When building curved walls, some of the most difficult aspects to deal with are those that don’t naturally curve. For example, large windows in a curved wall will create an area where the window either sits inside the plane of the wall or outside that plane. There is no way to bend the window, other than buying expensive curved windows, so you have to get creative in how you finish the window installation. You might add an over sized sill to handle the difference, or hang the whole thing out side the plane of the wall with a creative faux finish. It’s up to you.
One place that can be difficult is in the mudsills, or tow ups as they are often referred to in straw bale construction. As you can see in the photo above, bendable steel channel sills have been created to make the framing of conventional curved walls easy. These sills work great for stick framed homes and I have used them on several houses with great success. The problem for us bale builders is that the steel channel does not provide adequate nailing surface for the attachment of our mesh, nor does it give us any uplift for the bales to get them above the foundation or floor system. We need something else.
One of the easiest methods to handle this situation is to use laminated plywood cut to the radius needed. Because many, if not most, jurisdictions call for pressure treated lumber to be used in contact with the foundation, you will want to get a few sheets of pressure treated plywood for the bottom lamination or two. Each layer should be glued and screwed to the next. The screws are more of a temporary hold than anything as the glue, once dry, will be sufficient to hold the lamination together. Be sure to use strong glue and one that can handle getting wet without losing strength or bond. Gorilla Glue is a good option; however, it can be a little different to work with, so make sure you follow the product instructions.
In the above photo, you can see how a scarf joint is used at the linear connection points of the laminates. This gives a positive glue joint from end to end of the plywood as well as face to face. Be sure to stagger any end joints by at least two feet.
By creating a simple template for the curve, you can use a router or jigsaw to cut many layers in the exact same shape. Once the whole assembly is put together, you can move it to the location in which is is to be installed. Don’t try and do the entire sill in one big circle, using scarf joints all the way around. It will end up driving you crazy. Instead, create manageable lengths that can be laid in place like any other toe up/mudsill system. Use anchor bolts as you would with other systems to hold the mudsills in place.