, curved and very high walls. I think the strong pressure of the application really helps in those situations too.

The DragOn PRO can be purchased at either wholesale or retail. The retail cost is 279€ and there is always an additional 10% discount applied for anyone that is building with straw bales. They also offer a retail program if you have an interest in becoming a retailer. By the way, Straw Bale Innovations makes no financial gain from writing about the DragOn PRO sprayer. In fact, we are not retailers or otherwise associated with DragOn, we simply thought this was a great product created by an amazing couple trying to make a positive impact on how green construction is developing in the world and we wanted to share their story.

Plastering is no easy task. Anyone who has done it can attest to the intensity of the labor and the time that it takes to do a job well. The DragOn PRO is a solution for easing this process. So, we want to thank Branislav and Gorica for believing in the benefits of straw bale construction and for creating a product that will make the plastering of bale walls much more manageable for everyone! To visit their website, please go to www.dragongpb.com and to email Branislav with any questions, please write to sales@dragongpb.com.  To watch a video of the sprayer in action click here.

Dun, dun dun…… Can you feel the excitement? I just got back from working on a project in Portugal and the plaster machine was one of the coolest I’ve seen. This monster mixed up to 1 1/2 bags of lime plaster at a time (35 kg bags). The capacity was not what made the machine exciting as some of the mortar mixers I use here in the States will mix twice that capacity at once. What was cool was everything that the machine was able to accomplish. Here’s a run down.

The machine should be placed directly behind the pile of sand. The reason is clear: why carry sand when you can use an automatic, winched sled?! That’s right, this baby comes with a remote controlled sand sled that places the sand directly into the holding barrel. That brings us to the next cool part of the machine, the holding barrel. This section of the machine allows you to pre place all of your materials, even water if you choose although I think it’s better not to put the water in at this time, into a holding tank of sorts. That way when you are ready to mix your next batch you simply empty the mixing barrel of the machine and add the next batch, already pre measured and ready to go.

What’s also cool is that this holding barrel is hydraulically lifted, so there’s no strain to your back. You simply hit a switch and the hold thing lifts up and dumps the material into the mixing barrel. Add your water and you’re off. While the plaster is mixing, you can get the next batch pre measured and into the holding barrel. This is especially useful when using a material like Natural Hydraulic Lime because that material has to actively mix for 20 minutes. While it’s mixing, you prepare the next load. It’s a great use of time. Otherwise, a 20 minute mix takes 30 minutes start to finish. This way, it’s 20 minutes on the nose.

Another aspect of working with plaster is moving it from the machine to where you need it. Most machines require a wheel barrow, a string back, a well inflated tire, and some relatively good balance. This is usually not a problem (save the well inflated wheel barrow tire), but it is work. This machine would simply pump the mix from the mixing drum directly to where you need it. In fact, it can pump the material up to 10 stories up! Most of us won’t need that, but it does help to send the mud up a steep driveway or hill where a wheel barrow might end up costing you hundreds in chiropractic repairs.

So now we have mixed our plaster, placed it within inches of where it will be used, and the only thing we have had to lift is a bag and half of lime at 35kg each. Not bad. So how about applying it, any savings there? Of course. The machine uses the same compressed air that drives the mix up the hill to run a plaster spraying gun. This speeds application and does a great job of both leveling the wall on the first shot and penetrating the straw.

The down side to this machine? It costs about $8,000. If you can find one to rent or borrow, go for it. Otherwise, just think about how cool it is and marvel at what we have been able to build as humans!

Anyone looking for elephant dung for their natural plaster? If so, I have a good contact for you. Gavin Wuttken (gavin.wuttken@pdza.org) from the Point Defiance Zoo and Aquarium in Tacoma, Washington is looking for a good home for some elephant dung. Could you be that good home? If so, please contact him directly to set things up. Good luck.

I was recently asked about using different finishes for straw bale walls on the interior. I’m very often asked about using different exterior finishes, but not as often does the topic of interior wall finish come up. Of course, the type, texture and color of plaster is always in question, but more rare is a discussion about using metal panels as a wall finish, or perhaps drywall or even tile.

The answer is that yes, these things can be done; however, “is the amount of extra labor worth it to you?” may be the appropriate question to ask yourself. Keep in mind that straw bale is an organic building material. Where this is most obvious is around curves by windows and doors. At every workshop I teach, people ask me about making a template to measure each window well with to make sure all of the window curves look the same throughout the house. Possible? Sure. Plausible? Not really. The fact of the matter is that each curve will be slightly different. That’s part of the beauty of building with straw bales. So now consider how you might finish an 18″ deep window well that has a natural, undulating curve to it, slightly different on each side of the window, while using metal panels. How do you attach them to the house? How do you adjust for the irregular shapes within the window well?

You can see where the difficulty comes in. The same is true for any material you use that is rigid and “unforgiving” to uneven sub straights. The key is to provide backing that fully supports the material in mind to ITS specifications. This means that you may have to insert framing to support the material every 24″ on center, or provide for nailing on a specific pattern. You’ll need to weigh the pros and cons of each finish you choose and see just what is worth your efforts.

I have seen corrugated metal roofing used in shower stalls and in straw bale barns to great success; however, would I do an entire room with that material? Probably not. I think for specific spaces that require something like this, it is worth it. If you want your walls to look like drywall on the inside, then I suggest you build with drywall, and forgo the straw bale infill, or at least create a hybrid installation system that will eliminate all of the “extra” framing to be notched in to the bales, post stacking.

Mixing plaster is as much of an art as it is a science. It’s important to keep a consistent mix from one batch to the next and this isn’t always easy to accomplish. One of the most common mistakes people make when creating plaster mixes is to lose track of how much sand has been put into the mixer. Have you ever mixed plaster with a bunch of folks helping you? If so, you know exactly what I mean.

Picture this: you’re using a 5 gallon bucket to place sand into the mixer. You need 4 buckets of sand for every bag of lime you add to the mixer. Your helpful friends are filling up the bucket as fast as you empty it and you guys are totally cruising through the mixing process. Suddenly you ask your friend “is that 3 or 4 buckets?” In return you get what a friend of mine calls a “goat face,” a look of complete disconnect. Now you retrace your steps to try and figure out how much sand is in the mix. You look at the plaster tumbling on the paddles in hopes of recognizing the texture of the mix. IN truth, neither of you have any clue how much sand is in the mixer. There’s a better way.

Instead of using one bucket, use 4. Three buckets should be the same color and the last bucket a different color. This way know if you’ve made it to the end of a load. For example, when I mix plaster, I can usually fit 3 bags of lime in the mixer at a time which means I need 12 buckets of sand. Counting those out one by one would be hard to track, so I use the 4 buckets. I put in three white buckets of sand and then an orange one (you could always use another white bucket and just pray paint it or wrap it with tape). As soon as I see that orange bucket go by, I know I’m at the end of a run. Now my helper can fill all 4 buckets again while I add the bag of lime and we’re ready for the next grouping. It’s easy!

The sad and ugly aftermath! In a recent workshop we discovered that the sand that was ordered for the plaster was not acceptable. I blame myself for this mistake as I did not catch the problem in time and allowed the plaster to be mixed. As is always the case, a mistake can be either just that: a bummer, or it can be a learning piece. What I learned in this scenario is the importance of finding the right sand for your plaster mix.

I spoke with the sand yard directly and told them what I needed. The dispatcher seemed to understand completely and the next day, 10 yards of angular, variable size sand was delivered to our very remote building site. There was no sending it back if we didn’t like it, it was what it was and it turned out to be the wrong stuff. The issue was that it had no fines in it.

The right sand for the Natural Hydraulic Lime (NHL) plaster needs a gradation of sand from the fines to the more coarse. No silts and clays mind you, but the finer end of the gradation is important.

You can see in the picture how the plaster simply did not hold together well. The lack of finer sand made the plaster loose and almost gravelly. I did a stretch test on the plaster by pulling a hawk’s worth of plaster across the table with a trowel. In quality plaster, the mud will spread and stay together, creating a smooth surface. In this case, however, the plaster ripped and tore as I pulled it on the mud board. There was nothing to hold it together.

We tried for an hour or so to get the mud to stick to the wall and eventually the sad truth became all too apparent: the plaster was not good. The worst part of the story is that we had premixed 18 bags of plaster the night before and all of it was wasted! The host now has a very fancy NHL carport floor! I ordered 18 replacement bags for the host and have informed him of the correct sand he’ll need, moving forward.

When you order your sand, be sure it meets the requirements on the NHL bag. Where I live the perfect sand is called “concrete sand.” It is angular, has many different grits (including the finer sands), and is perfect for the proper adhesion of the plaster. The same has been true across the country as I teach. The term “concrete sand” is almost always understood. In this part of New Mexico, that was not true. My plan moving forward is to send future workshop hosts a sample of the sand they should be looking for so they can make sure, well in advance, that they have the right stuff.

Per Martin’s request, here’s an image of the appropriate style of sand to use (size wise). I’ve also heard from an expert that using a lime based sand can cause problems too as the free lime in the sand will take up the moisture and leave the NHL without the ability to attach itself to the sand.

It’s not unusual for people to get excited about plastering their straw bale structure. After all, this is when a simple structure starts to look like a home. When the plaster goes on, suddenly, the building gets a new lease on life and it really begins to feel like completion is near. This is also a very difficult aspect of the construction process and one that will be very visible for the life of your home. When people come to visit, the plaster is likely the first thing they will notice about the home. It will either look great and wow them, or it won’t. Of course, you’re likely hoping for the wow factor! Okay, back to my point. Take a look at the photo above. What problems do you see? I see one major problem that happens far too often.

The plaster has been applied by several different people and there are a series of wet edges all drying at once. Here’s the biggest problem: this picture was taken during a lunch break and so the “wet edges” were left to dry for a while. These dried edges will ultimately result in cold joints, areas where plaster of different wetness is in contact. The fresh, wet plaster will not fully bond to the drying plaster shown here and a cold joint will result. That’s not a huge deal in this case because we’re looking at a scratch coat and cold joints are common throughout the field on scratch coats. That said, it is always best to limit the cold joints in your scratch coat as best you can.

As you can see in the picture to the left, working with lots of plastering hands is easy to accomplish while keeping a wet edge. Each person simply sets up near each other and works from the bottom up (or top down if you prefer) so that they overlap on the edges. Section by section, the wall is completed with a constant wet edge in contact with a wet edge. Should you break for lunch or some other reason, it’s best to stop in a straight line that will be located differently in each coat. In other words, don’t always stop 1′ from the ceiling when you take a break as the cold joints will then travel through all of the coats in which you stop in that location. You can imagine though that if this crew stopped for lunch now, their cold joint would be much more manageable than the one shown in the first picture.

I received some information from a man in Alabama the other day. He has a sweet set up for spraying plaster that he is selling. It’s almost new and in great condition. Here’s the description, some photos and the price.

I have a used but in great condition Hyflex HZ-30g stucco pump with brand new never been used set of hoses and sprayer and a Essex 11 cy used mixer. All items were purchased for the construction of my personal home. We ended up running out of time and so I hired a stucco crew instead. The only thing used on our job was the mortar mixer and it works great. Take it from my experience and save yourself thousands by buying this equipment. It’s located in Birmingham Alabama.

My input on this is that he’s right, having your own equipment will save you lots of money in the long haul. What’s more, when you’re done with your project, you can sell the equipment to the next lucky soul! If you plan to plaster your own house, this equipment will be a great option for you. It will speed things up and allow you to hit the deadlines you need to keep your house on schedule and on budget. If you’re interested in purchasing this equipment, leave a comment here and I’ll get you in touch with the owner. The asking price for all the equipment listed is $8,750. This is a great deal based on the cost of this equipment new.

Picture this. Your bank loan is on a strict timeline. Your plastering job seems overwhelmingly huge because you plan to do it yourself. You don’t have any money in the budget to hire a professional crew, so your options are few. How do you speed up the process so that you can finish the job on time and avoid bank penalties? Good news, you have some options.

First option: Spray the Plaster. I’ve spoken about this before and how a simple spray gun can drastically speed up the plastering process, especially for a small group of people. By spraying the plaster, you can dramatically increase the application rate for the mud. You still need to trowel the plaster into the bales and “press it back” (see previous blog entry on this topic), but you’ll save a lot of time applying the mud.

Second Option: Premix. This is tied to the first option because it is absolutely required if you plan to spray the plaster since your application speed will be so amped up. By taking a day or so to mix a bunch of plaster, you’ll be able to focus on plastering all day long once you start as opposed to having to stop and start each time you need to mix a batch of plaster.

In addition, any hands you have helping you will be able to work on the wall, not just the mixer. Furthermore, premixing the plaster and allowing it to slake overnight (wrapped tight in plastic so it can’t dry out, of course) will make it much more workable the following day. That workability also increases the amount of time it remains plastic. This means that you can wait longer in between when you apply it, when you press it back, and when you scratch it. This is perfect for a small crew as it keeps you efficient all day long.

Third Option: Close the loan with only a brown coat. This is not ideal from the visual point of view, but it does create some positives. First of all, the brown coat will get a long time to cure and set up. This will give you a great base for your finish coat. Secondly, it will look “finished” to some extent so the bank can close the loan and you won’t have any delays on the end of construction financing. This is of course not the best option for several reasons including the fact that once you’re “done” with your project, you’ll really want to be. Not to mention the fact that your interior walls, if left incomplete, will require you to move furniture, protect floors, and so on down the road to complete the finish coat. Not much fun.

So, you have some options for sure; however, the best option is to hire a professional crew to take care of the plastering or plan for the extra time and effort it will take to do it yourself. To be sure, you’ll need to really consider how long it will take to do yourself before you lock yourself into a loan that is time sensitive. If you don’t, you may end up against the wall, to use a bad pun. You can’t simply ask a plastering crew how long a job will take and expect to complete the work in the same time period. They’re professional and will likely have a huge crew, 14 or more people in some cases. You need to know how many sets of hands you’ll have and how hard those hands can work. As I always say, if you can afford it in the budget, hire a professional crew. After all, the first thing your visitors will see is your plaster and the quality of that plaster will have a huge impact on the overall perceived quality of your house.

I remember visiting a job site some time ago during the plastering stage. I noticed that the plastering crew had removed the protection around the windows and the floor where it meets the wall. They had done this so that they could remove any debris from around the tape that was in place from the scratch and brown coats and thus apply a clean finish coat to the wall. This is a good idea as the little bumps of dried plaster can actually contaminate the finish coat quite easily. Remember that when you scratch the first coat, there will be a lot of debris left on the surface once those scratches dry. Similarly, when floating the brown coat, sand grains and other material can ball up and dry on the surface. If you don’t knock those pieces off of the wall, especially where they might be stuck to the tape around windows, etc… you can end up with these dry contaminants in your finish coat. Now, removing the tape as mentioned above is a great idea, yes; however, the plastering crew had begun applying the finish coat of plaster without replacing the window and floor protection. This is not a good idea at all.

In the areas the plasterers had already applied the finish coat, I found a number of places where the plaster had landed on the windows (wood frames with clear coat finish) and on the finish concrete floor (yet to be stained). I informed them that they would need to clean up the mess they had made under the supervision of my project manager and that they needed to tape everything off before they continued to plaster. It seems common sense to tape off before plastering, but don’t always count on common sense prevailing when you are not on the site to supervise. I make it a rule to be on site at least three times a day if I do not have my own crews there with a project manager. In other words, either my project manager or I will visit each site in the morning, after lunch, and in the afternoon before quitting time. Catching a mistake while it is still small is well worth the effort.

Ever wanted to use hydraulic lime but not been able to find it in your area? Or perhaps you can find it, but it is going to cost you more than the rest of the house combined to ship it to your location. Have hope, there is a way to produce your own hydraulic lime from a much cheaper and more readily available source: quick lime and brick dust.

Simply add up to 5% brick dust to your quick lime and voila, you have a lime with similar properties to hydraulic lime. To be clear, it is not hydraulic lime, per se, but it behaves in much the same ways. This is considerably cheaper and the ingredients are available in most areas.

Earth plasters are hard to work with. That’s not something that most people expect to hear when talking earth plaster. Most people expect that the natural plaster will be easy to work with and more fun because it is basically just glorified mud. Unfortunately, this is not the case. Earth plaster is an art form and a learned skill rolled into one. With all other plasters, your materials are consistent; however, with earth plaster, the ingredients can change from one square foot of your land to another. You have to fully understand the materials you are working with and how they fit the bigger picture of the overall mix.

There will be lots of trial and error learning when creating an earth plaster. For this reason, I suggest you build a small bale wall on which to test your mixes. This can be as simple as a stack of bales that will accept plaster. Make sure that you can put some pressure on the bales as you apply the mud without them falling over. You want a realistic test of the material. If you find that the plaster is too crumbly, then you don’t have enough clay in the mix. If it is cracking like crazy, you have too much clay in the mix.

A Mason Jar Test: Fill a mason jar half way with soil you think would work for your plaster and the rest with water. Shake the jar vigorously for a few minutes to mix everything up. Set the jar down and wait for it to settle out. The sand will settle out in the first few minutes. The silt will be next and will settle out in a few hours, maybe as long as 10. The clay will take a couple days to fully settle. Measure the sand layer, the silt layer and the clay layer. You can determine the percentage of each based on the relationship to each other displayed in the test. You want a soil that is roughly 80% sand and 20% clay and silts.

Testing your soil is a must. Find a large area that you are willing to dig up and use for your plaster. One option is to use the soil removed from any excavation for the house or structure. Screen it through several grades of screening starting at about 1/2″, moving to 1/4″ and ending at 1/8″. This will give you a quality material to work with when plastering with no rocks or major lumps. With the material screened you can run mason jar tests on the final batching of material as before, this time with an actual finished material that is all mixed together. If your testing shows weakness by either too much or too little clay, make adjustments with bag sand or bag clay.

You will want to stabilize the plaster as well. The most common stabilizers are cement, lime, and asphalt emulsion. Each should be added to your test mix until you have enough to resist a hose spraying on a dry test patch. The hose should not erode the plaster at all. Once you meet this grade, you are ready to plaster. You can add horse hair or chopped straw to the mix as well to give it more tensile strength. This does not effect the ratios of the materials, but it will require that you add more water to the plaster.