Flanged Window Replacement in a House with Wood Siding (3/5)

Step 3: Prepare the Opening

Learn how to clean the existing rough opening and inspect for damage before installing the new window. Also, see how to waterproof the opening with self-adhesive flashing tape.

Bill Robinson: When preparing the rough opening, it's important to look for damage, and moisture is the main culprit. To check for damage here, I'm going to use my chisel and stick it into the wood and pry on it. Look for the wood to splinter up like that. That means it is sound wood. If it crumbles, you have to replace it.

The felt paper you can see here is sound–it's actually solid behind. I didn't cut it when I removed the siding, so I'm not going to worry too much about water getting behind there. The felt paper is in pretty good shape.

The window opening is big enough to fit the window. And it's plumb, level and square enough that I can make some adjustments when I install the window. So we're good to go.

Bill fills in missing gaps of sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. around the rough opening. Then he vacuums all the dust and dirt so that he'll have no trouble installing the rigid and self-adhesive flashings.

Bill: This is vinylCommon term for polyvinyl chloride (PVC). In chemistry, vinyl refers to a carbon-and-hydrogen group (H2C=CH–) that attaches to another functional group, such as chlorine (vinyl chloride) or acetate (vinyl acetate). coil stock. It's real thin, and what's nice about it is that it pretty much won't react with moisture or anything. I cut this in narrow strips and slide it behind the siding. Any water that might get inside the window opening will be forced to the outside. I work the coil stock down behind the siding carefully and use a little pry bar to pull the siding away from the felt paper so that I can get the vinyl coil stock behind there. It's kind of a tight fit, but I need to get it in there.

So, I've done the bottom with the vinyl coil stock. I'm going to do both sides but leave the top open so that the flashing tape will actually stick to the raw framing itself on the wall. Everything installed while you're flashing should be installed shingle style, which means what's at the bottom should be covered by what's above it.

Next, put in the self-adhesive flashing. The first piece will be here on the sill, overlapping the vinyl coil stock, and it will roll into the sill a little bit. Self-adhesive flashing has release paper on it. I'm going to position it where I want it to be with a little bit wrapping up at each end. Get it to stick, pull the release paper off, and keep it in a straight line. Readjust it as you go. If you take all the release paper off at one time, you'll get it wrapped up into a sort of snowball or flashing ball. With the corner in, make cuts and fold it out along the face of the wall. Whenever you're applying self-adhesive flashing, it's critical to use a J-roller; that's going to make it stick. Next, flash the corners.

The flashing material is 6x6 flexible self-adhesive squares. Bill uses them to seal up the corners. They're a little tricky to work with, but after he's got them into position, he J-rolls them for a good seam. Next, he flashes the sides of the rough opening with more self-adhesive flashing material.

Bill: I have the side jambs flashed before I put the window in. Add a corner shield at the top for a little bit of extra insurance to keep water out of there.

Now that the top corners are in, it's time for the sill pan. The sill pan is especially important because it's going to drain any moisture that gets inside the opening to the outside. The sill pan is pretty cool. On the back is self-adhesive flashing that sticks to the frame. There's some foam in the back that won't absorb moisture. That's going to set into the inside of the window. Any water that gets inside will be forced to the outside–but here's the cool part. This has some sort of wicking material that, when it faces outside, will drain water to the outside even if the sill is sloped backward, toward the inside of the house.

To ensure the window and rough sill aren't damaged by leaks, Bill wants pan flashing running from the inside of the sill, out and down the sheathing. The flashing isn't quite wide enough, so he cuts strips from the sill material and layers them. Like everything else here, he starts at the bottom and works his way up, shingle style.

Bill: Now that the filler piece is in, I'm going to put in the sill pan piece itself. I've already cut it to length. Take off part of the release paper, pull it out of the way, set it down firm here, and pull the paper out slowly to keep it straight.

With the sill pan in, it's time to dry-fit the window. If the dry fit looks good, it's time to take it out and do the permanent install.

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