Why does my roof leak after heavy rain?
Short answer. A roof that stays dry in normal showers but leaks in heavy rain usually has a weak spot that only overflows under high volume: a valley carrying more water than it can handle, gutters backing water up under the roof edge, a debris dam behind a chimney or skylight, short flashing laps, or a low-slope section where water moves slowly. Wind-driven rain during the same storms adds pressure at walls and ridge vents.
Why volume matters
Shingle and flashing laps shed water by gravity. In a Georgia summer downpour, water can run deep enough in valleys and along walls to climb over a lap or slip under a flashing edge that stays dry in light rain.
The usual suspects
- Valleys: two slopes empty into one channel, so a worn or poorly cut valley leaks when flow peaks.
- Gutters and downspouts: clogged or undersized gutters overflow back under the drip edge into the fascia and decking. One downspout serving a large roof area is a frequent culprit.
- Debris dams: pine needles and leaves behind chimneys and skylights and in valleys hold water back until it finds a seam.
- Roof to wall corners without kickouts: heavy flow runs behind the siding.
- Low-slope sections: porch roofs, or shingles installed on a slope too low for them.
- Short step flashing or missing membrane at walls.
What to do
- During the next storm, watch the gutters from a window: water sheeting over the front or back edge means they are overflowing.
- Check the attic during heavy rain for the wet trail.
- Clear valleys and gutters, then have the valleys and wall flashing inspected.
- Consider larger gutters or extra downspouts where flow is heavy.
See gutter sizing and guards, pine needles and leaves on the roof and common leak causes.