Why Does My Roof Only Leak in Heavy Rain?

If the ceiling only drips in big storms, the roof is telling you something specific. Here's how to read it.

It's one of the most frustrating roof problems: everything is fine for weeks, then a big storm comes through and there's a drip in the bedroom. By the time the roofer arrives the weather has cleared and the stain is dry. The good news is that leaks like this follow patterns, and the pattern usually points to the cause.

Why a roof can be "fine" in normal rain

In a light, steady rain, water runs straight down the roof, over the shingles and into the gutters. Small gaps and weak spots don't get much water, so nothing gets through. A heavy or wind-driven storm is different:

• Wind pushes water sideways and uphill, under shingle edges and behind flashing
• Huge volumes of water overwhelm valleys and gutters, so water backs up where it normally wouldn't
• Gusts lift shingles whose seal is already broken
• Water hits the house from one direction, so only one side of the roof gets tested

The most likely causes

1. Flashing. Chimneys, walls, skylights and vents are the usual suspects. Small gaps in old or poorly installed flashing stay dry in gentle rain but let water in when it's blowing sideways. If the stain is near a chimney or an outside wall, start here. More on this in chimney and flashing repair.

2. Lifted or creased shingles. A shingle that's lost its seal lies flat in calm weather and lifts in the wind, letting rain blow underneath. Common on the windward side of the house and along edges.

3. Valleys. Valleys collect water from two slopes. In a downpour they carry a lot, and worn valley shingles or debris build-up can push water under the edges.

4. Clogged or undersized gutters. When gutters overflow, water can back up behind the gutter and under the first row of shingles, or run down behind the fascia into the wall. These leaks often show up near the top of an exterior wall rather than in the middle of a ceiling.

5. Siding and windows, not the roof. Wind-driven rain can get in around windows, through siding joints or at the top of a wall. It looks like a roof leak from inside but isn't one.

6. The chimney itself. Water can soak into old brick and mortar or come in through a cracked crown or missing cap, then drip down inside. That's a masonry problem, not flashing.

Clues that help pinpoint it

• Which storms? If it only leaks when the wind comes from the east or northeast (typical of nor'easters), the problem is likely on that side of the house.
• Where's the stain? Near a chimney points to flashing or masonry. At the top of an outside wall points to gutters, roof edge or siding. Middle of a ceiling usually means water traveled along the deck or a rafter.
• When does it start? Right away in a storm suggests a direct gap. Hours later suggests water slowly soaking in or backing up.
• What's in the attic? Go up during or right after a storm with a flashlight. Wet wood, drips or stained insulation will usually lead uphill to the source.

Write these down and share them with the roofer. It can save a lot of time.

How roofers find intermittent leaks

A good roofer will inspect the roof above the stain, check every penetration and flashing detail, and look at the attic for the water trail. When it's still not obvious, they'll often do a water test: running a hose on one small section at a time, starting low and working up, while someone watches inside. It's slow, but it finds leaks that a visual inspection misses.

Don't wait for the next big storm

A leak that only shows up a few times a year still soaks the decking, insulation and drywall each time. Over a season or two that leads to rot and mold. Get it looked at while you still remember exactly where and when it dripped. See roof leak repair.