Soil Stack Design
Branch connections to the stack
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
wherre these figures are exceeded, the branch pipe should be
ventilated, most commonly by using an anti-syphon trap.
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.
||Discharge units, l/s
|WC, 6l cistern (1.2 - 1.7 l/s)
|Shower tray (no plug)
|Urinal (cistern flush) per person
|Washing machine, up to 6kg
|Washing machine, up to 12kg
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
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
L = 450mm, up to three storeys high
L = 740mm, up to five storeys high
L = one storeyheight, over five storeys
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