Installing Flanged Windows – Part 2: Prepare the Rough Opening

This how-to video walks through the details that go into properly preparing a wall opening before a new window goes in, using an Andersen 400 Series window as the example. The builder cuts the opening in the wall sheathing, softens the sharp edges, and then focuses on two things that determine whether a window leaks over time: the air barrier and the water barrier. He ties the wall's air-sealing layer (in this case, Zip sheathing tape) across the opening so there's no gap once the window sits in place.

For water, he builds what's called a sill pan, a shaped, watertight tray at the bottom of the opening that catches any water that gets past the window and directs it back outside instead of into the wall. He uses a strip of beveled wood to create a slight slope and a raised "back dam" at the inside edge, then covers it with a stretchy adhesive tape, pressing it firmly so the glue bonds completely. He notes the difference between a simple strip of tape (a sill flashing) and an actual sill pan with a back dam, which offers real backup protection if the window ever leaks.

For a homeowner, the takeaway is less about doing the work yourself and more about understanding what quality window installation involves. A sill pan, back dam, and continuous air and water barrier around a window are details that affect how long a new window performs and whether small leaks turn into hidden wall damage. If you're planning a window replacement, these are the kinds of details worth asking about.