Roof Flashing: Where Roofs Actually Leak From

Quick answer

Asphalt shingles almost never leak in the middle of a slope. Leaks happen at penetrations - chimneys, vent pipes, skylights, walls, valleys - where the flat plane of the roof meets something else. Those joints are sealed with metal flashing. When a roof leaks, 90%+ of the time it's flashing that failed, not shingles. Replacing shingles without replacing flashing is the single most common corner-cut in cheap re-roof jobs.

The flashing locations

Step flashing (where roof meets a wall)

L-shaped pieces of metal woven into the shingle courses where the roof meets a vertical wall (dormer side, chimney side, adjacent gable). Each piece overlaps the one below by 2" and is covered by the next shingle course. When done right, water on the wall is shed onto the shingle below; when done wrong (one continuous strip, no weaving), it leaks within a few years.

Counter flashing (chimneys and sidewalls)

The vertical metal cap on a brick chimney or sidewall that covers the top edge of the step flashing. On chimneys it's traditionally cut into a mortar joint and sealed - the masonry sheds water over the metal. Reusing old counter flashing on a re-roof is the #1 source of chimney leaks.

Valley flashing

The line where two roof planes meet at an angle. Carries 2 - 4x the water of any other section. Either open (metal exposed up the center) or closed (shingles woven across with a wide ice & water shield underneath). Both work when installed right; closed cut fails first if the underlayment isn't robust.

Vent boots / pipe collars

The rubber-gasketed metal flange around every plumbing vent pipe. The rubber dries, cracks, and fails in 8 - 15 years depending on UV exposure - far faster than the shingles. The most common single leak source on any roof over 10 years old. New roof = always new boots.

Skylight flashing

Most modern skylights ship with a proprietary flashing kit from the manufacturer (Velux, Fakro). Old skylights often have site-fabricated flashing that's leaked since installation. On a re-roof, the skylight flashing should be replaced with the manufacturer kit - or the skylight itself replaced if it's more than 15 - 20 years old.

Drip edge (eaves and rakes)

Detailed in our drip edge guide. Required at every eave and rake; missing on pre-2012 homes; responsible for fascia and decking-edge rot when absent.

Ridge flashing

On metal roofs, the cap that runs along the ridge sealing the two slopes' top edges. On shingle roofs, the "ridge cap" itself serves as flashing - it must be ridge-cap- specific shingles, not cut-down field shingles (which fail at the bend).

Why flashing fails

• Old age & corrosion. Galvanized steel flashing rusts through in 25 - 40 years. Aluminum holds up longer; copper longer still.
• Reused on a re-roof. A cheap quote leaves existing flashing in place. The new shingles are 30 years; the flashing is on year 20 of its life. Leaks within a few years.
• Improperly installed. Step flashing as one long strip instead of woven pieces. Counter flashing surface- mounted with caulk instead of cut into mortar. These shortcuts look fine for a couple years, then leak.
• Caulk fails. Caulk is not flashing. Roofs are flashed with overlapping metal that sheds water; caulk is a last-resort sealant that lasts 3 - 5 years before cracking. Any roof "repaired" with caulk has a clock on it.

DIY caulk = death sentence

Climbing up with a tube of roofing cement to "fix a leak" almost always makes it worse. The caulk seals one path but redirects water to a new one, often inside the roof assembly where you can't see it. By the time the leak reappears in the ceiling, the decking is rotted. Get a real flashing repair - $300 - $1,200 - rather than a $20 tube of cement and a future $8,000 of decking work.

What "new flashing" should mean on your estimate

A complete re-roof should specify by line item:

• New step flashing where roof meets wall (specify linear feet)
• New counter flashing on all chimneys (cut into mortar or saw-cut reglet)
• New valley flashing or new ice & water shield in valleys
• New vent boots and pipe collars on every penetration (count)
• New drip edge at all eaves and rakes
• New ridge cap shingles (not field-shingle cut-downs)
• Skylight flashing kit per manufacturer spec, or skylight replacement

If the estimate just says "flashing as needed," that's a tell. Ask for the breakdown. See how to read a roofing estimate for the full line-item checklist.

The takeaway

New shingles on old flashing isn't a new roof - it's a cosmetic upgrade with a 5 - 10 year leak countdown. A real re-roof replaces every flashing on the roof. Run the calculator with full flashing replacement included so the budget reflects what you' re actually buying.

Sources: NRCA · InterNACHI roof SOP