Advice · Drains · 5 min read
Why the same drain blocks every winter
A drain that blocks once has something in it. A drain that blocks every January has something wrong with it — and jetting it again tells you nothing new.
We jetted a run off Quay Road three winters running. Same house, same chamber, same fortnight in January — and the third time the customer asked the question nobody had asked her before: why does it only ever do this in winter?
It is a fair question, and it has an answer. The blockage was not the problem. The fall was.
What the fall does
A 100 mm foul drain wants to fall about 1 in 40 — twenty-five millimetres down for every metre along. Below that, the water does not move fast enough to carry solids, so the solids stop and the water goes on without them.
The Quay Road run measured 1 in 110 over eleven metres. It was probably laid steeper around 1904, and a century of ground movement had done the rest.
Why only in January
A shallow run works right up until something makes it work harder, and in January three things happen at once:
- The water is colder. Fat that stayed liquid in September sets on the sides of a cold pipe, and every millimetre narrows the bore.
- Everyone is at home. Christmas fortnight roughly doubles what goes down a domestic drain, and it goes down in bursts.
- The ground is wet. Groundwater gets into old clay joints, and a run half full of water it did not ask for has nowhere to put yours.
None of that matters on a run laid at the right fall. On a run at 1 in 110, it is the difference between a drain and a storage tank.
What we did instead
Jetting would have worked — it works every time, for about eleven months. At £240 a go, that is £240 a year, forever, to be told nothing new.
Instead we ran a CCTV survey with levels recorded along the run. It found the eleven metres at 1 in 110 and a displaced joint at 6.2 m where a root had got in. That turned an annual mystery into a priced decision.
The customer chose to re-lay seven metres and leave the rest. It has not blocked since.
The rule of thumb
One blockage: clear it. Two in a year, or two winters running in the same fortnight: have it surveyed before you pay for a third clearance. Our price list has both.
Quick answers
How much does it cost to find out why a drain keeps blocking?
A CCTV survey with a written report is £190, or £120 on the same visit as a jet.
Can a shallow drain be fixed without digging?
A displaced joint or crack often can, with a patch liner. A pipe laid at the wrong fall usually has to be re-laid over the bad section.
