Drainage Stack & Building Drain Lab
Add up the discharge units of the fixtures, apply the frequency factor and size the soil stack to BS EN 12056-2, with or without a secondary vent. Then size the building drain by slope and fill with Colebrook-White, check self cleansing velocity, and watch a flush travel down the stack.
How to use: enter fixtures per floor and floors per stack. Discharge units are the BS EN 12056-2 System I values and you can edit the counts. Tap i for explanations.
1. Stack and fixtures
One stack serving the same fixtures on every floor.
2. Stack and drain
Stack loading against capacity, the drain cross section at design flow, and a flush in 4D.
3. Checks
4. Drain sizes at your gradient
5. How it works
Discharge units and K
Each fixture has a discharge unit (DU) in L/s. They are not simply added: the wastewater flow is Qww = K × √ΣDU, where K reflects how often fixtures are used. The flow can never be less than the largest single fixture.
Stack capacity
A stack carries water as a film on the wall with an air core in the middle. Too much water closes the core and pulls the water seals out of the traps. BS EN 12056-2 tables give the capacity of each stack size for primary or secondary ventilation and for square or swept junctions.
Building drain
Horizontal drains are sized with Colebrook-White for the gradient, running no more than half full (h/d 0.5) so air can move, and at least 0.7 m/s so solids are carried along.
Learning and checking tool. Some authorities use IPC or AS/NZS 3500.2 fixture units instead of discharge units. Use the method your authority adopts.