UK Soil Pipe Sizes & Gradients

For domestic properties, 110 mm (commonly sold as "4 inch" pipe, though true 4 inch is 101.6 mm) is the standard soil pipe size for WC connections and main soil stacks. 82 mm (trade "3 inch") reduced bore can be used for waste-only stacks (no WC) or as a secondary ventilation pipe. 160 mm (trade "6 inch") is used for larger buildings or where multiple WCs connect to a single stack. The soil stack must be at least as large as the largest branch connecting to it.

For a 110 mm WC branch, Approved Document H Table 2 sets gradient limits of 18 to 90 mm fall per metre (1:56 to 1:11), and Diagram 3 marks the branch slope as 18 mm per metre minimum with a maximum unventilated run of 6 m for a single WC. In practice most soil branches are laid at 25 mm per metre (1:40), which sits inside that window. Table 2 note 2 allows the 18 mm per metre minimum to be reduced to 9 mm per metre on long runs where space is restricted, but only where more than one WC is connected. The gradient must be sufficient to maintain self-cleansing velocity but not so steep that water runs away from solids. For underground drainage, Approved Document H Table 6 gives 1:40 as the flattest gradient for 75 mm and 100 mm foul drains, and 1:150 for a 150 mm drain.

Push-fit (ring seal) soil pipe uses rubber O-ring seals and simply pushes together — it allows thermal movement and is easy to disassemble. Solvent weld uses chemical adhesive to permanently bond joints — it creates a rigid, neat system but cannot be taken apart. Push-fit is more common for new installations; solvent weld is often used where a neater finish is required or for replacement of existing solvent systems.

The soil vent pipe (SVP) must extend at least 900 mm above the top of any opening window within 3 m of the pipe. If the pipe terminates near a window, dormer or roof opening, the height must be measured from the nearest opening. The top of the pipe must be fitted with a cage or cowl to prevent bird entry but must not be capped — it must remain open for ventilation. Air admittance valves (AAVs) can replace the open vent in some situations.

UK soil pipe sizes refer to the outside diameter: a standard 110 mm soil pipe (sold in the trade as "4 inch", though true 4 inch is 101.6 mm) measures 110 mm across the outside, and the internal bore of 110 mm PVC-U pipe is around 104 mm. The smaller 82 mm and larger 160 mm sizes are quoted the same way. When extending or repairing an existing stack, measure the outside diameter of the pipe before ordering, and check whether the existing system is push-fit (ring seal) or solvent weld, as the two joint types are not the same.

A toilet (WC) waste pipe must be at least 100 mm in diameter under Approved Document H, which is the minimum branch and stack size for a WC pan with an outlet over 80 mm, the standard UK pan. UK builders merchants sell this as 110 mm outside diameter PVC-U soil pipe, with an internal bore around 104 mm, comfortably meeting the 100 mm minimum. The WC connects through a pan connector rather than a separate trap fitting, because the trap is built into the pan itself. See the Waste Pipe Sizing Calculator to check pipe sizes for a full bathroom, or the UK trap sizes reference for how the WC pan trap compares with basin, bath and sink traps.

The soil stack, also called the soil vent pipe or SVP, is the main vertical pipe, usually 110 mm and sometimes 160 mm on larger buildings, that runs from ground level up through the building and out above roof level, collecting waste from every floor. A branch pipe is the shorter run connecting one or more individual appliances, a basin, bath, WC or sink, to the stack. Branch pipes are sized to the appliance: 32 mm for a basin, 40 mm for a bath, shower or kitchen sink, 50 mm minimum where two or more of these combine into one run, and 100 mm (110 mm outside diameter) for a WC. The stack itself must always be at least as large as the largest branch feeding into it, which in practice means 110 mm wherever a WC connects.

Where two or more appliances, for example a bath and a basin, discharge into a single combined pipe before it reaches the stack, BS EN 12056-2 sizes the combined run by adding up each appliance discharge unit (DU) value rather than just taking the largest individual branch. As a conservative domestic guide: combined waste up to 6 DU with no WC needs a minimum 50 mm pipe, 7 to 12 DU needs 65 mm, and above 12 DU needs 75 mm or larger. A 50 mm combined branch has a maximum unvented run of 4 m before an air admittance valve or vent pipe becomes necessary. Any branch that includes a WC is sized at 100 mm regardless of how many other appliances share it. The Waste Pipe Sizing Calculator works out the combined size for a specific fixture list.

No, although they often share the same diameters. Soil pipe is the above-ground pipework fixed to or inside a building that carries waste down from appliances and WCs to the point where it goes underground. Underground drainage pipe carries that waste onward from ground level to the sewer or a soakaway, and follows different bedding, cover depth and gradient rules under Approved Document H. Both commonly use 110 mm and 160 mm pipe, but a soil stack uses push-fit or solvent weld joints with bracket spacing rules, while underground pipe uses socketed, jointed runs laid to a bedding class. See the Drainage Pipe Sizes reference for the underground figures, cover depths and minimum fall gradients.