What is roof flashing in construction?
Part of the building envelope
A house sheds water in layers: shingles or panels, underlayment, the wall's housewrap or weather barrier, and siding or brick. Flashing connects those layers where they meet. The rule is that every upper layer laps over the one below it, so water moving down the building always stays on the outside.
Where the code requires it
The International Residential Code requires flashing at wall and roof intersections, wherever the roof changes slope or direction, and around roof openings. It also requires a flashing to divert water where the eave of a sloped roof meets a sidewall, the piece builders call a kickout. Chimneys wider than 30 inches, measured parallel to the ridge, need a cricket unless they sit on the ridge.
Sequencing between trades
- Roofers install drip edge, underlayment and step flashing before the siding goes on.
- Siding installers lap the housewrap over the step flashing and keep siding a small gap above the shingles.
- Masons cut reglets or mortar joints so counterflashing can be set into chimneys and brick walls.
- Window installers flash the heads and sills of windows that sit above lower roofs.
When one trade skips a step or caulks instead of lapping, the gap often appears years later as rotted sheathing below a roof to wall corner, a common finding on Georgia homes with vinyl or fiber cement siding.
Learn the parts in roof anatomy explained and roof flashing explained.