The walls of our owner built home are over a foot thick and are stressed skin panels framed with steel open web trusses of my own creation. When after 24 years of toil we were finally getting close to installing the siding, we had to install the permanent windows and window surrounds first. I knew that there would be some window placements that would require cutting out part of a truss and so the solution needed to be quite structural. I had considered a few possibilities and decided on these.

Like every panel of the structure itself, floors, walls, roof, and now many things inside, partition walls, doors, stair treads, dining table etc, these window surrounds are also built as stressed skin panels.

Supplies

1/4" baltic birch plywood, this material comes in both 4/8 as well as 5x5 sheets, I've seen reference to 5x10 sheets but don't have a source for those yet. We used 4x8 sheets for this project.

5/8" osb, I call it flake board

Wood glue, titebond 2

Step 1: Building the Panels

The window surrounds are built from 1/4" baltic birch plywood, osb, eps
foam, and wood glue. We purchased an entire unit of the plywood, which is 65 sheets . We knew we would consume about 40 of these just for the window surrounds alone, and we use this material for all sorts of other creations. Cutting this material was not the most efficient, as we needed 12-1/4" strips so we could only get 3 per sheet with about an 11" strip leftover, but this was no problem as we knew could make use of these elsewhere and have long since used them up. In fact this process, used for much of the interior panels and furniture we are making, creates very little waste as we can make use of quite small pieces of plywood and foam.

We glued two layers of the 5/8" osb, much of which were leftovers from the walls of the house, to make 1-1/4" material and then ripped them on the table saw to make 2" wide strips to match the rest of core which is 2" thick eps foam. We first glued the osb strips to one of the plywood skins and then glued the foam and top skin and nailed with a pneumatic gun. The entire surface of the foam is glued to either skin. A 3/32" notched trowel makes fast work of spreading the glue.

The end result is an 8 foot long plank, we made about 60 of these. We did make a few longer than 8' planks by splicing the skins with a 2" strip of plywood placed to the inside of the plank.

These panels are of course extraordinarily stiff and strong, we made a few extra and have used them as scaffolding planks.

Step 2: Assembling the Frames

I purchased a used radial arm saw from restore, after measuring and confirming that it could lop the planks which are 2-1/2"x12-1/4" in cross section. This worked out quite well. The frames were assembled by butting one piece to the next and attaching with glue and 4" screws.

Step 3: Installing the Pre Faricated Frames

The frames thus created were installed onto the structure from one side or the other, usually the outside. We used liquid nails to glue them and 2-1/2" screws every half foot or so. In places where I thought it likely that we would want to open up the wall later for plumbing or what not, we did not glue on the inside.

Once installed the osb or plywood that is the structural skin of the walls themselves was trimmed to flush with the window frame using a router and pattern bit.

Step 4: Finishing and Conclusion

One hesitation I had before choosing this solution, was that it would necessitate leaving the edge cut from the osb wall panel as finished material against the plywood, and was concerned about how this might look. In the end we are very pleased with how this detail looks. We stained both the plywood and the osb edge and it looks fantastic. we've only stained one frame so far. It's a mineral stain covered with a clear water based poly urethane applied with a brush. Which is mostly what we use for similarly constructed furniture.

We are very pleased with these, once we got started the process went very smoothly and the end result is beautiful and functional. The generous depth is great for the the cats and sometimes I will sit on one of the sills as well. Any doorways that we have made or moved going forward are now also done with these same surrounds, which is an improvement in both execution and end result, over the more laborious welding and truss framed openings that I made before.