The short answer
The clearest sign of a blocked gutter is water spilling or sheeting over the edge during rain instead of running to the downpipe. Other common signs include plants, grass or moss growing in the gutter, the gutter run sagging or pulling away from the fascia under the weight of debris, damp patches or green staining on the wall below, and water pooling near the foundations. Inside, blocked gutters can show up as damp on upper walls and ceilings. Because overflowing water runs down the wall and towards the foundations, a blockage is worth clearing promptly — it is far cheaper to clear a gutter than to fix the damp it causes.
Blocked gutters rarely announce themselves until it rains hard. Knowing the signs — outside and in — helps you act before overflowing water starts to mark or soak the building.
Signs to watch for
- Overflowingwater sheets over the edge in rain
- Plants growinggrass, weeds or moss in the gutter
- Saggingthe run droops or pulls off the fascia
- Staininggreen or damp marks on the wall below
- Insidedamp on upper walls or ceilings
What to look for outside
- Overflowing in rain: water pouring over the gutter edge rather than draining to the downpipe is the classic sign of a blockage.
- Plants in the gutter: grass, weeds, moss or even small saplings mean trapped soil and debris have built up.
- Sagging or pulling away: a gutter heavy with wet debris droops or detaches from the fascia.
- Staining and streaks: green algae, damp streaks or tide marks on the wall below show water has been running where it should not.
What it does if left
A blocked gutter sends rainwater straight down the wall and towards the foundations instead of away from the house. Over time that can cause damp patches on upper walls and ceilings, peeling paint, frost damage to brickwork in winter, and water pooling around the foundations. The longer it is left, the more likely the damage spreads beyond the gutter itself. Clearing a blockage early keeps it a quick, low-cost job rather than a damp-repair one.
Spotted the signs?
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Frequently asked questions
How do I know if my gutters are blocked?
The clearest sign is water spilling over the gutter edge during rain instead of draining to the downpipe. Other signs include plants growing in the gutter, the run sagging or pulling away from the fascia, and green staining or damp patches on the wall below.
Can blocked gutters cause damp inside the house?
Yes. When a gutter overflows, water runs down the wall instead of away from it, which can cause damp patches on upper walls and ceilings, peeling paint and staining. Clearing the blockage early helps prevent this.
Is a sagging gutter a sign of a blockage?
Often, yes. A gutter heavy with wet leaves and silt can droop or pull away from the fascia. It can also point to loose or failing brackets, so it is worth having both the blockage and the fixings checked.
Sources & further reading
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property. They are guidance, not a quotation.