Window and Outer Flashing
Before placing the window, I caulk the top and sides of the flashed opening to provide a bed for the flanges (see slideshow). I don’t caulk the area behind the bottom flange, though — that way, water can drain to the outside if necessary. For added insurance, I apply a continuous bead of caulk to the horizontal surface of the sill, about 2 inches in from the sheathing. This acts as a dam to prevent wind-driven rain from penetrating beyond it.
After installing the window and confirming that it’s plumb and level, I nail it in place according to the manufacturer’s directions.
Once the window is secure in the opening and operating freely, I’m ready to apply the outer flashings. I install the side pieces first, working from the top edge of the inner head flashing to the sill, then I go back to the top and add the head flashing. Finally, I pull down the folded strip of housewrap at the top and seal the corner cuts with strips of housewrap tape. I don’t tape the bottom of the housewrap, so that any water that penetrates the housewrap can drain rather than soaking into the sheathing.