Roofing component
What is roof flashing?
Shingles keep water off the field of a roof. Flashing keeps it out of every joint, and joints are where roofs leak.
Updated September 12, 2026 · Every figure links to its source · Sources
DefinitionComponentAlso called: flashing, step flashing, counterflashing
Roof flashing — components, usually of sheet metal or flexible membrane, installed to weatherproof and seal the joints and edges of a roof — where it meets a wall, chimney, curb or pipe, changes slope or forms a valley; the NRCA glossary defines flashing so, and the 2021 IRC, Section R903.2 requires it wherever the roof plane is interrupted.
The kinds you will hear named
The IRC, Section R903.2.1 lists the locations: wall and roof intersections, changes in slope or direction, and roof openings. Each location has its own piece.
- Step flashing. Small L-shaped pieces, one per shingle course, woven up a sidewall so each overlaps the one below — the standard detail where a roof runs along a wall or a dormer side.
- Counterflashing. A second piece let into the mortar joint of a chimney or fixed to a wall that laps down over the step or base flashing, so the joint between wall and roof is covered from above.
- Apron (headwall) flashing. A single bent piece where a roof slope runs into the bottom of a wall, as at the front of a shed dormer.
- Kick-out flashing. The angled piece at the bottom of a sidewall that turns water into the gutter instead of behind the siding; the directory's kick-out flashing entry shows why it is the most commonly omitted piece on a house.
- Valley flashing. The metal lining of an open valley, or the membrane under a closed one, governed by Section R905.2.8.2.
- Pipe boots, vent flashings and drip edge. Purpose-made pieces for plumbing vents, exhaust vents and the roof edge.
Materials are galvanised steel, aluminium, copper and lead for metal, and self-adhering modified-bitumen membrane where the joint is irregular. A flashing is only as good as its laps: every piece must overlap the one below it in the direction the water runs.
Why it matters
Ask a roofer where leaks come from and the answer is flashing, because a shingle field rarely fails on its own while the joints around it fail constantly. Flashing is also the part most often reused on a re-roof: old step flashing bent flat and re-covered, chimney counterflashing caulked instead of relet into the brick, a wall detail buried under sealant. A sealant bead is a maintenance item that lasts a few seasons; a properly lapped metal flashing lasts as long as the roof. The directory's guide to finding a roof leak walks through the joints in the order a leak hunter checks them.
What it costs and who does it
On a replacement, flashing is priced as line items by the piece or the linear foot — step flashing per wall, chimney flashing as a set, valley metal by the foot — and the honest quote states whether it is new or reused. A stand-alone flashing repair is a service call plus the metal and the shingles disturbed around it. Roofing contractors do the work; on masonry chimneys the counterflashing is sometimes let into the mortar by a mason, and on commercial roofs sheet-metal shops fabricate the pieces. The directory's roof repair cost guide describes how these repairs are estimated.
Frequently asked questions
Short answers; the sections above carry the detail and the sources.
Should flashing be replaced when a roof is replaced?
What is the difference between step flashing and counterflashing?
Can roof flashing be repaired with caulk?
Next step
Find a roofer who does this
Flashing is the part of a roof where craft shows, and a contractor's answer to "is the flashing new?" tells you most of what you need. The directory lists roofing contractors by state and city.
Sources
- Roofing glossary — National Roofing Contractors Association (NRCA)
- 2021 International Residential Code, Chapter 9: Roof Assemblies — International Code Council (ICC)