Basement flats and pumped drainage in central London
Below drain level, gravity stops helping. Everything then depends on a pump, an alarm and a discharge run that nobody thinks about until it backs up.
Why basements are different
A fitting below the level of the main drain cannot discharge by gravity. Waste collects in a sealed sump and a pump lifts it to a point where gravity takes over. Add a high water table, which much of central and riverside London has, and there is often a second pump handling groundwater as well.
That makes a basement bathroom a mechanical system rather than a passive one, and mechanical systems have failure modes that gravity drains do not.
The alarm usually is not the pump
When a pump alarm sounds, the instinct is to assume the pump has failed. In practice the more common cause is that the gravity run it discharges into is restricted, so the pump runs, cannot clear the chamber and trips the level alarm.
Replacing a working pump because the drain downstream is blocked is an expensive way to not fix a problem. The check order should be: does the pump run, does the discharge line take the flow, is the run beyond it clear.
Lightwell gullies, the forgotten part
Basement flats in stucco terraces almost always have a lightwell with a gully in it. Out of sight, out of mind, and full of leaves by November. When it silts up, surface water has nowhere to go but into the well, and from there into the flat.
Clearing lightwell gullies twice a year prevents a category of flooding entirely. It is the cheapest maintenance on the property.
Where we see this most
Deep basement conversions are concentrated in Kensington, Chelsea and Westminster, where the kitchen, utility room and often a wet room sit below drain level and discharge into nineteenth-century laterals.
Riverside areas add a high water table to the same picture: Battersea, Putney and the Nine Elms strip all have basements working against groundwater as well as waste.
What to specify if you are building one
- Twin pumps with automatic changeover, so one failure is not a flood
- A working alarm that someone will actually hear, not one in the plant room of an empty flat
- An accessible sump that can be cleaned, because silt and grease build up and shorten pump life
- A non-return valve on the discharge, checked periodically, since a seized valve causes the problem it was fitted to prevent
- An anti-flood device where the property sits low and the sewer is known to surcharge
More on drainage installation and what we design around.
If the problem is on your doorstep, we cover CCTV drain surveys in Westminster and blocked toilets in Westminster.