Mud sills over concrete walls.

In my last post I talked a little about the mud sill assembly that over-lies the concrete basement walls. The installation of this portion of the mud sill was pretty simple but I think it's important to mention again that it is very important to make sure that time is taken to measure out the adjacent stick framed walls carefully so that the second top plate trasitions properly.

Also, I think I'd like to embark on a slight tangent at this point. Having had some time to reflect on the hole process - I do a lot of thinking while I'm building - I have come to the conclusion that the walk-out basement complicates the construction process significantly. For instance, my entire point about paying careful attention to trasitioning this second top plate would be moot if I had decided to build a two-storey slab on grade. I could cite a list of complications that could have been side-stepped by going to a two-storey slab on grade... but since this is just a brief tangent, maybe I'll save it for another post.

Getting back to the mud sills... As with all the other assembly procedures, one musn't forget to ensure the proper application of the air stop tape and EPDM gaskets.

The procedure was as follows:

• The mud sills were pre-drilled for anchor bolts.
• Air-stop tape was applied to each piece of mud sill.
• EPDM gasket (BG 63) was applied to the mud sill.
• The mud sills were bolted to the top of the basement (concrete) wall.
• A chalk line was snapped to mark the exterior edge of the 2x4 plate common to these mud sills and the adjacent stick framed walls.
• The 2x4 plate was applied using adhesive (PL Premium) then straightend and nailed with 2 1/2" galvanized nails.
• The air-stop tape at the mud sill was "connected" to the 2x4 plate.

Lay out the pieces of mud sill, then mark and pre-drill the anchor bolt holes. In my case, the interior edge of the mud sill was set flush with the inside face of the concrete wall.

Apply 4" air-stop tape to the interior edge of the mud sill. Leave a 2" "flap" with its backing still on. Note that in this illustration, the mud sill will need to be flipped over before installation.

Apply the EPDM gasket.

Flip the sill assembly over and install each piece on the wall. Take some time to roughly straighten all the pieces of plywood then tighten the nuts on the anchor bolts.

Apply the 2x4 plate to the mud sill by first snapping a chalk line across all the pieces of mud sill. Put down a bead of adhesive (I used PL Premium) to secure the plate to the mud sill assembly. Lay the 2x4 plate on the adhesive then nail the 2x4 down making sure to straighten it to the chalk line as you go. Once the plate is secured, the "flap" of tape is adhered to the inside face of the plate - covering the plywood edge.

Putting together the mud sill assembly wasn't hard but it wasn't super quick either. After all, we're talking about an assembly, implying (obviously) that there is more than just one piece to the mud sill. The extra time involved in the installation can almost entirely be attributed to the fact that this isn't a traditional one-piece mud sill. There was also some extra time lost to the fact that the plywood only comes in 8' lengths - more pieces to work with... and again to the fact that the plywood had to be ripped to the proper width. But I'm not sure how significant these factors are over all.

As between seperate sections of  the stick framed walls, it worked out well to leave about an extra 1/8" of both gasket and tape at each end of the plywood.

Installing the plate over the mud sill assembly went quickly - 16' lengths of 2x4 were used here. I used galvanized nails since I was nailing into pressure treated plywood and I happened to have 2 1/2" galvanized nails on hand. I'm not sure it matters if the nails are galvanized or not since they really only serve to hold the plate along a straight line until the adhesive cures. The adhesive is doing the lion's share of securing the plate to the mud sill assembly.

2x4 plate makes a successful transition to the stick framed wall. Notice the "flap" of tape with it's backing still on next to a short section that has been adhered to the plate. Each separate legth of tape corresponds to a separate piece of treated plywood. Notice that tape has already been adhered to the plywood sheathing along all the gasketed concrete/wood interface. Notice that no tape has yet been applied to connect the top of the plywood sheathing to the top plate of the stick framed wall.

Mud sill/plate assembly at outside corner with floor framing above. The EPDM gasket is visible at this corner. Notice how the 4" tape bridges the gap between the plywood and the 2x4 with plenty to spare.