Soil Stack Design

It is generally recommended that the length of branch connections should not exceed those in this diagram. The slope of the branch shown is that recommended in BS 12056 -2:2000. In cases where these figures are exceeded, the branch pipe should be ventilated, most commonly by using an anti-syphon trap.

Branch connections to the stack

Permitted number of appliances in non-domestic buildings

A group of appliances consists of one WC and one wash basin. Congested circumstances are where the frequency of use of each appliance is approximately 10 minutes.


ApplianceDischarge units, l/s
WC, 6l cistern (1.2 - 1.7 l/s) 1.7
Wash basin 0.3
Bath 1.3
Shower tray (no plug) 0.4
Kitchen sink 1.3
Urinal (cistern flush) per person 0.2
Bidet 0.3
Dishwasher, domestic 0.2
Washing machine, up to 6kg 0.6
Washing machine, up to 12kg 1.2

A discharge unit is the numerical value assigned to each sanitary appliance to express it's load producing property.

Refer to BS EN 12056-2, 2000, system III for more information.


Bends at the base of stacks should be of long radius. Two 45° bends may be used as an alternative. The same principle should be used for offsets.

The distance between the centre line of the lowest branch connection and the invert of the drain should be at least 450mm.

L = 450mm, up to three storeys high
L = 740mm, up to five storeys high
L = one storeyheight, over five storeys

Stub stacks

A group of ground floor appliances may be connected to drain using an unventilated stub stack. The vertical drop to the invert level of the drain must not exceed 1.5m from a WC and 2.5m from a waste appliance.