PREPARING YOUR WALLS FOR PLASTER

A quality plaster job begins before the plaster is even brought to the site. The substrate is very important because the plaster can only be as strong as the material it is attached to. As we know, straw makes a great base for plaster due to its natural “tooth” or ability to hold the plaster. It is important to make sure that the wall surface is as straight and free of loose material as possible before plastering. It is equally important to make sure that all areas of stuffed straw are tightly compacted and firm, as mentioned earlier in the course. These simple details will help ensure a quality base for your plaster to adhere to.

Sewing LineI have mentioned the use of welded wire mesh in this E-course several times and will do so once again here. The mesh does an amazing job of reinforcing the plaster in a way similar to what rebar does in concrete. Because plaster is a fairly rigid material, it is strong in compressive strength, ie when forces push on it; however, it is not as strong in tensile strength: when forces pull on it. The straw itself helps increase the tensile strength of the plaster and making sure the plaster is pushed deep into the bales will increase this bond and strengthening. The use of mesh gives even more tensile strength to the plaster and makes for an incredibly strong finish for your walls. Make sure the mesh is pulled tight and stapled in place well. The mesh should be sewn from one side of the wall to the other about every two feet on center with roughly 6′ pieces of baling twine. Make sure the mesh is pulled tight to the bales and that there is no movement that would cause the plaster to pull away from the wall.

Ready for PlasterUnlike straw, wood does not make a good backing for plaster and must be isolated from the mud. I use a layer of roofing felt on top of any wood that will be behind plaster. As you can see in the image to the right, wood that is next to plaster and ultimately exposed does not need any felt on it; however, the post that is buried in the plaster does need to be completely covered. In addition, the transitions from bale to wood must also be detailed. Because these two materials expand and contract at different rates, many cracks will appear in the plaster if the transition is not properly detailed with plaster lath. The lath helps support the plaster at the union and minimizes movement as a result of expansion and contraction of the wood.

Covering the wood with felt and mesh is acceptable as long as the width of the wood does not exceed four inches. Anything over 4″ wide needs to be covered with a strip of plaster lath, in addition to the felt and mesh, to give the plaster something to hang on to. Do not extend the lath over the edge of the post into the straw as it will end up loose and may cause plaster to remain unsupported during installation. Plaster should never be asked to span a distance of more than a few inches on its own without added structural structural support. The mesh gives some support and is adequate when the span is 4″ or less but does not provide enough to reinforce the plaster in larger spans. This is especially important on horizontal areas like the top of window and door wells as without this added support, plaster will not hang in place.

Be sure to fill any large gaps in the wall that you may have missed during the stacking, straightening, stuffing and meshing stages. It is best to take care of all of these places before the mesh is installed as stuffing them after the mesh is up is tedious. I like to use a tightly packed and twisted clump of straw to fill the gaps. It is also acceptable to use cob, light straw clay, burlap, spray foam, or other materials to fill the gaps. You can see on the cabin to the right that burlap patches were used in the upper corner to cover larger gapes (straw was packed in behind the patches to give it a solid backing) and spray foam was used to fill some smaller gaps near the right corner post.

When you are ready to apply the plaster, the walls should be tight and solid. All of the wood should be covered and the mesh attached firmly to the wood structure and/or sewn through the bale walls. Any holes or gaps in the bales need to be filled firmly so that when you push the plaster against the patch, it does not deflate. Be sure the site is clean of straw and debris so you can walk safely around the structure. Cover the floors on the interior of the building so dropped plaster will not mar them. Hang tarps on the exterior to protect the plaster from wind, rain, and sun during installation and curing.

H. CurveThere are many choices for plastering from earthen plaster to lime to cement based materials. I could write an entire book about plaster choices and indeed people have done just that. I will keep it simple here and say that my favorite plaster to use on a straw bale structure is lime plaster. Specifically, Natural Hydraulic Lime or NHL. It is durable, easy to apply, it breathes well, it is flexible, it has self healing properties for at least the first year, and it is a natural material. If you decide to plaster with NHL, be sure to contact me as I have two suppliers (one on the East Coast of the US and one on the West) who will give you a discount on your order if you mention my name and vendor code. The material is not cheap, so the discount is worth it. That said, I would not use anything else on my own house and I strongly recommend it for yours.

 

Tomorrow we will look at special considerations when hanging cabinetry on straw bale walls.

Happy Baling!

Andrew Morrison