How to check your installer's window flashing before the siding goes back on

Start at the sill. The IRC sends installers to the window maker's flashing instructions, and where those are silent it requires a pan at the sill, sealed or sloped so water drains out to the housewrap or siding. In DuPont's method for flanged windows, a strip of stretchable FlexWrap is cut at least 12 inches longer than the rough sill, laid across it and at least 6 inches up each side, then stretched at the bottom corners onto the wall face and fastened. DuPont's drawings show no gap where sill and jamb meet, and the guide says the rough sill must be level or slope slightly outward. Flat pieces butted together at the corners do not match those drawings.

Then the sides. DuPont calls for sealant behind the flange at the head and both jambs but not across the bottom, so water that gets in can drain. Jamb flashing should run from 1 inch above the head flange to below the bottom edge of the sill flashing, and the head strip should extend past the outer edges of both jamb strips. The order is sill, sides, head, so each upper layer laps over the one below.

At the top, the housewrap should have two 45-degree slits at least 8 inches long, running up and out from the head corners, according to DuPont's guide. They form a flap that is lifted while the head flashing goes on, then laid back down over it and taped along the cuts. Ragged cuts at the upper corners, or a flap tucked behind the head flashing, let water running down the wrap get behind the window.

Ask the installer which instructions they followed, the window maker's or the flashing maker's, and to show you the page. Then photograph every opening before the siding crew arrives.