Flashing windows - don't worry, I keep my clothes on.

It was almost a full year ago that I described the installation of some of our windows in an earlier blog post. Well, I guess it's about time I described how I set up the window openings and flashing in preparation for the windows. For those of you who may not know, proper flashing of the window openings is a key durability issue since, eventually, all windows leak. When your windows begin to leak, you want that water to be able to find its way back to the outside where it can drain away - and definitely not down into the wooden assemblies within the wall where it may cause mold and rot.

Hard to believe but this scene took place almost exactly a year ago already... I remember how relieved I was after we safely installed all the big pictures windows - what a load off.

Ray LaMontange and the Pariah Dogs - Beg, Steal or Borrow

There are many different products that can be used for flashing and several ways it can be done, but I think the most important things to keep in mind are that it's the rough opening that needs to be flashed and make sure the flashing overlaps in the right direction. With that said, here's what I did:

Before flashing the rough opening (RO) against leaks, I first taped the corners of the RO for "airtightness" with the same 3M 8067 All-weather flashing tape I've been using for this purpose. Next I created a sloped sill using beveled cedar siding which was screwed in place along the outer edge of the RO with a continuous bead of caulk underneath. Finally, I used the 8067 tape (for its actual intended purpose) to flash the corners of the RO at the beveled cedar sill. The 8067 tape works well at these corners where it can be stretched outwards as it is pulled downwards to stick flat against the Typar on the outer surface of the wall.

After stretching the corner flashing into place, I noticed that the 8067 tape's backing has some memory - it likes to "unstretch" itself some. To keep the stretched end of the corner flashing in place, I applied a small patch of the same tape over that end (the idea being that if the corner flashing wants to "pull back" it will have to stretch the patch).

With the corner flashing in place, I cut a rectangular piece of poly-backed bituminous water proofing membrane and stuck it to the beveled sill so that membrane would extend up the sides of the RO by eight inches or so and drape over the outside surface of the wall. As the membrane was applied to the sill and the sides of the RO, I carefully slit the membrane at the corners - careful not to cut into the corner flashing I applied earlier.

With the membrane in place, I flashed the sides of the RO with the 8067 tape. Because I was using 4" tape, I flashed 2" to the outside and 2" to the inside. Notice how this side flashing laps over top of the membrane.

Another piece of 8067 tape was used to extend the flashing a little further back into the RO. Also, notice that this extra piece of flashing covers the loose edge of the Typar in the RO for "airtightness". With that last piece, the RO flashing is complete and ready to deflect any leaks back to the outside.

With the flashing done, the next process is setting up the bottom shims on the sill. I cut these shims from the same beveled siding that I used to slope the sill so that when installed they sit level on the sill. Also, I cut these shims extra wide to better bear the weight of the heavier windows.

The corner-most shims were installed level with each other across the RO so that the window frame would sit level between them. Additional shims were added (every 24" o.c.) and shimmed up under the window frame after the windows were installed in the RO.

Here is how the RO appears after the window is installed. Notice the 8067 tape at the corner of the RO and under the cedar sill to air seal the seam between the plywood bucks. Also notice the extra shims that were added in between the corner-most shims.

An exterior view of another window shows final exterior flashing of the window flanges at the jambs and drip edge. Again, the 8067 tape was used but also some standard sheathing tape ("tuck tape") to fasten the Typar at the window head where it overlaps the drip edge. While the flashing that I installed inside the RO is meant to steer water back to the outside, this final flashing is meant to help keep water from getting into the RO in the first place.