UK Soil Vent Pipe Building Regulations: A Summary

Before soil vent pipes are installed or worked on, it is important to understand the regulations that apply to them in the UK Building Regulations.

This article is a guide to current regulations and how soil vent pipes should be installed to comply with them. For full details, we recommend reading the government’s Drainage and waste disposal: Approved Document H.

Traps

A washbasin or bidet should be fitted with a 32mm diameter trap and in working or test conditions this should have a minimum water seal of 75mm. If a spray tap basin is fitted with a flush-grated waste, the water seal can be reduced to 50mm.

For a bath or shower, the trap diameter should be 40mm and the water seal should be 50mm. However, if the bath or shower discharges directly into a gully, the water seal may be reduced to 38mm.

Urinals, sinks, food waste disposal units, washing machines and dishwashers also require a 40mm diameter trap with a minimum water seal of 75mm. If a washing machine and dishwasher discharges directly into a gully, the water seal may again be reduced to 38mm.

If a toilet pan has an outlet diameter of less than 80mm the trap diameter must be 75mm, and for toilet pans with an outlet diameter of more than 80mm, the trap diameter should be 100mm. In both instances, the minimum water trap should be 50mm.

Branch discharge pipes

The maximum length, diameter and fall gradient of a branch pipe depends on what appliances, and how many, it is serving. For more information, we recommend referring to page 9 of the previously mentioned Approved Document H.

The diameter of a branch pipe serving a single appliance should be at least equal to that of the appliance’s trap.

Unless discharging into a gully, branch pipes should discharge into another branch pipe or discharge stack.

Gullies are generally at ground level or in a basement. A branch pipe at ground level may discharge directly into a drain, stub stack or discharge stack. If the pipe is only carrying wastewater (not sewage) it may also discharge into a gully.

A branch pipe from a ground floor toilet may only discharge directly into a drain if the distance between ground level and the invert of the drain (the lowest point of the drain’s internal wall) is 1.3 metres or less.

Branch discharge connections to stacks

If opposing branches without swept entries are connected to a stack, wastewater may crossflow out of one branch and into the other. To prevent this, there must be a no-connection zone above and below each branch connection.

If the branch pipes are 65mm or less, the minimum vertical distance between their centres should be 110mm on a 100mm diameter stack pipe and 250mm on a 150mm diameter stack pipe.

If branch pipes without swept entries have a diameter greater than 65mm, the minimum distance between centres should be 200mm whatever the diameter of the stack pipe.

Rodding points must be added to any length of discharge pipe that is inaccessible via removing a trap, or if the appliance is fitted with an internal trap.

Opposing horizontal branch connections should be avoided.

The minimum distance between the bottom inside walls of the lowest branch and the drain invert should be 450mm on buildings of up to three storeys. This distance should be increased for higher buildings.

Branch ventilation pipes

Branch ventilation pipes should be connected 

1. to the discharge pipe within 750mm of the trap, and
2. to the stack vent at a point above the highest spillover level of the appliances it is serving.

If they run directly to the outside of a building, they should terminate at least 900mm above any opening to the building (e.g. window or door) that is 3 meters or less horizontally from the vent pipe.

Branch ventilation pipes serving one appliance should have a minimum diameter of 25mm. If its length exceeds 15 metres or it contains more than 5 bends along its length, the diameter should be increased to 32mm.

Discharge stacks

All stacks should discharge to a drain. The bend at the foot of the stack should have a minimum radius of 200mm at its centre line, more if possible.

If possible, offsets should be avoided in the ‘wet’ section of a discharge stack. If this is unavoidable, no branch should be connected within 750mm of the offset. If the building exceeds three storeys, the discharge stack must be located inside the building and it may also be necessary to add a ventilation stack with connections above and below the offset.

The diameter of a discharge stack is determined by the maximum capacity of the discharge, measured in litres per second.

A stack diameter of 50mm will be required for a stack that doesn’t serve any toilets and needs to handle a maximum capacity of 1.2 litres per second. If the same stack needs to handle up to 2.1 metres per second, the diameter should be increased to 65mm.

If the stack serves one toilet with a maximum outlet diameter of 80mm and requires a maximum capacity of 3.4 litres per second, a stack diameter of 75mm should be used.

If the maximum capacity increases to 5.3 litres per second, a 90mm diameter stack pipe is required. for 7.2 litres per second, increase the diameter to 100mm.

Rodding points should be provided to allow access to all parts of a discharge stack. These should be located above the spillover level of appliances connected to the discharge stack.

Discharge stack ventilation

If a discharge stack is connected to a drain that is prone to surcharging or near an intercepting trap, pressure in the system can force water out of a trap and the water seal will be lost. Ventilating the discharge stack will prevent this from happening. A ventilating pipe with a minimum diameter of 50mm should be connected to the base of the stack, above the potential flood level.

A stub stack may be used, providing the system meets the following criteria:

1. the stub stack can be connected to a ventilated discharge stack or a ventilated drain that isn’t prone to surcharging
2. no connected toilet has a distance between ground level and the invert of the drain (the lowest point of the drain’s internal wall) of 1.3 metres or less
3. the centreline of no other branch into the stub stack is more than 2 metres to the invert of the drain. 

If the ventilation pipe exits the building to outside air, it must terminate at least 900mm above any opening to the building (e.g. window or door) that is 3 meters or less horizontally from the vent pipe. It must also be finished with a wire cage or perforated cover that doesn’t impede the flow of air. If there is a risk of this cover being attacked by rodents, it should be made of metal.

On a one- or two-storey building, the ‘dry’ section of a ventilation stack (above the highest branch) may be reduced in diameter but not less than 75mm.

A ventilated discharge stack that terminates inside a building must be fitted with an air admittance valve that complies with BS EN 12380:2002. This valve should not adversely affect the level of ventilation required for the underground drainage system, normally provided by open stacks in the sanitary pipework.

Air admittance valves should be located in an area where there is adequate ventilation and access for maintenance. They must also be removable in the event of blockages so these can be cleared. They should not be used outside or in dusty environments.

We hope you have found this information helpful and interesting. If you have any further questions you are always welcome to call the friendly team of drainage experts at Drainfast on 01420 555600 or email [email protected]. 

Also, look out for more articles in our ongoing series of blog posts, bringing you useful information, insights, guides and tips on all things drainage!