2 Protect against water infiltration

Cut a 6-inch-wide strip of self-adhering waterproof membrane (or a 9- to 12-inch-wide strip of 15-pound builder's felt) 18 to 24 inches longer than the window's width. Center the membrane under the rough opening and adhere it to the existing builder's felt or house wrap. Make sure its top edge doesn't extend above the edge of the opening.

Cut two more strips of membrane (or felt) 1 foot longer than the height of the opening. Center and attach them along each side of the opening, overlapping the strip under the window.

Cut another strip of membrane (or felt) 1 foot longer than the window is wide; center and attach it across the top of the rough opening so that it overlaps the two side strips.

TIP: When applying flashing, layer the material so that any water running down the wall is directed out: Seams should never face up.