How to Install a Waterproof Window

Cut a 9-inch-wide strip of builder's felt with a utility knife. Make it 18 inches longer than the bottom of the rough opening for the window. Center the felt under the opening and fasten it to the wood with a staple gun.

Take a 6-inch-wide strip of self-adhering waterproof membrane the same length as the builder's felt and remove the non-sticky backing. Center it over the sill of the opening. Let it overlap the felt by about 2 inches and extend into the empty space of the rough opening.

Use the utility knife to cut the membrane at the corners of the opening. Press the cut section of membrane down against the sill. Fill in any sections of the sill where wood remains exposed with small swaths of the membrane.

Cut two more strips of membrane a foot longer than the length of the sides of the opening. Center the strips on each side of the opening, ensuring that they overlap the strip on the bottom.

Take the window and center it in the rough opening.

Check the window for level with a carpenter's level. Place shims under the frame to get the window flush with the header. Check for plumb with the level. Check for square with a tape measure by measuring from one diagonal corner to the other. The two measurements should be with 1/16 of an inch.

Hammer in nails in the flanges of the window once the window is level, plumb and square.

Cut a strip of membrane 1 foot longer than the width of the opening and place it across the top of the rough opening.