SVP stands for soil vent pipe: the vertical pipe that carries waste from a building’s toilets, basins, baths and showers down to the underground drain, while venting sewer gases safely above roof level. It is also called a soil stack. In UK construction and plumbing, SVP almost always means soil vent pipe; other meanings of the abbreviation have nothing to do with building work.
Key facts
- SVP = soil vent pipe, also known as a soil stack or discharge stack.
- Standard UK size is 110mm diameter above ground; 160mm is used on larger buildings2.
- It does two jobs: carrying foul waste to the drain and venting sewer gases above the roof.
- Approved Document H requires the open end to finish at least 900mm above any opening window within 3 metres.
- An SVP can run externally on a wall or internally through the building, boxed in and plastered.
What does SVP stand for in construction?
In construction, SVP stands for soil vent pipe3. It is the large-diameter pipe, usually 110mm, that runs vertically from the underground drain up past the roof line. Every waste appliance in the house connects into it, either directly or through smaller branch pipes.
You will also hear it called a soil stack, a soil and vent pipe, or just the stack. They all mean the same pipe.
The name tells you what it handles. Soil is the trade word for foul water from toilets. Waste water is everything else, from sinks, baths, showers and appliances. Both go down the same stack, and the vent part of the name covers the pipe’s second job above the roof.
Older properties usually have cast iron SVPs. Anything built or replumbed since the 1960s will normally have plastic (uPVC), which is cheaper and easier to work with3. Cast iron is still fitted on listed buildings, and sometimes chosen in flats because it is quieter than plastic.
What does an SVP do?
An SVP does two jobs at once.
First, it carries soil water (from toilets) and waste water (from sinks, baths, showers and washing machines) down to the underground foul drain4.
Second, it ventilates the drainage system. The open top of the pipe, above roof level, lets sewer gases escape and lets fresh air into the pipework3. Without that open vent, water rushing down the stack would pull air behind it and siphon the water out of the traps under your sinks, baths and toilets. Empty traps mean drain smells inside the house.
That is why the pipe carries on past the highest connection and finishes above the roof instead of stopping at the top bathroom. Venting matters even more in taller buildings, where appliances sit a long way above the main drain4.
What is an SVP in plumbing?
In plumbing terms, the SVP is the main vertical discharge stack of the above-ground drainage system. A typical UK house uses a 110mm PVC-u stack, vented through or above the roof, with the toilet connecting directly and smaller waste pipes joining through bossed fittings2.
| Pipe | Typical UK size |
|---|---|
| Soil vent pipe (stack) | 110mm |
| Stacks and drains on larger buildings | 160mm |
| Basin waste | 32mm |
| Bath and shower waste | 40mm |
| Kitchen sink waste | 50mm |
Access matters as much as size. NHBC guidance calls for rodding access points at intervals of no more than three storeys, positioned above the spillover level of appliances, so blockages can be cleared without cutting the pipe1. On site, the stack should be supported with brackets every 1.2 to 1.5 metres5.
What are the SVP regulations in the UK?
Approved Document H of the Building Regulations covers soil and vent pipes in England and Wales7. The main rules are:
- The open end must finish at least 900mm above any opening window or rooflight within 3 metres of the pipe.
- The outlet needs a wire cage or perforated cover to keep birds and leaves out.
- A stack that serves a toilet must be at least 100mm internal diameter, which is why the standard 110mm external pipe is used.
- Every drainage system needs at least one vent open to the atmosphere.
Air admittance valves (AAVs, often called Durgo valves) can terminate a stack inside the building instead of running it through the roof. They must comply with BS EN 12380, and because they only let air in, not out, the system as a whole still needs an open vent somewhere6.
Moving or extending drainage counts as notifiable building work. Tell building control, or use an installer who can self-certify.
Internal vs external SVP: which is better?
Both are allowed. Which one you have mostly comes down to the age and design of the building.
| External SVP | Internal SVP | |
|---|---|---|
| Where it runs | Fixed to an outside wall, usually the rear | Inside the building, in a duct or boxed-in corner |
| Typical on | Older houses | New builds and flats |
| Cost and access | Cheap to fit, easy to inspect and rod | Needs access panels, harder to reach |
| Weather | Exposed to frost and UV | Protected |
| Appearance | Visible pipework | Hidden |
New builds usually run the SVP internally because buyers and planners prefer clean outside walls. An internal stack passes through floors and the roof, so the holes and fixings for it are cut as part of the builders’ work in connection (BWIC) on a job.
Once the stack is in, the boxing around it is normally plasterboarded and skimmed. If you are having this done, our guide to the first coat of plaster explains what a proper finish looks like.
How do you know when an SVP is blocked?
The usual warning signs are slow-draining sinks and baths, gurgling from plugholes when the toilet flushes, foul smells around the property, and a toilet that empties sluggishly8.
At the top of the pipe, the common culprits are bird nests, leaves and tree debris, and even tennis balls8. Lower down it is usually fat, wipes and scale.
Clearing a blocked SVP usually means rodding from the access point nearest the blockage, which is why the NHBC rules on access spacing matter. If the terminal is blocked by a nest, it can often be cleared from a ladder or through a roof access hatch.
A visual check of the vent terminal and a clear-out every 6 to 12 months keeps the stack working5. A persistent smell with no visible blockage can mean a cracked pipe hidden behind boxing. Get a drainage engineer to fix the pipe first, then have the boxing rebuilt and skimmed. Our guide to finding a reliable plasterer in Kent covers what to check before you hire.
If an SVP repair or refit has left you with damaged boxing, chased walls or holes to make good, our plasterers in Kent can quote for the making good.
Frequently asked questions
What does SVP mean in construction?
SVP means soil vent pipe, the vertical pipe that carries waste from toilets, basins, baths and showers to the underground drain and vents sewer gases above the roof. It is also called a soil stack. The standard size on UK homes is 110mm diameter.
What is an SVP in plumbing?
In plumbing, the SVP is the main vertical discharge stack of the above-ground drainage system. Toilets connect into it directly, and smaller waste pipes (32mm, 40mm and 50mm) join it through bossed fittings. It runs from the underground drain up past the roof line, where the open end lets the system breathe.
What does an SVP do?
An SVP does two jobs: it carries foul water down to the drain and it ventilates the drainage system. The open top lets sewer gases escape above the roof and lets air in behind flowing water, which stops the water seals in traps being siphoned out. Without it you would get drain smells indoors.
What are the SVP regulations in the UK?
Approved Document H of the Building Regulations covers SVPs in England and Wales. The open end must finish at least 900mm above any opening window within 3 metres and be fitted with a cage or perforated cover. A stack serving a toilet must be at least 100mm in diameter, and every drainage system needs at least one vent open to the atmosphere.
Should an SVP be internal or external?
Both are allowed under UK Building Regulations. External SVPs are cheaper to fit and easier to inspect, which is why most older houses have one on the rear wall. Internal SVPs are standard on new builds and flats because they keep outside walls tidy, but they need access panels and boxing-in that is usually plasterboarded and skimmed.
Is an SVP the same as a soil stack?
Yes. Soil vent pipe, soil stack, soil pipe and discharge stack all describe the same vertical pipe in domestic drainage. Soil refers to the foul water from toilets, and vent refers to the pipe’s second job of ventilating the drain. Builders and plumbers use the terms interchangeably.