Wednesday, December 13th, 2006 | Popularity: 16%
There are specialty coating systems available that can be used if you have the skills, but in general, they are much more difficult to finish well. In fact, it may be easier to simply finish the concrete in such a way that the foundation slab can be used as the finish floor of the home. Be sure to protect the floors during construction. If you plan to acid stain the floor, make it a point to know the details of that art. For example, you would want to know that a stack of lumber left on the concrete during construction will cause the stain to fail in that area leaving a blank or discolored straight line that will not look very good at all in the finished floor. You will also have to be very clear with any subs that the slab is the finished floor and inform them that they cannot write notes on the floor with a construction crayon or snap lines with red chalk. There are many things to keep in mind here and some basic knowledge is a must. Knowing what to watch out for ahead of time will be a life saver in the end!
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I was recently asked if it is possible to do an earthen or cement skim coat over a slab foundation. The answer is yes, you can do a skim coat of either clay/straw (earthen floor) or cement over the top of a concrete slab; however, there are minimum thicknesses that need to be maintained. At least 3″ of concrete are required unless a specialty skim coat is used (this takes extra skill).
There are specialty coating systems available that can be used if you have the skills, but in general, they are much more difficult to finish well. In fact, it may be easier to simply finish the concrete in such a way that the foundation slab can be used as the finish floor of the home. Be sure to protect the floors during construction. If you plan to acid stain the floor, make it a point to know the details of that art. For example, you would want to know that a stack of lumber left on the concrete during construction will cause the stain to fail in that area leaving a blank or discolored straight line that will not look very good at all in the finished floor. You will also have to be very clear with any subs that the slab is the finished floor and inform them that they cannot write notes on the floor with a construction crayon or snap lines with red chalk. There are many things to keep in mind here and some basic knowledge is a must. Knowing what to watch out for ahead of time will be a life saver in the end!
When applying a skim coat of stray and clay, it is a good idea to use about 2″ of material to get proper adhesion and compaction. Be sure to apply the earthen floor in lifts. In other words, apply one inch of material and then let it cure. Moisten the surface of the cured floor and apply another 1/2″ for proper adhesion to the base coat and then apply the final 1/2″ while the previous 1/2″ is still moist. This will ensure a tight bond between all the coats. Compact and finish the floor as you would any earthen floor. In most cases if an earthen floor is used, you can eliminate the concrete slab altogether and build the earthen floor over compacted gravel lifts. A foundation can be poured to carry the loads of the home and then the gravel and earthen floor can be built up inside the foundation.
If an existing concrete slab is to be covered, many options are available and the ones listed above are just two of many.
Popularity: 16%
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Andrew Morison is a licensed contractor specializing in straw bale and green construction. He has shown thousands of people how to build their own straw bale projects through his comprehensive series of instructional straw bale, concrete foundation, and plastering DVDs. You can check these out at http://www.LearnStrawBale.com.
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September 13th, 2007 at 1:29 pm
Can an Adobe/Cob floor just be poured in a similar fashion to a concrete slab floor?
Would troweling in a 2×4 baseplate for interior non-loadbearing walls give adequate support? Would composite lumber perform better to avoid expansion and shrinking?
September 13th, 2007 at 1:29 pm
I do not have a lot of experience with earthen floors; however, I can speak from what experience I do have. The process for an earthen floor is different from that of a concrete floor. For an earthen floor, you need to build in lifts. IN other words, a few inches at a time and with different materials. The first lift is drain rock that creates the base for the floor. Drain rock does not have to be compacted, because of the nature of the stone and how it nests with itself. On top of the drain rock you need to add about 4″ of 3/4″ gravel that should be well compacted. On top of that goes the earthen floor material which also needs to be compacted in place. It is crucial that the moisture content of the floor be perfect as too much water will cause the earth to squish and too little water means the earthen material will be too dry to compact. Either way you will not be able to get adequate compaction. As you install these lifts, be sure to use a screeds board to ensure the material is installed level and evenly. The last 1/2″ - 3/4″ is the skim coat of earthen material which acts as the finish for the floor. It smooths everything out like a final coat of plaster and yields an even texture. After that has all dried, you want to put several coats of linseed oil on the floor and burnish it in. As you can see, it is a different and labor intensive process. You will also have to perform annual maintenance on the floor to keep it in good working order. One major advantage of an earthen floor (other than the natural materials of course) is that it is softer under foot than concrete. If you spend a lot of time standing on the floor, the earthen floor is much gentler on your body.
December 26th, 2007 at 2:26 pm
Andrew -
I am replacing carpet on a concrete slab and would like to do an earthen floor. I want to limit the earthen floor applied on top of the slab to 1 1/2 inches at most and 1 inch is preferred.
I am not sure from the article what the minimum amount of earthen skim I would need. Could you point me to any resources you may have?
December 27th, 2007 at 10:13 pm
Rudy,
I do not see it being a problem to stay with a one to one and a half inch earthen floor if the concrete is stable. If the slab is failing, you will need to fix it before you move forward with the earthen floor. In general, a structural concrete slab should enable you to finish the floors at the thickness you desire. I do not have a specific source for earthen floors that I would recommend. The truth is I don’t do that many of them in this area. You might try connecting with Bill and Athena Steen at the Canelo Project. I hope that helps.