Question 1

What size soil pipe do I need?

Accepted Answer

For domestic properties, 110 mm (4 inch) is the standard soil pipe size for WC connections and main soil stacks. 82 mm (3 inch) reduced bore can be used for waste-only stacks (no WC) or as a secondary ventilation pipe. 160 mm (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.

Question 2

What gradient does a soil pipe need?

Accepted Answer

For 110 mm soil pipes serving a single dwelling, Approved Document H specifies a minimum gradient of 1:40 (25 mm fall per metre) and a maximum of 1:110 (roughly 9 mm per metre) for branch pipes up to 6 m long. The gradient must be sufficient to maintain self-cleansing velocity but not so steep that water runs away from solids. For underground drainage, 1:40 minimum for pipes up to 100 mm, 1:80 for 150 mm pipes.

Question 3

What is the difference between push-fit and solvent weld soil pipe?

Accepted Answer

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.

Question 4

How high should a soil vent pipe extend?

Accepted Answer

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.

Question 5

Is 110 mm soil pipe measured by inside or outside diameter?

Accepted Answer

UK soil pipe sizes refer to the outside diameter: a standard 110 mm (4 inch) soil pipe 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.

Nominal Size (mm)	Outside Diameter (mm)	Application	Standard Lengths
82	82.4	Waste-only stacks, vent pipes	3 m, 4 m
110	110	Standard soil stack, WC connections	3 m, 4 m
160	160	Large buildings, multiple WCs	3 m, 6 m

Type	Joint Method	Pros	Cons
Push-fit (ring seal)	Rubber O-ring seal	Easy, allows movement, demountable	Slightly bulkier sockets
Solvent weld	Solvent cement adhesive	Neat, slim profile, rigid	Permanent, no movement allowance
Boss connector	Strap-on or solvent boss	Connects waste pipes to existing stack	Must be correctly positioned

Branch Type	Pipe Size (mm)	Min. Gradient	Max Length
WC branch	110	1:40 (min 18 mm/m)	6 m
Basin branch	32	1:12 to 1:40	1.7 m (unventilated)
Bath/shower branch	40	1:12 to 1:40	3 m (unventilated)
Kitchen sink branch	40	1:12 to 1:40	3 m (unventilated)

UK Soil Pipe Sizes & Gradients

Standard Soil Pipe Sizes

Connection Types

Branch Connection Gradients (Approved Document H)

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