Part H Drainage Calculator — Pipe Sizing & Gradient

Each sanitary fitting has a discharge unit (DU) value: WC=7, basin=1, bath=7, shower=2, kitchen sink=6, washing machine=3, dishwasher=3. These integers are the domestic column of the discharge unit table in BS 5572 (withdrawn in 2000 but still the set in everyday UK use); BS EN 12056-2 Table 2 expresses discharge units in litres per second instead. The DU total tells you the flow the drain has to carry, and Approved Document H para 2.33 sets the floor on diameter: a drain carrying foul water needs an internal diameter of at least 75mm, and a drain carrying effluent from a WC or trade effluent at least 100mm. So: no WC and up to 10 DU, 75mm. Any WC at all, or more than 10 DU, 110mm (110mm OD uPVC is the DN100 size, i.e. 100mm internal). Over 100 DU, 160mm.

AD H Table 6 gives the recommended minimum gradients. A 100mm drain at a peak flow under 1 litre per second is 1:40, which is 1000/40 = 25mm of fall per metre. A 100mm drain at a peak flow over 1 litre per second may go as flat as 1:80, but that row is footnoted 'minimum of 1 WC', so it only applies once the drain carries WC discharge. A 75mm drain under 1 litre per second is also 1:40. Steeper is acceptable. The gradient must be consistent with no low points. Design the gradient to be self-cleansing, meaning water flows fast enough to carry solid waste.

For 110mm pipes, access points (inspection chambers or rodding eyes) should be no more than 12m apart. For 160mm pipes, 22m maximum. Access is needed at every change of direction, every junction and at the head of each drain run. This allows blockages to be cleared with drain rods or jetting equipment.

For drain depths up to 600mm: 450mm diameter (shallow access chamber) is sufficient. For depths 600mm-1200mm: 475mm diameter or larger. Over 1200mm: 1050mm diameter manhole with step irons. All chambers need suitable covers rated for the expected traffic loading (pedestrian, light vehicle or heavy vehicle).

Yes. All new drains must be tested before backfilling. The standard test is an air test, and AD H para 2.60 sets it out in two stages: pressurise the pipe to 110mm water gauge and hold it for approximately 5 minutes prior to testing, then start the test from an initial 100mm pressure and confirm the maximum loss of head on the manometer is no more than 25mm over a period of 7 minutes. The 5 minutes is the conditioning period, not the test period. Alternatively a water test can be used. Building Control will inspect drainage at this stage.