Roof water should discharge into a gully connected to a surface water drain, or into a correctly sized soakaway at least five metres from the building. A downpipe ending in a shoe over open ground delivers a roof's worth of water to the base of the wall every time it rains, and it is a common cause of internal damp.
- A downpipe discharging onto the ground is the commonest fixable cause of damp at the base of a wall.
- Foul and surface water drains are different systems. Connecting roof water to foul is usually not permitted.
- A soakaway only works if the ground drains. Dig a test hole before building one.
- A blocked gully makes a perfect gutter useless.
Why it matters more than it sounds
A modest house roof sheds a very large volume of water in a heavy shower, and all of it arrives at the bottom of two or three downpipes. Where that water goes is not a detail.
When it goes somewhere wrong, the symptoms show up indoors and get attributed to something else:
Damp at the base of a wall, internally, often diagnosed as rising damp and treated with an injected damp-proof course that does nothing, because the cause is a downpipe soaking the outside of that wall several times a week.
A permanently wet patch of ground beside the house, which keeps the wall saturated and can bridge the damp-proof course.
Saturated ground against the foundation, which in clay contributes to seasonal movement.
Water tracking into a cellar or under a suspended floor, and blocked airbricks where silt washes against them.
Moss, algae and spalling brickwork on the wall below the discharge point.
A neighbour's complaint, where the water runs onto their land.
The fix is usually inexpensive — connecting a downpipe to an existing gully, or clearing one that is blocked. It is among the best-value repairs on a house, and it is routinely ignored because the damage appears somewhere other than the cause.
The two drainage systems
Houses have one or two underground drainage systems, and knowing which you have determines what you can connect to.
Surface water drains take rainwater from roofs and paved areas, and discharge to a watercourse, a surface water sewer or a soakaway. The water is clean and it needs no treatment.
Foul drains take waste from toilets, sinks, baths and appliances, and go to the foul sewer for treatment.
A combined system carries both in one pipe, which was standard in older areas and still exists widely.
Where the systems are separate, roof water should go to the surface water system. Connecting it to a foul drain is generally not permitted, because it loads the treatment works with clean water unnecessarily and can cause the foul system to surcharge in heavy rain. The reverse — foul waste into a surface water drain — is far more serious, since it discharges sewage to a watercourse.
How to tell which you have:
Lift the inspection chamber covers and look. A foul chamber has a channel at the bottom with benching either side, and it will smell. A surface water chamber is usually cleaner with no benching.
Follow the downpipes and see what each one enters.
Look at the gullies. A gully taking a kitchen waste pipe as well as a downpipe is on the foul or a combined system.
Ask the water authority, which holds records of the public sewers in the street.
Getting this right matters if you are adding a downpipe, an extension, or a new paved area, because the wrong connection can be required to be undone.
The options, and when each applies
| Destination | When it applies | Notes |
|---|---|---|
| Gully to surface water drain | Where a surface water system exists | The straightforward answer |
| Gully to combined drain | Older areas with one system | Acceptable where that is the system |
| Soakaway | No surface water sewer available | Only where the ground actually drains |
| Watercourse | Property adjoins one | May need consent |
| Rainwater harvesting butt | Partial; overflow still needed | Supplements, does not replace |
| Shoe onto open ground | Never acceptable against a wall | Common and causes damp |
| Onto a neighbour's land | Not acceptable | Liability and disputes |
| Into a foul drain | Generally not permitted | Loads the treatment works |
Rows six and seven describe what a great many houses actually do. A shoe discharging a metre from the wall onto a border is better than one discharging at the wall, and neither is a connection. Both are worth putting right.
Soakaways: test before you build
A soakaway is a void in the ground — traditionally a rubble-filled pit, now more often a plastic crate structure wrapped in geotextile — that holds roof water temporarily and lets it disperse into the surrounding soil.
It only works if the surrounding soil accepts water. On free-draining sand or gravel it works very well. On heavy clay it can become an underground pond that stays full, and then the roof water has nowhere to go and backs up.
So the test comes first, and it is the same percolation test used for garden drainage:
Dig a hole roughly 300mm square and 300mm deep where the soakaway would go.
Fill it with water and let it drain away completely. This wets the surrounding soil so the second reading is realistic.
Fill it again and time how long it takes to empty.
Under four hours is good drainage and a soakaway will work well. Four to twelve hours is workable but the soakaway needs to be larger. Over twelve hours, or still holding water the next day, means a soakaway is not viable there.
The other requirements:
Distance from buildings. A soakaway should be at least five metres from a building, and further where the ground is clay, so that the water it disperses is not being introduced next to foundations.
Sizing. It has to be sized for the roof area draining into it and the local rainfall, not guessed.
Away from boundaries, so the dispersal does not simply become the neighbour's problem.
Clear of services and trees.
Where the test fails and there is no surface water sewer, the options narrow to a larger or deeper system, a connection elsewhere, or attenuation — and that is the point to get proper advice rather than dig.
Gullies: the five-minute check
- Find every gully. There is usually one at the base of each downpipe, and often one taking kitchen or bathroom waste.
- Lift the grating and look. A gully has a trap below it, and silt, leaves and grit collect there.
- Clear it out by hand or with a trowel. This is the single most useful five minutes of drainage maintenance on any house.
- Pour a bucket of water in and watch it go. It should drain away immediately. If it backs up or drains slowly, the run beyond is blocked.
- Check the shoe is actually in the gully. Downpipe shoes get knocked out of position and then discharge beside the gully rather than into it.
- Check for cracks in the gully surround, where water escapes into the ground beside it.
- Watch it in heavy rain, which shows whether it copes or surcharges.
- Look for a wet patch around it in dry weather, which suggests the gully or the run below has failed.
A blocked gully makes a perfectly good gutter and downpipe useless, and it produces exactly the same symptoms as a roofline fault — water at the base of the wall, damp inside. It is worth eliminating before anyone goes up a ladder. Gutter and downpipe faults are the other half of the same diagnosis.
Paving, extensions and added run-off
Worth flagging because it is where people unintentionally create a problem.
Every time a permeable surface becomes an impermeable one, the water that used to soak in has to go somewhere. A new patio, a widened drive, a conservatory or an extension all add to the volume arriving at the drainage system.
At the front, laying more than five square metres of hard surfacing between the house and a highway needs planning permission unless the surface is porous or the water is directed to a permeable area within the property.
An extension adds roof area, and its rainwater needs a designed connection rather than a shoe onto the lawn. Where it connects near an existing drain, a build-over agreement may be needed.
Existing drainage may not cope. Adding a conservatory roof to a downpipe already at capacity produces overflow at the head of the pipe in heavy rain.
Falls on new paving must run away from the house, and towards a drainage point that exists.
Never direct it over a boundary. Water you shed becoming a neighbour's problem is a liability and reliably the start of a dispute.
The sensible approach on any project that adds hard surface or roof area is to establish where the water currently goes, work out how much more is being added, and design the connection rather than discovering the shortfall during the first heavy autumn.
What it costs
These are the roofline figures published across this site:
| Job | Typical Coventry guide price | Notes |
|---|---|---|
| Gutter clean & reseal | £80–£200 | Clears blockages, stops overflow |
| Labour | ~£30/hr | Qualified roofline fitter |
| Per elevation (fascia + gutter) | £400–£700 | Front or rear |
| Full uPVC fascia, soffit & guttering (semi) | £1,000–£1,500 | Supply & fit |
Clearing a gully and refitting a downpipe shoe into it is among the cheapest work on this site and among the most useful, because it frequently resolves damp that would otherwise be treated as a wall problem. Soakaway construction and new underground connections are groundworks and are priced per job.
Look at the downpipe before the wall. A shoe discharging onto open ground is the commonest fixable cause and it costs very little to put right.
Frequently asked questions
Where should a downpipe discharge?
Into a gully connected to a surface water drain, or into a correctly sized soakaway at least five metres from the building. A shoe discharging onto open ground delivers a roof's worth of water to the base of the wall every time it rains and is a common cause of internal damp.
Can roof water go into a foul drain?
Generally not, where separate systems exist. Surface water should go to the surface water drain; connecting it to foul loads the treatment works with clean water and can cause the foul system to surcharge in heavy rain. In older areas a single combined system takes both, which is acceptable.
How do I know if I have separate or combined drains?
Lift the inspection chamber covers. A foul chamber has a channel with benching either side and will smell; a surface water chamber is usually cleaner with no benching. Follow each downpipe to see what it enters, and the water authority holds records of the public sewers.
Will a soakaway work in my garden?
Only if the ground accepts water. Dig a hole 300mm square and deep, fill it and let it drain, then fill again and time it. Under four hours is good; over twelve hours, or still holding water next day, means a soakaway will become an underground pond. Heavy clay often fails this.
How far from the house should a soakaway be?
At least five metres, and further on clay, so the water it disperses is not being introduced beside the foundations. It should also be clear of boundaries so the dispersal does not become a neighbour's problem, and away from services and trees.
Why is my gully overflowing?
Usually silt and leaves collected in the trap below the grating. Lift it, clear it by hand, then pour a bucket of water in — it should drain immediately. If it backs up, the run beyond is blocked. A blocked gully makes a perfect gutter and downpipe useless.
Follow the water to the ground
Most damp blamed on walls starts at a downpipe or a blocked gully. We check both before recommending anything expensive.