Why does my roof only leak in heavy rain?
Updated August 2026
A roof that stays dry in ordinary showers but drips during a wind-driven downpour is pointing at failed flashing, the metal that seals the roof's weak spots at valleys, chimneys, skylights, and vent pipes. Under heavy, sideways rain, water gets driven up and under flashing that a gentle rain never tests. The shingles themselves are often fine; it is the sealed transitions that give way. Because these leaks hide until the next storm and quietly rot the wood beneath, tracing the flashing is worth doing before the decking pays for it.
Safety first: a heavy-rain leak is deceptively serious because it wets the roof decking and framing you cannot see, and hidden moisture grows mold and rots sheathing long before the ceiling shows a stain. Stay off the roof in the rain and off steep or wet roofs entirely, since most leak tracing is done safely from inside the attic with a flashlight. If water is running near a light fixture or ceiling fan, cut that circuit at the panel before you poke at the wet drywall.
Causes, most likely first
| Cause | How likely | The fix | Typical cost |
|---|---|---|---|
| Failed flashing at valleys, chimney, or vents | Most common | A pro reseals or replaces the step, valley, or pipe-boot flashing. Cracked rubber vent boots are a frequent, cheap culprit. | $300–1,000 |
| Clogged gutters backing water under the eaves | Common | Overflowing gutters push water up beneath the first course of shingles. Clean the gutters and confirm they drain. | $100–250 |
| Worn or lifted shingle seals | Common | Wind-driven rain slips under shingles whose seal strips have failed with age. A roofer reseals or replaces the affected area. | $200–700 |
| Cracked or lifted pipe boots | Occasional | The rubber gasket around a plumbing vent splits after 8 to 12 years. Replacing a boot is an inexpensive, targeted fix. | $150–400 |
| Ice-dam path in cold climates | Occasional | Melt refreezing at the eave forces water backward under the shingles during storms. Improve attic sealing and ventilation. | $300–1,500 |
Check these first
- Get into the attic during or right after a heavy rain with a flashlight and trace the wet trail uphill to where it enters.
- Remember water travels along the rafters before it drops, so the entry point is usually uphill of the drip.
- From the ground with binoculars, scan the flashing at the chimney, valleys, skylights, and every vent pipe for gaps, rust, or cracked rubber boots.
- Check the gutters, since overflow that only happens in heavy rain can mimic a roof leak by backing up under the eaves.
- Note whether the leak needs wind, not just volume; wind-driven leaks point squarely at flashing and shingle seals.
Prevent it next time
Flashing and seals fail slowly, so a twice-yearly look is what keeps a $200 reseal from becoming a decking replacement. Inspect the roof each spring and fall, and after major storms, using the asphalt shingle roof maintenance checklist, paying special attention to the flashing and the rubber vent boots that dry out first. Keep the gutters clean so water cannot back up under the eaves. Catching a leak early is the whole game, as the cost of ignoring a small roof leak lays out, and it also tells you where you sit on the roof's lifespan.
When to call a pro
Call a roofer when you have traced water to the flashing or shingles but the roof is too steep or high to work safely, when a vent boot or valley is visibly cracked, or when the decking under the leak feels soft. A flashing reseal or vent-boot swap runs $150–1,000, a section of shingle repair $200–700, and decking replacement climbs fast once rot sets in. If the roof is past 20 years and leaking in more than one spot, price a replacement using how often to replace an asphalt shingle roof instead of chasing leaks.
Most of the causes above are maintenance problems wearing a disguise. OnOtto keeps the boring prevention on schedule so the 2 a.m. version doesn't happen.
Stop chasing symptoms, free for 30 daysFrequently asked questions
How do I find where a roof leak is coming from?
Start inside, not on the roof. During a rain, go into the attic with a flashlight and follow the water stain or drip up the sheathing and rafters to its highest wet point, because water runs downhill along the wood before it falls. Mark that spot and measure from a reference like the chimney or a vent so you can find it from outside. On a dry day a helper can run a hose over suspected areas one at a time while you watch from the attic. Flashing and vent boots are the usual entry points.
Can a roof leak in heavy rain but not light rain?
Absolutely, and it is a telling pattern. Light rain falls straight down and the roof sheds it normally, but a heavy, wind-driven storm pushes water sideways and uphill, under flashing and past shingle seals that a calm rain never challenges. Volume matters too, since a downpour can overwhelm a marginal valley or back water up behind a clogged gutter. When a leak needs both wind and volume to appear, look first at the flashing at valleys, chimneys, and vents rather than the field of the shingles.
How much does it cost to fix a roof leak?
Most single-point leaks are affordable if caught early. Replacing a cracked pipe boot runs $150–400, resealing or replacing flashing $300–1,000, and patching a section of shingles $200–700. Costs climb only when the leak has been ignored long enough to rot the decking, which adds sheathing replacement and sometimes interior drywall and insulation. That gap between a $250 fix and a multi-thousand-dollar repair is exactly why a heavy-rain leak is worth chasing down the first season it shows up.