Roof Anatomy
A roof is not a surface, it is an assembly. When something leaks, the question is almost never 'what is wrong with the shingles' — it is which part of this list gave up, and what it took with it.
2 components
The plywood or OSB substrate everything fastens to. Deck condition decides what kind of roof you can install at all.
How it fails
Soft spots, delamination, and old fastener holes that no longer hold. A new roof over a tired deck is a short-lived roof.
The secondary water-shedding layer over the field of the roof. If shingles or tiles are breached, this is what still keeps the house dry.
How it fails
Tears from foot traffic, wrinkles that create channels, and age. On tile roofs the underlayment fails long before the tile does.
5 components
Self-adhering membrane at eaves, valleys, and penetrations — the places most exposed to wind-driven rain and standing water.
How it fails
Missing or short runs at eaves, and adhesion lost to dust or a dirty deck.
Directs the concentrated flow where two roof planes meet. A valley carries more water than any other part of the roof.
How it fails
Open valleys clogged with debris, and closed-cut valleys cut short of the centerline.
Individual metal pieces woven into each course where a roof meets a wall, shedding water out and over the shingle below.
How it fails
Nailed through the face, installed in one long bent piece, or missing a kick-out at the bottom.
The upper half of a wall or chimney flashing, cut into the masonry or siding so water cannot get behind the lower flashing.
How it fails
Reglet loose or unsealed, and flashing relying on caulk instead of a mechanical turned-in edge.
Pre-formed boots and collars that seal vents, plumbing stacks, and conduit where they pass through the roof.
How it fails
Rubber collars that crack after years of UV. This is one of the most common small leaks we find.
2 components
Metal at the eaves and rakes that pushes water clear of the fascia and into the gutter instead of behind it.
How it fails
Absent on older roofs, or installed under the underlayment instead of over it.
A factory-sealed first row that locks the field shingles along the eave and rake, and seals the rake edge against wind.
How it fails
Cut field shingles used as starters, which seal poorly and lift in wind.
2 components
The visible weathering surface — shingles, membrane, panels, or tile. It takes the sun, rain, and impact.
How it fails
Granule loss, blistering, cracked tiles, lifted panels, and seam failures.
Purpose-made caps that cover the peak and the outside corners of a roof, the two places wind works hardest.
How it fails
Cut shingles folded over the ridge instead of a real cap — a wind-lift and leak point.
2 components
Exhaust venting along the ridge that lets hot, moist attic air escape, so the roof deck dries instead of condensing.
How it fails
Vent installed without matching intake, or a filter clogged with paint and dust.
Eave or soffit vents that let cool air in. Without intake, a ridge vent has almost nothing to exhaust.
How it fails
Blocked by insulation pushed into the soffit bay — very common after an attic insulation upgrade.
1 components
Gutters carry water away from the building, and apron flashing protects the roof-to-gutter and roof-to-wall junction that feeds them.
How it fails
Undersized or clogged gutters that back water up under the bottom courses.
The details, on video
Step flashing, kick-outs, drip edge, and counterflashing. If you watch one set of videos on this site, watch these.
GAF · Mastering the Roof
Wall, sidewall, and step flashing detail from the manufacturer whose system the flashings are integrated with.
Field demonstration
Step flashing and the kick-out at the bottom of a wall — the two details behind a large share of the leaks we diagnose.
This Old House
A clear look at eave drip edge and how it ties into step flashing at a sidewall, with the sequencing that makes it watertight.
Get Started
Send us a photo or book a site visit. We will identify the component, explain why it failed, and scope the repair.
Service Areas
We provide roofing & waterproofing for residential and commercial properties in every city and community across San Francisco and the Peninsula south to Hillsdale in San Mateo. Select your area for local building conditions, exposure, and details.
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