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Roof leaks · Heavy rain

Why Does My Roof Only Leak in Heavy Rain?

Because heavy rain tests parts of the roof that a light rain never reaches. A downpour can send more water across the roof than the gutters and valleys can carry, and storm wind pushes rain sideways under shingles, behind flashing and into roof vents. The usual suspects are the gutters, the valleys, the flashing at walls and chimneys, the roof vents and the pipe boots.

Coastline Roofing · Florida license CCC1331076 · Updated

A leak that only shows up in a downpour is frustrating, because by the time the sun comes out there is nothing to see. The good news is that when it leaks tells you a lot about where it leaks. Here is how to read the clues, what to check from the ground, and what to do before the next storm.

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Why light rain is fine

Light rain runs off. Heavy rain tests the weak spots.

A roof is built to shed water running downhill. Each row of shingles or tile overlaps the row below it, so rain flows from layer to layer down to the gutter. In a light rain that system has plenty of room. The water moves slowly, the gutters keep up, and nothing gets pushed where it does not belong.

Heavy rain changes two things. The first is volume. A downpour puts far more water on the roof at once, and every place where water collects, like a valley, a gutter or the top of a wall, has to carry more of it than it ever does in a drizzle. The second is wind. Storm wind drives rain sideways and even uphill, under the edges of shingles, behind flashing and into vents that only have to keep out rain falling straight down on a calm day.

Flat and low-slope sections

Flat roofs drain slowly. Florida's residential code expects a roof to drain within 48 hours of rain, and water still standing after that is called ponding, which ARMA says can wear the membrane down until the roof fails. Clogged drains and scuppers make it worse. More on ponding water.

So a roof that leaks only in heavy rain usually has one detail that works at a normal flow and fails under a big one. That is a much shorter list than "anywhere on the roof," and the timing of your leak is the best clue you have.

The usual suspects

Six places heavy rain gets in

Overflowing gutters and downspouts

When gutters are clogged, or there are too few downspouts for the flow, the water has nowhere to go. It can back up onto the roof under the shingles, or spill down inside the wall. Stains from this often show up near an outside wall.

Overloaded valleys

A valley is where two roof slopes meet, so the runoff from both collects there. In a downpour, fast water can shoot across the valley and run up under the roofing on the other side, and leaves caught in the valley slow the flow and back it up. More on valley leaks.

Wind-driven rain under shingles

Shingles are made to shed water that runs down the slope. Storm wind can push rain sideways and up the roof in sheets, under shingle edges and behind flashing, most often on the side of the house facing the storm. If the leak only shows up when the wind blows from one direction, start here.

Roof vents

Ridge vents, off-ridge vents and gable vents let hot air out of the attic, and wind can drive rain in through the same openings. FEMA notes that some vents leak only when the wind blows from certain directions. A wet spot near the high point of a ceiling, or water on the attic floor under a vent after a windy storm, points here.

Flashing at walls, chimneys and skylights

Where the roof meets a wall, chimney or skylight, Florida's building code requires flashing to bridge the seam. A downpour sends a sheet of water down the wall and across it. If the flashing is loose, short or missing, water gets behind the siding or stucco. How flashing fails.

Pipe boots

Every plumbing vent pipe through the roof is sealed by a pipe boot, and aging boots dry out and crack. A small crack may let in so little water in a light rain that nobody notices, and enough in a downpour to stain the ceiling. How to spot a vent pipe leak.

Narrow it down

Five clues you can check without a ladder

You do not need to climb anything to shrink the list. Everything below happens from the ground, a window or the attic.

  1. When does it leak?

    Only when the wind is up, and mostly from one direction: think wind-driven rain, roof vents and the shingles or flashing on the side facing the storm. Only in the hardest downpours, wind or not: think gutters, valleys and any flat section. In every rain, even a light one: something is already open, like a missing shingle or a split boot, and it should be looked at soon.

  2. Where does the water show up?

    Near the top of an outside wall or in the soffit: gutters or wall flashing. Under a bathroom, kitchen or laundry room: vent pipes usually come up near those rooms, so a pipe boot is a good bet. Near the high point of the ceiling: roof vents. Keep in mind that water runs along the framing before it drips, so the stain is rarely right under the hole. How a leak is traced.

  3. Watch the gutters while it pours

    From a window or a covered porch, look at the gutters during a downpour. Water spilling over the front edge, or pouring out behind the gutter, means they cannot keep up. A downspout that barely trickles is probably clogged.

  4. Check the attic after the storm

    Only if you can do it safely: step on the joists, never the drywall between them, and bring a flashlight. Look for wet or dark wood, damp insulation and drip marks. The highest wet spot you can find is the closest clue to where the water got in.

  5. Write it down

    Note the date, how hard it rained and which way the wind blew, and photograph every stain. A garden hose on a calm day cannot push water uphill the way storm wind does, so your notes from the storm are worth more than they seem.

Tile roofs

On a tile roof, heavy rain reaches the underlayment

Concrete and clay tile roofs are common on the Gulf Coast, and they handle water differently. Florida's tile installation manual describes a tile roof as a water shedding system. The tiles shed most of the rain, but tile is not waterproof, so the underlayment beneath it carries the water that gets past. Heavy, wind-driven rain pushes more water under the tiles, so a worn underlayment, a cracked or slipped tile, or a gap at a valley or a wall can stay hidden until the worst storms.

That is also why tile leaks are hard to find from the ground, and why walking on tile to look for one can crack more of it. Coastline usually flies a drone over a tile roof first, which shows cracked and slipped tiles without anyone stepping on them. If you need to know more before anyone goes up, read how to walk on a tile roof without cracking it.

What to do now

Catch the water now. Find the source on a dry day.

While it is raining, stay off the roof and off ladders. Catch the drips, move what you can and take photos. Our guide on what to do until the roofer arrives covers each step, including a ceiling that starts to sag.

Then call us, or text a photo of the stain. If water is still coming in, we can put on a temporary repair or a tarp to stop it, then trace the real source once the roof is dry. We typically answer every call, and if we miss yours, we call back the same day.

If the cause turns out to be small, like one pipe boot or a short run of flashing, it gets fixed as a roof repair. If the roof is close to the end of its life, you will hear that too, with photos, so you can decide what makes sense.

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Keep reading

Related leak help

Where Is My Roof Leaking?

Why the stain on your ceiling is rarely under the hole, and how a leak is traced back to where it gets in.

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Leak Around a Vent Pipe

How to tell if a plumbing vent pipe is your leak, and what replacing the pipe boot involves.

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Roof Repair

Once the source is found, a repair on the part that is actually failing, with photos of the work.

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FAQ

Common questions about leaks in heavy rain

Why does my roof leak in heavy rain but not in light rain?

Heavy rain puts more water on the roof at once, and it usually comes with wind. Spots that handle a light rain, like a half clogged gutter, a worn valley or a flashing seam, get more water than they can shed, and the wind pushes rain sideways into gaps that falling rain never reaches.

Why does my roof only leak when it is windy?

That is wind-driven rain. Storm wind can push water sideways and up the roof, under shingle edges, behind flashing and into roof vents, and FEMA notes that some vents leak only when the wind blows from certain directions. The leak usually starts on the side of the roof facing that wind.

Can clogged gutters cause a ceiling leak?

Yes. When gutters are clogged, or there are too few downspouts for the flow, water can back up onto the roof under the shingles or spill down inside the wall. These leaks often show up near an outside wall.

Why can nobody find my leak with a hose?

A hose runs water down the slope on a calm day. Storm wind drives rain sideways and even uphill, and a downpour floods valleys and gutters in a way a hose cannot copy. That is why your notes and photos from the storm matter. Our guide on tracing a roof leak explains the rest.

Is a leak that only happens in heavy rain urgent?

It should be fixed before the next big storm. Each heavy rain soaks the roof deck, the insulation and the drywall a little more, and wood that keeps getting wet can rot. A small repair now usually costs less than the damage it prevents.

Do you repair leaks that only happen in heavy rain?

Yes. Call (941) 896-7793 or text a photo of the stain to (941) 345-0072. If water is coming in now, we can put on a temporary repair or a tarp, then trace the source when the roof is dry.

Sources

  1. FEMA P-499, Home Builder's Guide to Coastal Construction (December 2010), Technical Fact Sheet 7.5: Minimizing Water Intrusion Through Roof Vents in High-Wind Regions
  2. IBHS FORTIFIED Home: Roof (wind and rain resistant attic vents, reinforced soffits, sealed roof deck)
  3. Florida Division of Emergency Management: Hurricane Retrofit Guide, Roof and Attic Water Intrusion
  4. InterNACHI: Water Management and Damage Prevention, homeowner guide (Ben Gromicko)
  5. InterNACHI: Mastering Roof Inspections, Flashing, Part 5 (valleys)
  6. NRCA, Professional Roofing: Essentials (Joan P. Crowe, September 2003)
  7. Florida Building Code, Residential, 8th Edition (2023), Section R903.2.1, Locations (flashing)
  8. Florida Building Code, Residential (2023), Section R202, definition of positive roof drainage
  9. ARMA: Ponding Water Basics (March 10, 2017)
  10. FRSA-TRI Florida High Wind Concrete and Clay Roof Tile Installation Manual, 7th Edition
  11. InterNACHI: Inspecting Tile Roofs, student book (2012)

Leaks only when it pours? Let's find out why.

Call, or text a photo of the stain. If water is coming in now, we can stop it first and trace the source when the roof is dry.

Call (941) 896-7793 Text a photo to (941) 345-0072
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