Branch limits, stack capacities, discharge-unit flow estimates and drain gradients used to size foul drainage.
Unventilated branch limits
Use the Approved Document H branch limits together: bore, length, fall and connected appliances. An unsuitable unventilated branch can run full and siphon its own trap. An unventilated 32 mm basin branch may be 1.7 m at 18 to 22 mm per metre, 1.1 m at 18–44 mm per metre and 0.7 m at 18–87 mm per metre. A 40 mm basin branch can reach 3 m, or a 50 mm branch 4 m, at 18–44 mm per metre. Sinks and baths may run 3 m in 40 mm pipe or 4 m in 50 mm pipe at 18 to 90 mm per metre. A single WC branch may run 6 m at no less than 18 mm per metre. An unventilated 100 mm branch can serve up to eight WCs over a length of no more than 15 m. Where more than one WC is connected on a long run and space is tight, the fall may drop to 9 mm per metre. For the larger branch options marked in Diagram 3, retain the normal trap diameter and extend its tail by 50 mm before increasing the bore.
Stacks
Table 3 gives stack capacities of 1.2, 2.1, 3.4, 5.3 and 7.2 l/s for 50, 65, 75, 90 and 100 mm stacks. The 50 and 65 mm sizes take no WCs. A 75 mm stack takes no more than one WC with an outlet under 80 mm, and WCs with outlets over 80 mm need at least 100 mm. A stack should not reduce in the direction of flow; in one- and two-storey houses the dry vent section may be reduced, but not below 75 mm.
Estimating flow with discharge units
For larger systems, BS EN 12056-2 (System III is the UK arrangement) gives each appliance a discharge unit (DU): 0.3 for a basin and 1.3 for a bath or kitchen sink, for example. The expected flow is Qww = K × √ΣDU, where K is 0.5 for dwellings and offices, 0.7 for schools, hospitals and hotels, 1.0 for public toilets and 1.2 for laboratories. Do not let the appliance-flow allowance fall below the largest connected appliance’s DU value. Add any continuous and pumped inflows separately at their applicable full rates. For example, with a given total of 25 DU in a school, the diversified appliance flow is 0.7 × √25 = 3.5 l/s, provided the largest single-appliance value is no higher. Additional inflows are not included in that 3.5 l/s.
Drains and sewers
A foul drain has a minimum internal diameter of 75 mm; WC or trade effluent requires at least 100 mm. In Table 6, the below-1 l/s peak-flow rows give 1 in 40 for 75 mm and 100 mm pipe. The above-1 l/s rows allow 100 mm at 1 in 80 with at least one WC, or 150 mm at 1 in 150 with at least five WCs. Their listed capacities are 6.3 and 15.0 l/s respectively. Read the flow band, bore and WC footnote together.
For a laid drain, subtract downstream invert level from upstream level to find the fall. Use the same units for length and fall, then divide length by fall to obtain the denominator of 1 in n. A larger denominator means a flatter run. Sewers serving up to 10 dwellings normally need at least 100 mm diameter, or at least 150 mm above 10 dwellings. These are minimum sizes, not a full capacity calculation.