What is flashing?

Flashing is thin, water-resistant material — usually metal — installed at joints, transitions, and penetrations to direct water away and keep it out of the building.

Anywhere two surfaces meet or something pierces the envelope — roof valleys, chimneys, skylights, walls, windows, doors, and deck ledgers — is a leak waiting to happen. Flashing laps the materials so water runs over the top and sheds outward instead of getting behind them.

Proper flashing is about shingling the layers in the right order so gravity keeps water on the outside. Most leaks aren't a failure of the material — they're a failure of flashing detail.