Common roof leak sources and how they start

Roof leaks feel mysterious when you are standing under a drip, but to a roofer they are not. The vast majority come from a handful of predictable spots, and knowing where they are makes it easier to spot trouble early and to understand what a roofer is checking. Here are the common sources of roof leaks and how each one begins.

Flashing at the chimney, walls, and valleys

The single most common source is the flashing, the metal that seals the joints where the roof meets a chimney, wall, or valley. Flashing flexes with heat, corrodes over the years, and is often the spot a previous owner patched with caulk instead of proper metal. When it lifts or fails, water runs straight into the gap. This is why a stain near a chimney is such a classic sign, covered on our pages on flashing failures and chimney leaks.

Worn underlayment beneath tile

On a tile roof, the tile can look perfect while the waterproof underlayment beneath it has aged out. Since the underlayment is the real barrier, its failure leaks even though the tile is fine, which baffles homeowners, covered on our page on roof underlayment. This is one of the most common leak sources on older LA tile roofs.

Slipped, cracked, or missing tiles and shingles

A tile knocked out of place by wind or a branch, or a shingle lifted and torn, leaves an opening for water. On tile it exposes the underlayment; on shingle it exposes the mat. Either way, the next rain finds it, covered on our pages on broken tiles and damaged shingles.

Where leaks usually start

Source	How it starts
Flashing at joints	Lifts, corrodes, or was caulked over
Worn underlayment (tile)	Ages out beneath good-looking tile
Slipped or broken tiles/shingles	Wind, branches, or foot traffic
Clogged valleys and gutters	Water backs up under the roof
Skylights and vents	Seals around them fail

Clogged valleys and gutters

When valleys and gutters fill with debris, water cannot leave the roof and backs up under the edges and tiles, especially in heavy rain. This is behind many leaks that only show in a downpour, covered on our page on leaks only in heavy rain. Clearing them is one of the highest-value maintenance tasks.

Skylights and roof penetrations

Anything that punches through the roof, a skylight, a vent, a pipe, is a potential leak point where the seal around it can fail. Skylights in particular are common culprits, covered on our page on skylights and leaks and our skylight repair service.

Finding your leak

Because leaks come from this short list, a methodical roofer can usually trace one quickly, remembering that where a leak shows inside is rarely where it enters. That is the work of leak detection, followed by a lasting roof repair. Catching any of these sources early keeps a small fix small, as our page on delaying a repair shows.