Flashing types — step, continuous (apron), counter, chimney, skylight — failure modes

Flashing is the most workmanship-sensitive component of a roof system because it lives at the transitions where water has the most chance to enter the assembly. Step flashing is used at sidewalls — where the roof slope meets a vertical wall — and consists of individual L-shaped pieces interleaved with each shingle course. Each piece kicks the water down to the next shingle and away from the wall. Continuous or apron flashing is a longer single piece used at headwalls and similar long horizontal transitions. Counter flashing covers the vertical leg of step or apron flashing where it meets a masonry surface, embedded in a cut mortar joint of brick or stucco — the cut-and-embed detail is what makes it weathertight. Surface-mounted counter flashing screwed and caulked to the brick face (rather than mortar-cut) is a common shortcut and a common future leak source. Chimney flashing is a system of multiple pieces working together: a base (apron) on the downhill side, step flashings along the sides, counter flashing covering the vertical legs, and on chimneys wider than 30 inches, a saddle or cricket on the uphill side that sheds water around rather than damming behind the chimney. Skylight flashing is typically a pre-engineered manufacturer kit (Velux, Fakro, and others) sized for the specific skylight model — installing without the manufacturer's kit is a leak risk and often a warranty void. Valley flashing has its own dedicated entry (cross-reference KB-D2-009). Common observable failure modes: visible rust on valley metal, surface-mounted counter flashing peeling away from a brick chimney, missing step flashings between the wall and the shingles (often visible as siding meeting shingles directly), and missing kick-out flashing at wall-to-roof intersections (cross-reference KB-D2-010). [Source: IRC Section R905.2.8 flashing; NRCA Roofing Manual flashing chapter; Asphalt Roofing Manufacturers Association flashing details; Velux skylight flashing kit install guide]