Drain blocking

Yorkie1

Thanks @FreeBear . I will be replacing the cast iron downpipe and bathroom hopper, and will get the others done better at the same time.

Section62

https://forums.moneysavingexpert.com/discussion/comment/82011231#Comment_82011231

Assuming the shared pipe is at least 4" then the additional rainfall runoff from one property won't be the cause of the flooding unless the shared pipe is seriously defective already. Although it can be a major problem in district and trunk sewers (and sewage works) some surface water runoff can help to keep foul/combined sewers clean by giving them an occasional 'flush'.

Albermarle

https://forums.moneysavingexpert.com/discussion/comment/82011312#Comment_82011312

In any case, the rainwater should not normally be going into the foul sewer anyway ( although it sometimes does).

Yorkie1

I think I'd need a full drain survey to know that! Although I could ask my neighbour if he knows where his new extension roof gutters flow into.

WIAWSNB

Additional rain/surface water shouldn't cause this issue, except temporarily. That's 'clean' water, sans solids, so should flush the sewer through if anything...

Yes, it can obviously overwhelm the system at times due to sheer volume, but it should dispel quickly too. Bottom line - if your drain isn't drainin', and if it ain't rainin', then you have a blockage.

Gonk1967

https://forums.moneysavingexpert.com/discussion/comment/82011312#Comment_82011312

Not necessarily, the properties were built in the 1930's it was usual in those days to build "combined" drainage systems. There may be several extensions upstream which could exacerbate the problem. Anyway, the OP has stated that the problem doesn't occur during heavy rainfall.

Section62

https://forums.moneysavingexpert.com/discussion/comment/82011372#Comment_82011372

I wouldn't risk creating potential neighbourly difficulties by asking… the runoff from an extension is not going to make the difference between flooding and not flooding with a 4" or larger pipe. Where combined drainage is common it isn't unusual to have maybe a dozen or more 1930's houses served by one 4" pipe which takes all the foul flow plus about half the surface runoff.

It appears your problem is building up over time - which points towards a blockage formed by things which shouldn't be flushed, or fat, or an outside chance there's a partial collapse with soil being able to partly block the pipe.

If the pipe has been affected by building waste then this might be making a build up of other 'stuff' more likely. For example if the invert of the pipe has a layer of mortar/concrete in it then the velocity of the flow may have dropped below self-cleansing velocity.

Yorkie1

https://forums.moneysavingexpert.com/discussion/comment/82011395#Comment_82011395

I think Section62 is right, although to be fair it hasn't rained since I noticed the issue … !

WIAWSNB

If you - or someone - can lift the lid, we will almost certainly have the answer. It's either your 'issue', or the WB's.

Section62

https://forums.moneysavingexpert.com/discussion/comment/82011395#Comment_82011395

Combined systems are the result of local bylaws more than time period. 1930's housing can be combined or separate, depending on the whim of the borough surveyor or sanitary inspector employed by the local authority at the time.

A 3m extension 5m wide will give a 30-year design peak flow of maybe a third of a litre per second. A 4" foul drain laid for self-clensing velocity should have a maximum capacity of >6 litres/sec. So it would take the equivalent of about 20 misconnected extensions of that size to reach full-pipe capacity (less the foul-water flow rate at the time).

Even then that doesn't mean the drain will flood. The pipe will start operating in surcharged condition, and as the water is coming out of a drain gulley (in Yorkie1's case) it means the surcharged head would have to be the equivalent of the drain invert depth.

If the pipe forms part of a combined system then it would have been designed for runoff from probably 50% of the main roof area, plus a percentage of any 'yard'. Adding some extensions to that load would only be an issue for storm intensities toward the upper end of the design capacity. I.e. maybe once in 25 to 30 years, or for older drainage systems perhaps once in 50 years.