Because you have to cover the edge of the foam. This is done with extension jambs, just like finish carpenters do inside.
With the Tyvek, flashing tape, and styrofoam installed, the windows are already watertight, so the trimwork isn’t passing itself off as a water barrier.
But it’s not NOT trying to be a water barrier, either.
The point is that the trim doesn’t have to be watertight, it mostly just has to look good and protect the flashing tape from UV damage.
This wall assembly is designed to drain water out if it gets in, and the bulk water drainage plane is the face of styrofoam.
Also considering that the trim and siding are Boral TruExterior made with polyash, which is completely impervious to water, this wall system is unbelievably redundant, resilient, and robust. Almost everything can go wrong and it will still work.
The best way to approach the project is to preassemble the casings, sill, and extension jambs.
The first part is the hardest: the sill, which will be different for almost every window. These sills have a couple of notches on each end and they must fit tightly between two stops.
Begin by making a template
I rip both edges to 15-degrees so that the sill slopes down and away.
There’s nothing magic about 15, it is just easy to remember and my saws have marks at 15 degrees on them.
Sliding the sill template against the face of the window, I mark the edge of the window stop and the outer edge of the Drainvent rainscreen.
I also measure how deep the stop is, so I’ll know where to cut the notch.
Back at my worktable, I square up the marks and draw the profile.
Cuts that are parallel to the outer edge are cut with the same 15-degree bevel.
Because my jigsaw is from the 20th century, it is cumbersome to switch between 15-degrees and 0 degrees, so I just tilt the saw plumb for the square cuts.
I usually end up back-cutting with a utility knife, so I’m not looking for 100% accuracy yet.
I slip the template into place to see if it fits, which it does, and then I check out the fit of the extension jamb using a scrap with a 15-degree cut on the bottom.
So far, so good.
I mark out and cut out the other end of the template piece and then slide it into place to test-fit it and test the extension jamb depth and angle, again.
As it should be, the jambs seem consistent.
With an accurate template, I have almost enough information to make the actual sill.
I just need to measure the critical fit, between the stops.
And then I write down the number, because, you know, sometimes I forget stuff.
Back at the table with my parts, I trace the extension jamb and side casing onto the template for a couple of reasons.
One, to determine the overall length of the sill -- right now, the ears are running long and I need to trim them.
And two, so I’ll know where to locate the screws for the extension jambs.
I square up the marks on the template and transfer them to the sill and begin drawing the final piece.
And then I cut those notches.
With the sill cut to size and shape, I test the fit. Seems like it fits.
Now, I can slip in the extension jambs and mark the tops for length. They’ll extend about ⅜ inch above the window stops to maintain a consistent reveal around the perimeter.
Now I’m ready to begin assembling the parts
I’ll screw the jambs to the sill with stainless steel finish screws. But first, I drill pilot holes.
I drill through the top within the jamb marks that I made earlier.
I drill just enough to push the drill bit through the sill.
Then I flip it over and countersink the holes so I can bury the screw heads.
As I screw the frame together, I am careful not to mess up while the camera is rolling.
Next, I add the side casings, which are beveled to 15 degrees at the bottom, and about ⅜ inch longer than the extension jambs, again to create a consistent reveal.
I attach the side casing with a brad nailer.
The head casing goes on next. To keep the face flush with the side casings, I’ll pocket screw the head casing to the sides.
So here, I’m marking where the pocket screws should go.
I set the stock in the pocket hole jig and drill the holes.
On a flat surface, I lay the window frame on its face and even up the head casing.
And I screw the head casing into the sides.
With the whole thing assembled, I nudge the side casing square with the sill and I hit it with a brad nailer.
Now I can slip the frame into the hole and check the fit.
It shouldn’t come as a surprise that it fits, because there has been so much back and forth, but what do you know, IT FITS!
I’ll hold it in place with a screw for now but I’ll bring it back to the bench for painting, where I will also add the sloped drip cap to the top.