Flashing Under Doors and Windows

If water gets past a window or door, where does it go? Without the right barrier underneath, it soaks into the framing below, and that damage can go unnoticed for years. A recent Q&A explains why a "sill pan" matters: a layer built into the rough opening under a window or door that catches leaks and directs them back outside instead of into your walls or floor.

The homeowner who asked had discovered rot under a sliding patio door while working on their deck, along with water damage to the flooring inside. The expert's answer confirms this is a known failure point. Builders create sill pans using flexible butyl tape, liquid-applied flashing, or preformed metal or plastic pans. A good installation slopes the sill slightly so water drains outward, and adds a small raised strip called a back dam behind the unit, sealed in place, as extra insurance against water pushing back inside during storms. In some cases, especially in wet or coastal areas, installers combine metal pans with flashing tape to prevent both leaks and condensation buildup.

If you have an older door or window, particularly a patio slider, and notice soft flooring nearby, discoloration, or a musty smell, it is worth having someone check what is happening underneath. Proper sill-pan flashing is a standard part of a correct replacement job, so if you ever have a door or window replaced, this is a detail worth asking about.