Roof Flashing
Definition: Flashing is thin, water-shedding material — usually metal — fitted wherever the roof covering is interrupted: against walls and chimneys, in valleys, around vents and skylights, and along the edges. Shingles shed water across the open roof; flashing handles the joints, where most leaks begin.
Shingles, tiles and metal panels keep water out across the open field of a roof. They cannot seal a joint: the line where a roof meets a wall, a chimney, a skylight, a vent pipe or another roof slope. Flashing does that job. It is lapped into the roof so that water running down always lands on top of the next layer, never behind it.
The model building code puts it plainly: the International Residential Code (section R903.2) requires flashing at wall and roof intersections, wherever the slope changes direction, and around every roof opening, installed so moisture cannot get into the wall or roof.
The kinds of flashing on a house
Step flashing. Short L-shaped pieces woven into each course of shingles where a sloped roof runs up beside a wall or a chimney side. Each piece overlaps the one below. (More in the step flashing entry.)
Apron (headwall) flashing. One continuous piece where the top of a wall meets the bottom of a roof slope, or along the low side of a chimney.
Counter flashing. Metal set into the wall or the chimney's mortar joints and folded down over the step or apron flashing, so the top edge of that flashing is covered. On brick it is let into a cut joint and sealed.
Chimney cricket (saddle). A small peaked roof built behind a wide chimney so water parts around it instead of pooling. The IRC (R1003.20) calls for one behind any chimney more than 30 inches wide that is not on the ridge.
Valley flashing. Metal lining a valley, where two slopes meet and the most water on the roof runs. Open valleys leave it exposed; closed valleys hide it under shingles with an ice-and-water membrane beneath.
Drip edge. The L-shaped strip along the eaves and rakes that throws water clear of the fascia. Required on shingle roofs under IRC R905.2.8.5.
Kick-out (diverter) flashing. A small bent piece at the bottom of a roof-to-wall run that steers water into the gutter instead of down the siding. Missing kick-outs are a classic cause of rotted wall sheathing below a roof line.
Pipe boots and penetration flashing. A flanged collar, usually with a rubber or lead seal, around plumbing vents, exhaust pipes and other things that poke through the roof; skylights come with their own flashing kits.
What it is made of
Galvanized steel is the common default; aluminum is light and easy to bend but should not sit against fresh mortar or treated lumber; copper costs most and outlasts the house. Where the code sets a minimum for metal flashing it is corrosion-resistant metal at least 0.019 inch thick (No. 26 galvanized sheet). Rubber boots on vent pipes are usually the first flashing to fail: the rubber cracks in the sun years before the shingles wear out.
Why it matters when you replace a roof
Most roof leaks come from flashing, not from shingles. On a re-roof, ask the roofer whether step flashing, pipe boots, drip edge and valley metal are being replaced or reused, and whether counter flashing on a chimney is being re-cut. Reusing old step flashing saves an hour and often costs a ceiling. A good quote names the flashing work line by line.