How is roof flashing installed around a chimney?
Short answer. Chimney flashing is a set of pieces, not one: an apron on the downhill face, step flashing woven into each shingle course up both sides, counterflashing set into the mortar joints and lapped over the steps, and a back pan or cricket on the uphill side. When any piece is missing or replaced with caulk, water gets in, most often at the uphill corners.
The pieces and their jobs
- Apron flashing: wraps the downhill face of the chimney and lies over the shingles below it.
- Step flashing: L-shaped pieces, one per shingle course, up each side. Each piece is lapped by the shingle above it.
- Counterflashing: metal let into a mortar joint cut into the brick, bent down over the step flashing so water cannot run behind it. On the sides it is stepped to follow the brick courses up the slope.
- Back pan or cricket: a metal pan on the uphill side, or, for chimneys wider than 30 inches measured parallel to the ridge, a small peaked cricket that splits water around the chimney, as the International Residential Code requires unless the chimney sits on the ridge.
How to judge yours
- Counterflashing held on only by caulk smeared across the brick face, with no cut joint.
- Black roof cement troweled over everything, a sign of earlier leaks.
- Rust at the bottom edge of the step flashing.
- No cricket behind a wide chimney, with leaves and pine straw piling up there.
- Water stains on the attic framing at the uphill corners.
Framed chases are different
Many Georgia homes have a wood-framed chimney chase covered in siding rather than brick. Those are flashed like a wall, with step flashing tucked behind the siding and a kickout at the bottom, and topped with a metal chase cover that needs its own attention. Robust Roofing's guide to chimney flashing leaks shows what a real fix includes.
See also chimney leaks and crickets and roof flashing explained.