If a gully or drain works fine most of the time but overflows when it rains hard, the problem is usually that too much water is arriving, or not enough can get away.
The usual causes
A partly blocked gully or drain
Rainwater from roofs often runs into the same gullies and drains as kitchen and bathroom waste. A gully narrowed by grease, silt or leaves may cope with a sink, but not with a downpipe in a downpour. Clearing the gully and drain usually solves it. See how to clean an outside drain gully.
Blocked gutters and downpipes
Leaves and moss in gutters wash down into gullies in heavy rain. If the downpipe empties into a gully with a leaf guard missing, debris builds up quickly, particularly in autumn.
Combined sewers under pressure
Many older urban areas, Portsmouth being a well-known example, are served by combined sewers that carry both rainwater and foul waste. In very heavy rain, the sewer can fill up and back up into the lowest connected drains, often a gully, a downstairs toilet or a cellar. The water may smell, because it contains sewage.
If the sewer itself is overloaded or blocked, the water company is responsible. Report sewer flooding to Southern Water, or to Wessex Water in Bournemouth, Poole, Christchurch and Salisbury.
A failed soakaway
Some homes send rainwater to a soakaway rather than a sewer. When a soakaway silts up or the ground is waterlogged, water backs up into the gullies and downpipes.
A defect in the drain
A dip, root ingress or a partly collapsed section restricts flow. In dry weather it may not show, but in heavy rain the restriction causes water to back up. A CCTV drain survey shows whether there is a defect.
How to tell which it is
- Only your gully overflows, and only in rain: likely a partly blocked gully, drain or downpipe.
- The water is clean rainwater: often a gully, downpipe or soakaway issue.
- The water smells or contains sewage: possibly a sewer problem. Check with neighbours, and see blocked drain: Southern Water or me?
- Neighbours are affected too: a shared drain or public sewer is more likely.
Preventing it
- Keep gutters and gullies clear, especially in autumn.
- Fit leaf guards on gullies under downpipes.
- Have drains cleaned if they are slow in dry weather.
- In properties that flood from the sewer repeatedly, ask the water company about non-return valves, which can stop sewage backing up into the property.
If your drain is overflowing, see what to do if a drain overflows, or contact us about blocked drains.