Blocked Soil Stack? Signs, Risks and What to Do Right Now

If an upstairs toilet is backing up, or waste appears in a bath or shower on a lower floor when someone flushes, the soil stack is the likely problem. Stop using every toilet, bath and basin on the floors above the problem, check the nearest manhole outside, and do not keep flushing to see if it clears. Each flush adds more waste to a pipe that is not emptying.

What the soil stack does

The soil stack is the large vertical pipe that carries waste from toilets, baths and basins down to the underground drain. It usually runs up the outside wall on older London houses and inside the building, boxed in, on newer ones and conversions.

At the bottom it turns through a bend into the underground drain. At the top it is normally open to the air above the roof line, or finished with an air admittance valve inside the loft or a boxing. That open top lets air in so water can flow down smoothly without pulling the water out of the traps under your sinks and toilets. Most blockages form at the bend at the bottom, at a branch joint where a toilet connects in, or in an old cast iron stack that has narrowed with scale over the years.

Signs the blockage is in the stack, not the drain below

The pattern of which fittings back up tells you where the blockage is. Fittings above the blockage fill up; fittings below it keep working. If the manhole outside is empty while an upstairs toilet is rising, the problem is in the stack or its bend, not the underground drain.

• Upstairs toilet rising or slow, ground-floor toilet fine. The blockage is in the stack above the ground-floor connection.
• Waste coming up in a bath or shower when a toilet above is flushed. The stack is blocked below the point where the bath branch joins it, so waste from above takes the easiest way out.
• Every fitting in the house is slow and the outside chambers are full. That is the underground drain, not the stack. Our guide to internal and external blockages covers how to tell the two apart.
• Gurgling from basins and toilets, smells, but water still drains. This often points to a blocked vent at the top of the stack rather than a blockage in it.
• Water or sewage weeping from a joint on the stack. A joint that leaks only when fittings above are used is often being pressurised by a blockage lower down.

What to do right now

Stop the water going in and contain what has already come out. Then work out whether anyone else shares the stack.

• Stop using every fitting on the floors above the blockage. That includes the washing machine and dishwasher if they drain into the same stack.
• Check the nearest manhole outside. Empty means the blockage is in the stack. Full means it is in the underground drain, which is a different job.
• If the stack is shared, tell your neighbours. In a house converted into flats, one stack usually serves every flat, and the people upstairs may not know that their flushes are coming up in your bath.
• Do not pour chemical drain unblocker into a toilet or bath. It sits on top of the blockage, will not clear waste in a stack, and can burn whoever has to open the pipe.
• Do not go on the roof or up a ladder to rod down from the vent. It is dangerous, and rods pushed down from the top rarely clear a blockage at the bend at the bottom.

When a blocked soil stack is an emergency

It is urgent tonight when sewage is coming out into a bath, shower or across a floor, when a boxed-in stack is leaking inside the building, or when it is the only toilet in the home. If the only symptom is a slow upstairs toilet and everything below works, it can usually wait until morning with that toilet out of use.

A stack leaking inside a wall or boxing is worth taking seriously even if the amount looks small. Sewage soaking into plasterboard, timber and floor voids is far harder to deal with than a spill on a tiled floor. Our rates are set by the time of day: 7am to 5pm at the standard rate, 5pm to midnight at the evening rate, and midnight to 7am at the night rate. Current figures are on the pricing page, and the price is agreed before anyone travels.

How a blocked stack is cleared

A stack is cleared from an access point, not through the toilet. Most have a rodding eye or access door near the base or at branch joints, and the bend at the bottom can often be reached from the first chamber outside.

Where the blockage is at the foot of the stack, jetting upwards from the chamber is usually the quickest route. Where it is higher up, the engineer works from the nearest access door, which on an internal stack is often behind a panel or boxing that has to be opened. A toilet may occasionally need lifting to reach a branch blockage. Once the stack is flowing, the underground drain below is flushed through, and a camera check is worth doing if the blockage keeps coming back. Our blocked toilet and internal drain clearance covers stacks, branches and the drain below.

If the stack is running but the house still gurgles and smells, the vent at the top may be blocked by leaves, a nest or a cap that has been fitted over it. Clearing a vent at roof level is a job for someone with safe access to the roof, not something to try from a ladder.

When a drainage engineer is the wrong call

If the stack itself is cracked, split or leaking at a joint that stays wet when nothing is being used, the pipe needs repairing or replacing. That is above-ground plumbing work, and a plumber is the right trade for it.

Old cast iron stacks on Victorian and Edwardian houses often corrode from the inside, which narrows the bore and makes blockages come back. Clearing one will restore the flow, but it will not fix a pipe that has rusted through. We will tell you if that is what we find, rather than keep coming back to clear the same stack. In a block of flats or a converted house, the stack usually belongs to the common parts, so the freeholder or managing agent should be told about repairs, even if you had to call someone out yourself to deal with the emergency.