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This guide explains how to install step flashing at sidewalls, continuous wall flashing at headwalls, and counter-flashing at chimneys and skylights. Each step flashing piece is woven between shingle courses with the upper edge tucked behind siding or counter-flashing. Apron flashing covers the front of chimneys and headwalls. The guide includes diagrams for brick chimneys, dormers, skylights, and pipe penetrations. Properly installed flashing is the difference between a 25-year roof and a 5-year leaker.
Flashing is what keeps the roof dry everywhere it stops being a simple plane. Almost every leak on an otherwise sound roof is a flashing detail that was improvised, reused, or replaced with a smear of roof cement.
| Flashing | Where | Rule |
|---|---|---|
| Step flashing | Sloped wall intersections | One piece per shingle course, woven in |
| Kickout flashing | Bottom of a wall intersection | Throws water into the gutter, not behind the siding |
| Apron flashing | Down-slope face of a chimney or wall | Laps over the shingles below |
| Counter flashing | Masonry walls and chimneys | Let into a cut reglet, over everything below |
| Cricket / saddle | Up-slope of a chimney over 30 in wide | Splits water around each side |
| Valley metal | Every valley | Fastened at the edges only, never the centre |
| Pipe boot | Every vent penetration | Flange under the shingles above, over those below |
| Wall length | At 5 in exposure | At 5⅝ in exposure |
|---|---|---|
| 8 ft | 20 | 18 |
| 12 ft | 29 | 26 |
| 16 ft | 39 | 35 |
| 20 ft | 48 | 43 |
| 30 ft | 72 | 64 |
A small flared piece at the bottom of a wall-to-roof intersection that throws water sideways into the gutter instead of letting it run down behind the siding. Its absence is one of the most common causes of hidden wall rot, because the damage happens inside the wall where nobody sees it for years. It is required by code on new work.
Flashing lifted off a roof is bent, work-hardened and full of nail holes in the wrong places. It is among the cheapest components on the roof and the one most likely to be the source of a leak in an otherwise new installation. Replace all of it, every time.
No. Continuous flashing up a sloped wall channels water behind the shingles at every course. Step flashing, one piece per course woven in with the shingles, is the only correct method and is what codes require.
A small flared piece at the bottom of a wall-to-roof intersection that throws water into the gutter instead of behind the siding. Its absence causes wall rot that goes unseen for years, and it is required by code on new work.
On any chimney more than 30 inches wide across the slope, yes. Below that an apron flashing across the up-slope face is normally enough.
Almost never worth it. Flashing lifted off a roof is bent, work-hardened and full of nail holes in the wrong places, and it is the most common source of a leak in an otherwise new installation.