Whether preformed or site-built, pan flashing channels water out where it won’t rot the sills under windows or the floors under doors. Pan flashing can be made from plastic sheeting, EPDM membrane, or something more durable, such as lead-coated copper, or even stainless steel. Nearly as important as head and pan flashing is how it’s tied into the wall behind the siding. Leaking water should roll down to the pan, lip over to a drainage plane, and come out the bottom. Water that leaks into wall cavities causes rot and mold.
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We’re going to look at sill pan flashings, probably the most important part of a window installation. This is what’s going to collect any water that leaks in around the jamb itself either due to an installation problem or due to damage that occurs to the window, or just a defective window. We want to collect the water and drain it out. You can get factory-made or form them on-site. A couple of different products are available that are preformed sill pans for windows; they come in a variety of sizes and shapes and thicknesses. The advantage of these is that they’re preformed, so you don’t have to do any forming on the site. The disadvantage is you have to order them or make sure the lumberyard has them in stock, because that’s not always the case. This is a real simple system with a built-up back dam in the back, and the joints overlap; you either tape or caulk these to create a seal at the overlap point. A more heavy-duty option and larger all the way around has separate corners and a separate sill pan. You’d use either a sealant or tape to seal the joint between the corner pieces and the main body of the sill pan. These have wide flanges at the perimeter, so it kicks the water out onto your drainage plain, and it has a built-in back dam. And they come in different widths, depending on the window size that you’re going to be installing, for 2×4 construction, 2×6, or an extended situation like we have here with the foam insulation on the exterior of the wall. The more common products that are used to make sill pans come in a roll, and they’re flexible flashing tapes like this one, or nonflexible flashing tapes.
Preformed sill pans:
Flexible flashing membrane:
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