New Construction Window Installation: A How-To Guide

Once the WRB has been fastened to the wall, cut the portion of the WRB covering the window opening in a capital I shape. First, cut across the top and the bottom of the rough opening and then down the center. Fold the WRB back across the vertical sides of the opening. Then cut a 4- to 6-inch diagonal slit in the WRB up from each of the top corners of the window rough opening. Fold up the flap and tape it to the WRB above with a small piece of tape.

To flash the window, use rubberized flashing tape and flash the base of the window rough opening from the window frame down over the WRB. Make sure that the piece of flashing tape is 6 to 8 inches longer than the bottom of the window rough opening. That way it'll be long enough to fold up the sides of the trim studs 3 to 4 inches on each side.

First, tape the bottom of the frame and then up the sides. Make a downward cut in each of the corners and then fold the tape over the WRB. Use a J-roller to ensure that the flashing tape is firmly secured to the window frame and the WRB. Cap staples may also be used to ensure that the flashing tape is held in place.

Following the manufacturer’s instructions, install the window. Each manufacturer has specific instructions for installing their product, including appropriate fasteners, caulking, sealing and insulation specifications. It's important to follow these instructions exactly, as deviating from them will void the warranty of the window. Check to make sure that the window is plumb and level.

Now that the window has been installed, it’s time to install the side and top flashing. When choosing a flashing tape, be sure to choose one that's 2 to 3 inches wider than the window trim. This will ensure that any fasteners that go through the trim or the siding will penetrate the self-sealing membrane of the tape.

Starting with the sides, cut a piece of tape long enough to cover the relief slits that were cut in the sill flashing but not so long that it'll go past the top of the flashing tape. Use a J-roller to make sure that the tape is securely sealed to the window flanges and WRB.

Once the side flashing has been installed, it’s time to install the top flashing. Cut a piece of tape that's long enough to go a couple of inches past the sides of the side flashing. Place the tape over the top window flange and the side flashing, and use a J-roller to make sure that it's securely sealed.

Install metal Z-flashing over the upper edge of the top piece of window trim. Be sure to use the type of fastener that corresponds to the material of the Z-flashing that's being used.

Fold down the piece of WRB that was cut and taped above the window so that it goes over the top of the metal Z-flashing. Tape the corners to ensure that it lies flat.