Why does it only leak when the upstairs toilet is flushed?

Because that is the only time water passes that joint at volume. Working out which fitting triggers the leak is the fastest way to locate the failure, and it is something you can do from the garden with someone else indoors.

Can a cast iron soil pipe be repaired or does it need replacing?

Individual joints and short sections can be repaired, but corrosion is rarely isolated on a stack that has reached that stage. An engineer will look at the whole run before deciding, and on many older houses replacement with plastic is the sensible outcome.

Do I need permission to replace a soil stack?

Drainage work is covered by Building Regulations Part H, and on a listed building or in a conservation area changing a cast iron stack for plastic on a visible elevation can need consent. Check with your local authority before ordering materials.

Caution

Updated 30 August 2026

Soil Pipe Leaking

The large vertical waste pipe on the wall is wet, stained or dripping when a toilet upstairs is flushed.

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What's happening?

The soil stack is the large-diameter vertical pipe — usually 110mm, plastic on modern houses and cast iron on older ones — that takes the toilets and, on many houses, the bath and basin wastes down to the underground drain. It is also the vent: it runs up past the eaves to terminate above the roof so air can enter and leave the system without pulling the traps out. A leak on it is easy to spot because it only happens when something upstairs discharges, and easy to smell. Cast iron stacks fail at the caulked joints and corrode from the inside out at the base; plastic stacks fail at push-fit seals that have hardened, at brackets that have let the pipe drop, and at connections made without solvent cement where they should have been.

What usually causes it?

A perished rubber seal in a push-fit socket on a plastic stack
Corrosion at the base or at a joint on a cast iron stack, often hidden behind paint
A cracked or split branch connection where a toilet or bath waste enters the stack
Brackets that have pulled out of the wall, so the stack has dropped and pulled a joint apart
A blockage in the stack causing it to surcharge and force water out of a joint
Thermal movement on a long plastic run with no expansion allowance
Damage from a ladder, scaffold or falling roof tile

Is it dangerous?

Moderate. What is escaping is foul water, so it is a hygiene issue and it will stain and eventually damage the wall behind it. A leak into a boxed-in internal stack is worse, because it goes unseen into a floor void or a stud wall until the ceiling below shows it. On older properties a corroded cast iron stack is also a falling hazard if the base has rusted through and the pipe is no longer properly supported. Do not stand under one that is visibly corroded at the fixings.

Can I fix it myself?

Diagnosis, yes; repair, mostly not. Get someone to flush each toilet and run each upstairs fitting in turn while you watch the stack from ground level, and note exactly which discharge produces the leak — that identifies the branch. Check whether the brackets are secure and whether any joint has visibly opened. Clear vegetation and check that the top of the stack above the roof is not blocked. Replacing a push-fit seal or a bracket on a ground-floor section is within reach of a competent DIYer; anything at height, anything cast iron, and anything internal is not.

When should you call a drainage engineer?

Call a drainage engineer when the leak is above ground-floor level, when the stack is cast iron, when it is boxed in internally, or when the leak only happens under heavy flow — that last one usually means a blockage causing the stack to surcharge, and the joint is a symptom. Call sooner if there is staining on an internal ceiling or wall, because the damage will already be ahead of what you can see.

What will the drainage engineer actually do?

Watch the stack under test discharge to identify the failing joint or branch
Replace perished seals, cracked branches, or a length of pipe, and refit proper brackets
Cut out and replace corroded cast iron sections, or replace the stack where it is beyond repair
Check the stack for blockage and jet it where surcharge is forcing the leak
Inspect the vent terminal above the roof and clear or replace it
Pressure or smoke test the finished stack and check the traps hold under discharge

Frequently asked questions

We’re happy to answer all your questions.

Sources & References

Drain Smells Of SewageCaution Gurgling DrainsCaution Blocked Outside GullyCaution Outside Drain OverflowingHigh risk

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