Wet Cellar in Sydenham: Leak or Ground Water?

In most Sydenham cellars, water that appears after a wet week and seeps in low down on the uphill wall is ground water, not a leak. The test that settles it is your water meter: with every tap and appliance off, it should not move at all. If it does move, you have a leak on your side and it needs tracing. If it does not, you need a damp or drainage specialist rather than a plumber, though it is still worth checking the pipes that run through the cellar while you are down there.

Why Sydenham cellars get wet

Sydenham Hill rises to around 112 metres on the Norwood Ridge, and Sydenham itself is built down its slopes on clay. The Victorian villas and terraces on those slopes were often built with cellars or semi-basements, dug into the hillside. Rain that falls further up the hill soaks into the ground and moves downhill through it. When it meets a cellar wall, it pushes through the brickwork and up through the floor, especially where the old lime mortar has weathered.

So in a wet autumn or winter, a damp patch on the uphill side of a Sydenham cellar is normal. The question is whether a pipe has started leaking as well.

What does ground water look like?

• It arrives after rain, usually a day or more after a long wet spell, and goes away again in a dry one.
• It comes in low, at the base of the walls or up through the floor, often on the side facing up the hill.
• It is clear and cold, with no smell.
• It leaves a tide mark on the walls that rises and falls over the season.
• Your neighbours’ cellars are often wet at the same time.

What does a leak look like?

• It does not care about the weather. Wet in a dry August is a strong sign.
• It is local, with a definite wettest point, often near where a pipe runs or where the supply pipe comes in through the front wall.
• It may be warm, if it is a hot water or heating pipe.
• The meter moves with everything off.
• Your water bill has jumped, or the boiler pressure keeps dropping.

How do I check, step by step?

1. Turn off every tap and appliance in the house, and make sure no toilet is refilling.
2. Find your water meter, usually under a lid in the front path or pavement, and photograph the reading.
3. Wait 30 to 60 minutes without using any water, then photograph it again.
4. If the meter has moved, turn off the stopcock and watch it again for ten minutes: still moving means the underground supply pipe; stopped means a pipe inside the house.
5. In the cellar, put a dry sheet of kitchen roll on the wettest spot and look at it after an hour: wet and cold suggests ground water, wet and warm suggests a heating or hot water pipe.
6. Feel along any pipes in the cellar for dampness, and look at the joints and the stopcock.
7. Keep a simple diary for two weeks: date, rain or dry, and how wet the cellar is.

The diary sounds old-fashioned, but it is often what makes the picture clear. Ground water follows the rain with a lag; a leak does not.

Check the pipes while you are down there

In many Sydenham houses, the cellar is where the water supply comes in from the street, and where the stopcock is. The supply pipe in these houses is often the original lead. It is dull grey, soft (a coin scratch shows silver), with no screw joints and bulbous swellings at the joins. Thames Water, which supplies Sydenham, says you are unlikely to have lead only if the house was built after 1970 or fully replumbed since, and it can tell you about its lead pipe replacement scheme on 0800 316 9800.

An old lead pipe in a damp cellar wall is exactly the kind of pipe that starts weeping without anyone noticing. While you are there, check that the stopcock turns. If it does not, have it replaced before you need it.

What if it is ground water?

A plumber cannot stop ground water. Your options are building work:

• Tanking the cellar walls and floor with a waterproof system.
• A cavity drain membrane that lets the water in behind it and channels it to a sump.
• A sump pump to lift water out to a drain.
• Clearing the outside: make sure gutters, downpipes and gullies on the uphill side are not pouring water into the ground next to the cellar.

A specialist damp or basement waterproofing company is the right call for that. Check your address on gov.uk’s long-term flood risk service, which includes groundwater.

What if it is a leak?

Then it needs tracing before anyone digs or lifts anything. Acoustic listening, tracer gas and thermal imaging can usually pin a leak down to a small area. If it is on the underground supply pipe, Thames Water says it is your responsibility and expects it fixed within four weeks; if you are on a meter and fix it in time, you may be able to claim a leak allowance.

Orpington has a chalk version of the same puzzle, and our guide to whether water in an Orpington garden is a leak or groundwater covers it. More about SE26 on our Sydenham page.

Need a plumber now?

If your meter moves or water is coming into the cellar in dry weather, Call 07868 281022. Tell us your postcode, what the meter is doing, where the water is coming in, and whether the stopcock is off. We do leak detection in Sydenham and water leak repairs across SE26, and we agree the price with you before any work starts.