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!”
The fact of the matter is that the plaster is what people will see when they walk up to your house. The value of your home and any judgment of its overall quality will be made in large part as a result of what the plaster looks like. This should be incentive enough to do a good job on this part of the process. If it’s not enough incentive for you, then consider that weaknesses in the plaster can lead to damaged bales and ultimately, to overall wall system failure. Surely that’s enough to get you fired up.
There are so many details about plastering that must be addressed each and every time you apply it to the wall. Covering all of those details in one blog post will not do them justice. I have covered many of the details in other blog posts, so I’m going to continue that trend by adding another aspect here: the direction of application. Believe it or not, it actually matters how you apply the mud to the wall. There are several reasons. First, if you apply the mud the wrong way, you will actually change the physical characteristics of the plaster in the process which will weaken it. Secondly, applying the plaster in the right direction will make you more efficient in your task. Let’s take a look at the proper direction of application and how it effects the overall plastering process.
Maintaining the Physical Characteristics of the Plaster
I mentioned above that applying plaster in the wrong direction can actually change its physical properties, causing weakness within the plaster. This is because overworking the plaster separates the fines from the larger grains within the mix. In the field this means that the “cream”, in this case the lime, is drawn to the surface of the wall and the aggregate (sand) is pushed away from the surface. This separation causes the plaster to fail over time because the binder (lime) is no longer properly mixed with the aggregate (sand). Chalking will occur on the surface of the plaster and the wall will be prone to general failures such as delimitation, where the plaster literally falls off of the wall, and/or excessive cracking.
So how to not overwork the plaster? This brings us back to the direction of application. Mud that is applied to the wall should be placed so that it is pushed into any plaster that is already on the wall. For example, if you are working from the top of the wall down, then you would apply the mud to the top edge of the wall, cutting it into the ceiling, soffit, or rafters. Once placed, your next application of mud would start about one to two feet below the initial placement (depending on how much plaster you apply in one swipe) and would be worked back up into the existing mud. This allows you to join the two sections of plaster together without overworking the material. If you had placed your next trowel full of mud on top of the existing plaster and then pulled it away, you would risk overworking the material.
Efficient Plastering Technique
Applying plaster in the right direction helps to improve your efficiency as well. By placing the mud directly on the bales, and not on the existing plaster, you are able to physically push it into the straw and thus create a solid “keying” of the plaster to the wall. This makes the plaster/straw bond stronger and helps speed the process by limiting the adjustments you will need to make to properly key the plaster. Applying the plaster in the right direction also means that you can see your progress easily and that you can gauge the plaster thickness as you continue down the wall.
When working from the top down, you also minimize the need to correct drips and plaster falls. Consider that if you plaster the wall from the bottom up and drop plaster off of your hawk or trowel as you move up the wall, it might land on the area you just finished plastering. Now you have to go back and repair an area you just finished. That’s wasted time. Plastering from the top down means you are always working on a fresh wall and there is no chance of damaging plaster you have already placed.
Finally, most of us are right-handed and so will apply the plaster in a stroke from the bottom up and from left to right. Lefties would apply from the bottom up and from right to left. This is the most efficient and comfortable stroke to make. It also lends itself perfectly to applying the mud from fresh bale into plaster when the wall is worked from the top down. Picture the top three feet of the wall already plastered. As you stand on the ground and apply the next swipe of plaster, you would start roughly two feet below the fresh plaster line. The plaster would then be pressed into the wall and lifted from the bottom up and to the right (or left if you are lefty) in a sweeping motion. The arc of plaster would overlap the line of previously applied plaster just enough to blend the two lines together. In this way, there is no overworking of the plaster and your stroke is simple, consistent, and efficient.
Window Well Headers
One place where the application of plaster is difficult to get right is the interior well headers above windows and doors. They are hard to plaster because gravity wants to fight the plaster all the way to the end. It’s not easy to get plaster to hang upside down. In fact, many people opt to use lintels, a solid material such as wood, over the tops of their windows and doors. This eliminates the need to plaster upside down and is a beautiful look. If, however, you choose to plaster the wells, you will definitely need some guidance.
The first ingredient is an adequate substrate. You cannot ask plaster to hang upside down without providing it something to hang on to. I like to use either a self-furring plaster lath, one that has dimples in it to hold it off of the wall surface, or a combination of welded wire mesh and standard plaster lath. In the latter scenario, I place the welded wire mesh onto the window well head first and then cover it with plaster lath. This gives a little separation between the window well head and the plaster lath and allows the plaster to attach more easily. Be sure that all materials are stapled firmly to the window well head box (box beam above the window or door) as any movement in the substrate will make it harder to keep the plaster from falling off.
When applying the plaster, I like to do the head wells only after I have plastered the sides of the opening and before I plaster the bottom sill/seat of a window. The plaster on the sides helps to hold the top plaster in place while leaving the window sill/seat unplastered until the end avoids issues created by falling plaster which would otherwise damage the sill if you have already completed it. Furthermore, the amount of plaster that falls may be just enough to smooth out and properly seal your sill. No wasted mud!
Work from the sides towards the middle of the well head applying only a thin layer of plaster. Be sure to press it fully into the plaster lath so that it can hold onto something. The key is not to apply it too thick. The thicker it is, the heavier it is. Instead, apply a thin layer across the entire well head and then come back a few minutes later and apply a thicker layer on top. Typically the time it takes to place a thin layer across the well is enough to turn around and start at the beginning again adding the second layer. Be sure to work from the edges in once again.
The hardest part is typically the curve out and away from the top of the opening. I like to apply the plaster on the vertical section of wall above the opening after I apply the well head mud. This gives the “upside down” mud a little time to set up and really hang on tight. Once the vertical section of wall above the well head is done, I add plaster to the curve, again a little at a time. I then pull from the center of the curve away, in each direction, towards the existing mud. Once placed, I leave it alone for a few minutes. After it has had a chance to set up, I come back and smooth the entire curve out, making sure the plaster is well seated into the straw. It’s really important that you not create suction with your trowel when working these areas as that will pull the mud right off of the wall. Be sure to keep the leading edge of the trowel (the one towards the direction you are moving) slightly elevated off of the wall surface. This allows you to smooth the wall but breaks any potential suction.
Like I said earlier, there are a lot of details to keep track of. These will help you get moving in the right direction. If you want to learn more about plastering, please check out my DVD, the How to Guide to Plastering with Natural Hydraulic Lime Plaster and/or consider joining me at a hands-on workshop. Above all, be patient with yourself and have fun. After all, if it’s not fun and rewarding, it might not be worth all the effort. You can always hire it out!