Installing a Flanged Window - Preparing the Opening
In the last few years window manufacturers and builders have realized the tremendous importance of proper flashing around window and door installations to ensure that water and moisture are properly directed away from the opening. So always read and follow the flashing instructions that come with the window.
Here are the steps that we generally follow. (We call our theory of flashing, “Be the Raindrop”. In other words, we imagine how moisture will run down the surfaces and we try to make sure that the raindrop will always be directed away from the framing and sheathing of the house.)
Cut the house wrap and wrap it back around the sill and the trimmer studs. Peel the house wrap back from the top of the opening and secure it out of the way. (Once the window is installed we’ll flop it back over the top nailing flange of the window.)
Put a short piece of adhesive flashing across each of the bottom corners of the opening, slice it, and fold it back onto the sill and trimmers.
Place a piece of adhesive flashing across the sill, running it up the trimmers about 3 inches. Slice it and fold the rest down across the wall below the opening.
Put caulk along the sides and top of the opening, but NOT across the bottom.
Setting the Window
Set the window in place and center it in the opening. Determine which side is higher and secure the window on that side at the bottom of the opening with one nail through the flange. (Use galvanized roofing nails.) You’ll need a helper or a free hand to hold the top of the window in place while you take this and the next step.
From the inside, shim the low side of the window up until the
bottom of the window is level. Then secure that side at the bottom corner with one nail through the flange.
Square the window up by checking the sides for plumb, and checking the diagonal measurements to make sure they are equal. When the window is square, nail the top two corners.
Open and close the window to double-check that it is operating properly. Remove nails, adjust and re-nail the corners if needed. Then finish nailing the window all the way around the flange, usually with a nail every 8 inches or so.
Finishing the Flashing
Run a strip of adhesive flashing up each side of the window, covering the flange and nail heads and lapping out onto the housewrap. Run these pieces about 5 inches higher than the window.
Apply the final piece of adhesive flashing across the top flange, running 6-7” past the window on each side.
Flop the upper piece of house wrap down over the top flange, and seal the cuts in the house wrap with house wrap tape.
Once the siding material is installed, caulk between the siding and the window along the sides and top, but not along the bottom.
One of the window manufacturers we work with has and excellent set of window installation instructions on its web site. The technique is a variation of what we’ve described here, but with a few more features. Again, be sure to read and follow the instructions that come with any window you install.