Soil stacks crack, separate at joints, lose their seals and block. The symptom is usually a smell, sometimes gurgling from fittings, and occasionally staining outside. Venting faults and physical faults look similar and need telling apart.
The symptoms and what they mean
| Symptom | Likely cause |
|---|---|
| Smell outside near the stack | A cracked or separated joint, or a failed seal at a branch connection |
| Smell inside, near a fitting | A dry or siphoned trap far more often than a soil pipe fault – run the tap for thirty seconds first |
| Smell inside, persistent, not near a fitting | A soil pipe fault in a void, or a boxed-in stack with a failed joint |
| Gurgling when another fitting drains | A venting problem – the stack cannot get air, so it pulls it through a trap seal |
| Staining or damp on an external wall below a joint | A leaking connection |
| Slow draining throughout the house | A blockage in the stack or below it, rather than at one fitting |
| Waste backing up at ground floor | A blockage below, possibly in the drain rather than the stack |
| Flies or insects around a soil pipe | A crack or an open joint |
Most drainage smells indoors are a trap that has lost its water – dried out in a rarely used bathroom, or siphoned empty by another appliance discharging on the same branch. Running the tap for thirty seconds refills it. If the smell goes and stays gone, that was it. If it returns within days, that is siphonage and it is a venting problem worth looking at.
Venting, and why gurgling matters
A soil stack must be vented, normally to open air above the roof, so that water discharging down it does not create a partial vacuum that siphons the water out of nearby traps.
When venting is inadequate:
- Fittings gurgle when another one drains. That noise is air being drawn through a trap seal.
- Traps lose their water, which is why smells return days after somebody refilled them.
- It is a design fault, not a blockage. Clearing something will not fix it.
Air admittance valves are an alternative to an open vent in some positions – they let air in but not out. They must remain accessible, they do fail, and there are limits on where they can be used in place of open venting. A failed AAV in a boxed-in position is a recurring cause of smells that nobody can locate.
What fails, by material
Cast iron
Common on older Norwich and Norfolk property. It corrodes, particularly at the back where nobody looks and where it sits against the wall, and it cracks. Repairs are possible with proprietary couplings; a badly corroded stack is a replacement. Cast iron is heavy and working on it needs proper support – a stack is not something to cut into casually.
Plastic
Push-fit joints separate through thermal movement, particularly on long vertical runs with inadequate expansion provision. Seals perish. Solvent-welded joints fail where they were made badly. UV degrades plastic on south-facing elevations over decades, leaving it brittle.
Connections and branches
Boss connections for basins, baths and washing machines are a common leak point, particularly where a boss was cut into an existing stack and the seal was not made properly.
The base of the stack
Where it turns to horizontal and connects to the drain. A tight bend here is a recurring blockage point, and this is the section most likely to be affected by ground movement.
Repair or replace
| Finding | Usually |
|---|---|
| A single failed seal or joint | Repair – a coupling or a new seal |
| A cracked section of plastic | Cut out and replace that section |
| A separated push-fit joint | Remake, and address the expansion provision that caused it |
| Corroded cast iron at one point | Repair coupling, if the surrounding metal is sound |
| Generally corroded cast iron | Replace the stack. Patching a failing stack is a sequence of visits |
| Failed boss connection | Remake properly, with the correct fitting |
| Inadequate venting | A design fix – open vent or a correctly sited AAV. Not a repair |
| Blockage | Clear, then establish why. Repeat blockages are structural |
Internal stacks are a different proposition from external ones. An internal stack in a boxed duct needs access, and a leak there does damage before anybody notices. External stacks are easier to inspect and easier to work on, which is one of their genuine advantages.
Below ground is somebody else’s trade
We work on the above-ground drainage – the stack, the branches, and the connection down to the gully or the point where it goes below ground.
Beyond that point – collapsed clay pipework, root ingress, camera surveys, drain relining and excavation – is a drainage specialist’s work, and we will say so rather than guessing at it. The distinction matters because the symptoms overlap: waste backing up at ground floor can be a stack blockage or a collapsed drain, and telling them apart determines who you need.
Also worth knowing: shared drains and the sewer beyond your boundary are generally the water company’s responsibility. If a blockage is affecting more than one property, call them first – it may cost you nothing.
Smell but no visible leak? Run every tap for thirty seconds first. If it comes back within days, that is venting rather than a crack.
Questions we get asked about this
Why can I smell drains outside?
A cracked or separated soil pipe joint, or a failed seal at a branch connection. It is usually visible on inspection once you know what to look for.
Why does my bath gurgle when the toilet flushes?
A venting problem. The discharging soil is drawing air through the bath’s trap because it cannot get air anywhere else, which means the trap is being siphoned and smells will follow.
Can a cast iron soil pipe be repaired?
A single corroded point, yes, with a repair coupling if the surrounding metal is sound. A generally corroded stack is a replacement – patching it becomes a sequence of visits.
Is an air admittance valve as good as an open vent?
They work and they are permitted in defined situations. They are mechanical and they fail, they must remain accessible, and there are limits on using them in place of open venting.
Do you do below-ground drainage?
Above-ground drainage and the connection to the gully, yes. Collapsed clay pipework, camera surveys and excavation are a drainage specialist’s trade and we will tell you rather than guessing.
Who is responsible for a shared drain?
Shared drains and the sewer beyond your boundary are generally the water company’s. If a blockage affects more than one property, call them first – it may cost you nothing.
What happens when you call
Call and describe it
You speak to an engineer, not a call centre. Most jobs can be narrowed down on the phone.
We look at it properly
A real diagnosis before a price, and a straight answer if it is not worth doing.
A written quote
Itemised, so you can see what you are paying for.
The job, and the paperwork
Certificates where the work needs them, and what will need attention next.
Where we do this work
Across Norwich and the whole of Norfolk. If you are on the edge of the county, call and ask — we will tell you honestly whether we are the right people.
- Norwich
- Wymondham
- Dereham
- Attleborough
- Aylsham
- North Walsham
- Cromer
- Diss
- Loddon
- Long Stratton
- Watton
- King's Lynn
Need this doing?
Call and describe the problem — you will speak to an engineer, not a call centre. Written quotes, no estimates over the phone.