A gutter overflows when water cannot leave as fast as it arrives. The usual causes are a blocked outlet or downpipe, debris along the run, a gutter that has lost its fall, or one too small for the roof. Watch it in rain to see where the water spills.
Watch where it spills
The spill point tells you most of what you need. Water pouring over one stretch usually means debris or a low spot there. Water over the whole front means the outlet or downpipe is blocked. Water at a corner can mean the angle fitting is clogged.
Fall and size
A gutter should slope very gently towards the outlet. If brackets have sagged, water lies in the low spot and overflows. On a large roof, or one with a valley that feeds a single outlet, even a clean gutter may struggle in a downpour.
Other causes
- Moss washed from the roof into the gutter.
- A cracked or leaking joint that looks like overflow.
- Roof tiles overhanging the gutter incorrectly, throwing water past it.
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Common questions
Is an overflowing gutter urgent?
Not an emergency, but persistent overflow soaks walls and can rot fascias, so deal with it soon.
Why does it only overflow in heavy rain?
A partial blockage or poor fall copes with light rain and fails when the volume rises.