A drain that blocks once has something in it. A drain that blocks every January has something wrong with it, and jetting it again is charging you twice for the same information.
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 in front of her: 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. That number is not a preference. Below it the water does
not move fast enough to carry solids, so the solids stop and the water goes on
without them. Above it the opposite happens: the water runs away and leaves the
solids sitting on a dry invert.
What a 100 mm foul run does at each gradient
1 in 40 — what the pipe wants
carries
1 in 80 — shallowest we will lay, straight runs only
carries, just
1 in 110 — what we measured off Quay Road
silts
Dead level — the control
nothing moves
The Quay Road run measured 1 in 110 over eleven metres. It had probably been
laid at something nearer 1 in 60 in about 1904 and the ground had done the rest.
Then why only in January
Because a shallow run works right up until something makes it work harder, and
in January three things do at once.
None of those would matter on a run laid at 1 in 40. On a run at 1 in 110 they
are the difference between a drain and a storage tank.
If the answer is yes, the thing to buy next is not another jetting.
What we did instead
The decision, in her case, was to re-lay seven metres and leave the rest. Seven
metres at £210 came to £1,470, plus a chamber, plus reinstating the path. It is
not a small bill. But it is a bill that ends, and the annual one does not.
What to do before you ring anyone
If you do not know any of that, ring anyway. The person who answers is the
person who will drive out, and they will work it out on the phone.
Written up by the person who did the job. If your drain is doing what
this one did, the survey is £190 and it is £120 if we jet it the same
visit — both on the rate card.