Mud Sill Installation for Cabin Log Walls

With the foundation finally finished, it was time to start on carpentry.

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I’m really glad to be finished with concrete work. I don’t have experience with concrete, so I spent a lot of time grinding my teeth about how it would turn out. Concrete seems so final compared to carpentry. Mud has its own stopwatch, and anything you don’t finish before it cures is something you have to live with. Do-overs are expensive and rarely an option.

Carpentry, on the other hand, is fun. I understand it, it’s relatively simple, and I like the smell of sawdust. The first carpentry project will be to lay down mud sills on top of the cast-in-place foundation stem walls.

Mike drills for bolts

The mud sill is a layer of 2X8 pressure-treated lumber. It fits over bolts that protrude from the concrete. Normally it is then secured with a washer and nut on each bolt. Our technique is a little different, and I’ll write about that in future posts. Our anchor bolts are our thru rods.

The rim and floor joists will sit on top of the mud sill. Log walls then sit on top of the floor framing. I inset the mud sill ½ inch from the exterior of the foundation. The rim joists, subfloor edges and bottom plate all will be indented. It adds up to a little over a foot in height.

When logs are stacked on top of all this, they will be brought out to flush with the foundation. That ½ inch indentation below will be filled with trim board, probably a tinted cementitious-fiber board that never has to be painted. Then we will have 3 feet of fireproof structure between the ground and the first log.

Back to the mud sill. Between the concrete and the board is a foam strip of sill sealer. It’s there to prevent capillary leaks of water into the crawlspace, and to keep bugs out. A bead of caulk between the butt ends of the boards serves the same purpose, for example at corners.

Mud sill installed

My ski buddy Mike came over for a day-long work party. Working as a team we measured X and Y distances for each bolt and drilled to the boards to fit. This is the time to level up the foundation if it's off anywhere. My foundation was dead level (within 1/8 inch) all around.

Plan ahead to keep the boards true on the foundation. When I delivered the lumber to the site I stacked it on stickers and clamped the ends to keep it from warping. However once it was installed it began to curl. I was able to clamp the ends to the concrete, except I didn’t have enough clamps for all of the ends. The warping caused problems later when we were trying to level the floor joists.

In retrospect it would’ve been worth wet setting some anchor bolts where the board ends would meet, just to keep them true until the weight of the floor framing takes over.