Walls - Day 8-11 - Top and Cap Plates

First piece of cap plate: installed

We already finished installing the walls. Yay! But there are two more elements needed to create the transition between the walls and the rafters: the top plate and the cap plate.

How about a vocabulary lesson?

The top plate is the 2x4 lumber that fits inside the groove in the foam along the top of the walls. It is required.  It spans the seams between panels and adds another layer of holding-it-all-togetherness, like an extra spline.

The cap plate is the optional (in my particular SIP system) piece of 2x6 lumber, ripped down to 4.5" wide, that goes on top of the top plate. The point of the cap plate is to add extra wall height and/or point-load strength (i.e. so the weight that is transferred down the rafters doesn't deform the wall).


Before installing the top plate, we went around with a can of Great Stuff spray foam insulation and filled the electrical chases that we would not be using. The less air circulating inside the walls, the better! 

There are two ways to go about this step. The first one is pictured here: spray the Great Stuff in, let it cure fully (8 hours, according to the can), and then use a serrated blade to cut off the blurp of excess foam that spills out the top of the chase. My floor was soon littered with little pillows of Great Stuff.

Blurp of Great Stuff...

Blurp be gone!

The second, better, less wasteful way would be to get everything in place for the top plate installation, right down to the beads of mastic, then spray the Great Stuff into the chases and immediately install the top plate, before the foam has time to harden at all. That way the foam will bond to the top plate and be forced to expand downward more.

I don't have any pictures of the top plate installation. We installed them just like we installed the lumber splines in the corners of the walls, with the additional concern for making sure that each break between pieces of top plate lumber was at least 24" from any break between SIPs. This allows the top plate to brace the wall system.

The cap plate then goes on top of the top plate, but since it's 4.5" wide, it lies flush with the whole wall. Its seams should be staggered relative to the seams between pieces of top plate lumber.

I opted to install a cap plate along the long walls of the house, but not on the gable ends. This was for two reasons:

1) The rafters will not be supported by the gable walls, since they run parallel to the gable, so the extra point-load strength is not necessary.

The cap plates overlap the gable walls

2) If I run the cap plates over the corners of the gable walls, it creates a uniform ledge from which the all the rafters, including the end rafters, can hang. This way I don't have to do anything special to attach the end rafters--just cut the same birdsmouth in them as all the rest.

However, since the gable walls are--of course--sloped, and the gable wall SIPs were cut with pointed corners, it was a struggle to figure out how to cut the cap plates so that they would run over the corner and be flush with the gable wall.

Same corner, different angle

A better-equipped workshop would have had just the tool for this cut. It is not unprecedented to rip (i.e. cut lengthwise) a piece of lumber on the diagonal, after all. However, nothing we had really did the job--and trying to rip the pieces by hand, with a regular hand saw, was not going to work (I tried! How I tried!).

I eventually remembered that to create a lengthwise opening, you can make multiple crosswise cuts and then knock or chisel out the thin pieces that are left in between the cuts. Dad made a sloped guide for the Skilsaw and we did all four corners like that. It worked okay.

Of course, the weak link is the thin edge of wood, which you can see in the picture above. We had to be very careful not to break those edges when we handled and installed the cap plate pieces!

Lesson 49: When in doubt, pre-drill your nail and screw holes.

 We attached the cap plates using a single bead of construction adhesive and a double row of 12d or 16d, hot-dipped galvanized nails, 6" on center, driven into the solid wood of the top plate below. I am confident that those cap plates are not going anywhere!

The next steps, to create the roof, will be, in order: rafters, blocking between the rafters, outriggers (or lookouts) to create the gable end overhangs, fascia boards and verge rafters all around the edges of the rafters and outriggers, plywood sheathing over top of everything, ice and water barrier over top of that, and finally the metal roofing and edge flashing.