After much research, this is how we did it:
First, we took a razor and made an inverted “I” shape in the tar paper. We did this from inside the house, and then bent those pieces inwards around the frame. (We basically made a peace sign without the bottom vertical line – maybe a pie of thirds?) We also cut two diagonal lines at the top corners of the window opening, so that the window’s nailing fin can fit underneath the paper. This prevents water from slipping behind the fin on its downward path.
Next, we nailed in the tar paper with 4D nails (small nails we happened to have leftover from working on the rafters). We used 2-3 on each section, depending on how large the window was.Then, we made pan flashing out of window flashing tape and attached it along the bottom sill of the window. We made sure 1/3 to 1/2 of it was hanging out the window, and then used the razor to cut the corners so it would fold down onto the paper. The ends of each piece of tape extended 6 inches above the sill for added protection.
For tape, we chose to use a butyl-based tape. We researched flashing and found that aluminum is easily corroded by the tannins found in cedar (what we’re using for our siding), and that PVC tape is toxic (because, well, PVC is usually full of all kinds of additives like lead and pthalates). So we chose a standard butyl-based tape, which uses a simple poly-propylene backer, so it’s not quite as toxic. But, as we found, it doesn’t stick very well to tar paper. Drew used a heat gun to better adhere it, which helped, but we’re still working out how to increase its adherence to the tar paper. It probably would have worked even better had we used plastic house wrap for our house, but cedar siding is notorious for tearing that to shreds. So, for now, we’re happy with the tar paper.
After this, we siliconed around the top and side edges of the window. We siliconed as close to the edge as possible, since we’re using windows with different size nailing fins. On the top edge, we siliconed underneath the flap we had cut earlier.
Next, it was time to insert the window! We made sure it was under the top flap and that the nailing fin rested on the silicone.
Then, we inserted shims from the inside of the house to make the window angle outward slightly, so that rain won’t pool up on the edge or underneath the window frame. Shims are also used for making the windows level. (Note: before we began this process, we made sure our whole house was still level on the trailer, so that our window leveling would be accurate… There’s a lot to think about.)
For the shims, we used some cedar shims Drew’s dad offered us. If you want to make your own, Drew’s dad said you can cut pieces of wood at a very shallow angle (~4 degrees). We usually inserted only two shims per window.
Once that was complete, it was time to nail in the window. We used 8D nails and nailed into all the nailing fin holes around the edge. We ran into problems where our metal house strapping is, because we couldn’t get the nails to go all the way through. In these places we ended up pre-drilling the holes before nailing.
Then, it was time to tape the edges of the window. We taped the bottom first, going about six inches past the ends of the nailing fins, for extra coverage. Then we taped over the the vertical sides, covering the bottom tape (again, water always flows down). We cut a smaller piece of tape and used it to cover the top, and also attached a couple along the lines we had cut diagonally in the tar paper earlier.
And here is the finished result:
Leave questions and comments below!
Next, we will be installing our metal roofing. Wish us luck!