How high above the roof does a soil stack need to be?

Approved Document H requires the open end of a ventilating soil pipe to be at least 900mm above any opening window within 3 metres of the stack, to prevent foul gases entering the building. In practice this means the stack should extend at least 900mm above the eaves or parapet, and the open top should be fitted with a wire cage or a proprietary air admittance valve (AAV) cage where direct venting is not possible. In very exposed locations, a louvred terminal is used.

What is a single stack drainage system?

A single stack system uses one main soil stack to carry both soil (WC) waste and grey water (bath, basin, sink) waste together, relying on careful pipe sizing and branch gradients to prevent the siphonage of trap seals. This replaced the older two-pipe system (which had a separate ventilated soil pipe and a separate waste pipe discharging to an external gully) and is now the standard approach for new dwellings. The single stack system requires precise compliance with AD H connection rules to work correctly.

Can I use an air admittance valve instead of venting to the open air?

Yes - an air admittance valve (AAV, sometimes called a Durgo valve) can be used instead of running a pipe through the roof, subject to conditions in Approved Document H. At least one open-vented stack must serve each building to provide a path for positive pressure relief. AAVs must be installed in a ventilated space, must be accessible for maintenance, and must comply with BS EN 12380. They are not permitted as the only vent for a system if there is a risk of positive pressure (e.g. downstream of a pumped system).

Drainage & Utilities noun

Soil Stack

Q: What is a soil stack?

A soil stack (or soil and vent pipe, SVP) is a vertical pipe - typically 110mm diameter PVC in domestic construction - that collects foul waste from WCs, baths, basins, showers, and other sanitary appliances and carries it down to the underground foul drain. The stack extends upward through and above the roof to vent the system to atmosphere, preventing the build-up of gases and the siphonage of water trap seals that would otherwise allow foul smells into the building.

/ sɔɪl stæk /

Also known as: SVP, soil and vent pipe, discharge stack, main stack

Definition

A soil stack (soil and vent pipe, SVP) is a vertical pipe - typically 110mm diameter PVC in domestic construction - that collects foul water from WCs, baths, basins, showers, and other sanitary appliances and carries it down to the underground foul drain. The stack extends upward through or on the outside of the building and terminates above roof level to vent the drainage system to atmosphere, preventing pressure build-up and the siphonage of water trap seals.

In practice

Modern UK dwellings use a single stack system where all sanitary appliances - WCs, baths, basins, kitchen sinks, washing machines - connect to a single 110mm stack. Branch connections must follow strict rules in Approved Document H regarding gradient, pipe diameter, branch length, and connection position on the stack to avoid siphonage of traps. WC branches connect directly into the stack; basin and bath waste branches must enter below the WC connection or at specific offsets.

The stack is usually run internally in a service duct or boxing to limit noise transmission and protect it from frost. External soil stacks are permissible but less common in new build. At the base of the stack, a large-radius bend connects to the underground drain, and a rodding eye is fitted at this point to allow jetting and clearing of the lower stack and drain entry. The stack terminates above the roof with either an open vented top with a wire cage or an AAV (air admittance valve) in the loft space.

Building Regulations

Relevant PartPart H - Drainage and waste disposal

Minimum diameter (domestic)100 mm (110 mm PVC in practice)

Vent height above windowsMinimum 900 mm above any opening within 3 m

AAV standardBS EN 12380 - air admittance valves

Approved Document H Section 1 covers above-ground sanitary pipework including soil stacks. Branch pipe gradients, lengths, and connection positions are all prescribed to prevent trap siphonage and ensure adequate self-cleansing flow. At least one soil stack per building must be open-vented to atmosphere. AAVs may be used for secondary stacks or branch venting but not as the sole vent for the system.

Full Building Regulations guidance

Soil Stack

Just so you know...