Deep dive · Updated October 17, 2026

Roof leak detection: how to find where water is getting in

Local versions: San Rafael, Novato, Mill Valley, Larkspur, Corte Madera, San Anselmo, Fairfax, Sausalito, Tiburon, Kentfield, Greenbrae, Ross, Belvedere, Stinson Beach and West Marin

Local versions: San Rafael, Novato, Mill Valley, Larkspur, San Anselmo, Fairfax, Sausalito, Tiburon

Local versions: San Rafael, Novato, Mill Valley, Larkspur, Corte Madera, San Anselmo, Fairfax, Sausalito, Tiburon, Kentfield, Ross

A stain on the ceiling tells you there's a problem, but not where it starts. Water can enter at a flashing joint, run down rafters and the roof deck, and drip onto the ceiling several feet away. Here's how leak detection works.

Step 1: Rule out the impostors

Not every wet ceiling is a roof leak:

• Condensation: widespread dampness in cold weather, often worse above bathrooms. See roof leak vs. condensation.
• Plumbing: an upstairs bathroom, supply line or drain leak
• HVAC condensate: an attic air handler's drain line or pan overflowing

A leak that appears only during rain, especially wind-driven rain from one direction, points to the roof.

Step 2: Look in the attic (if safe)

With a flashlight, look above the stain and uphill from it for:

• Dark, stained wood on the roof deck or rafters
• Wet or matted insulation
• Daylight around penetrations
• Rusted nail tips, or drips during rain

Step only on framing, never between joists, and avoid wiring.

Step 3: Check the usual suspects outside

From the ground with binoculars, or by a professional on the roof:

• Chimney flashing, including step flashing and counter-flashing
• Skylights (see skylights and flashing)
• Plumbing vent boots, which crack in the sun
• Valleys filled with debris
• Wall-to-roof transitions on additions
• Solar mounts and other penetrations
• Low-slope seams and drains, common on Eichlers and mid-century homes

Step 4: Professional tracing

Roofers may:

• Run a controlled water test, applying water to sections from the bottom up while someone watches inside
• Use moisture meters on decking and ceilings
• Use thermal imaging to spot wet areas, which then need to be confirmed

Marin patterns

• Under heavy trees in Mill Valley, Larkspur and Fairfax, debris dams in valleys and gutters push water under shingles.
• Coastal homes in Sausalito and Tiburon see corroded flashing.
• Eichlers in San Rafael have low-slope roofs where leaks show directly on ceiling planks.
• Older homes in San Anselmo and Ross have chimneys with aging flashing.

Document as you go

Photograph stains and mark their edges with a dated pencil line, and note the weather when leaks appear. These notes speed up diagnosis and help with insurance.

Reading the stain

The shape and timing of a stain carry clues:

• A ring that grows during storms and dries between them points to an active roof leak.
• A stain that appears only with wind from one direction, often the south in winter storms, points to wind-driven rain at flashing, siding transitions or windows above the roofline.
• Stains near exterior walls may come from gutters overflowing behind the fascia, or from ice-dam-like backups at the eave. Ice dams are rare in Marin, but debris dams cause similar backups.
• Small, scattered spots on a cold morning suggest condensation rather than a leak.
• Stains below bathrooms can be plumbing, such as a failed wax ring, tub drain or shower pan, even when they show up during rain.

Making a water test useful

Controlled water testing works best when it's slow and methodical:

1. Start low on the roof, below the suspected area, and run water for a set time while someone watches inside.
2. Move up in sections: the wall above a chimney, then each side, then above.
3. Wait between sections. Water can take several minutes to travel.
4. Stop when water appears inside, and note the last section tested.

This takes time, and roofers may charge for it, but it can save money compared with replacing components that weren't leaking.

Low-slope and Eichler roofs

On flat and low-slope roofs, water can travel a long way under the membrane before entering. Common sources include:

• Seams and laps in torch-down or single-ply membranes
• Drains and scuppers that clog with leaves, causing ponding
• Flashing at walls, skylights, and HVAC curbs
• Penetrations for vents, solar, or conduit

Because many Eichlers have exposed beams and tongue-and-groove ceilings with no attic, there's no place to inspect from inside. Roofers rely on the exterior and on moisture readings.

Leaks that aren't the roof

• Windows and doors above a lower roof can let water in that runs down onto the roof deck below.
• Siding and stucco cracks let water behind walls.
• Decks over living space depend on waterproof membranes and flashing at the house.
• Chimney masonry can absorb water and pass it inside even when flashing is fine.

A good roofer will tell you when the problem is outside their trade and point you to the right one.

Temporary measures while you wait

• Put a bucket under drips, and poke a small hole in a bulging ceiling to drain it into the bucket only if it's safe and the ceiling isn't about to fall. Otherwise stay clear.
• Move belongings and cover them with plastic.
• If the roof is open, arrange emergency tarping.
• Document everything with dates.

When the roof is fine but water damaged the house

Even after the leak is fixed, wet insulation and drywall need to dry. If water got into walls or ceilings, a restoration provider can measure moisture and dry the structure. Our sister site Marin Water Damage covers drying.

Related reading

• The complete roof guide
• Roof leak repair

Frequently asked questions

This guide is general information, not legal, insurance or medical advice. Sources are linked in the text. Marin Roof Repair is a referral service and doesn't perform the work.