What is chimney flashing, and how is a chimney flashed?
Short answer. Chimney flashing is the metal that seals the joint between a chimney and the roof. A chimney is flashed in layers from the bottom up: apron flashing across the front, step flashing woven into each shingle course along the sides, a back pan or cricket on the uphill side, and counterflashing set into the mortar joints and lapped over everything below it.
The order of work
- Prep: remove the old flashing and the shingles around the chimney, replace soft decking, and check the brick and mortar.
- Membrane: run self-adhered membrane onto the deck and a few inches up the chimney sides.
- Cricket: behind chimneys wider than 30 inches measured parallel to the ridge, frame and sheathe a cricket, as the International Residential Code requires unless the chimney sits on the ridge, and cover it with membrane.
- Apron: bend a piece across the downhill face that runs onto the shingles below and up the brick.
- Step flashing: as each shingle course reaches the chimney, set a step piece against the brick and under the next shingle, nailed only to the deck.
- Back pan: metal over the cricket or uphill side, wrapped around the upper corners, with the shingles above lapping over it.
- Counterflashing: cut a reglet roughly 1/2 to 1 inch deep into a mortar joint, insert the bent edge of the counterflashing, and seal it. On the sides, stepped pieces follow the brick courses down over the step flashing.
The test of a good job
Water poured at the top should run across metal and onto shingles without ever depending on a caulk joint.
Shortcuts that cause leaks
- Counterflashing caulked to the brick face with no reglet.
- One long L-shaped piece up each side instead of individual steps.
- No cricket on a wide chimney.
- Rusty old step flashing reused under new shingles.