Drainage - Design Points

Design Points

There are some points to note when designing any drainage scheme, these are:

Foul Water

1.     Foul water is soil water from toilets and waste water from basins, baths, showers, etc.

2.     The one-pipe system is favoured over the two-pipe system because there are fewer pipes and it is more hygienic.


3.     The two-pipe system uses a separate vent from each sanitary appliance, which are then joined into a combined vent stack, whereas the single-stack system is simplified.

4.     All systems are vented and trapped to exclude smells and foul air.

Traps are devices, which contain a water-seal of about 50mm to 75mm to prevent gases escaping into sanitary fittings like wash basins, water closets, sinks, baths, showers, etc.

Foul water pipes exceeding 6.4 metres long are usually required to be vented.

5.     If the waste pipe from a wash basin is at too steep a gradient, self-siphonage may occur. This is where the contents of the trap are sucked out into the waste pipe because the water flows away too quickly thus emptying the trap.

6.     Induced siphonage can occur if a suction pressure develops in the drainage system. A suction pressure of 500 N/m2 (50mm water gauge) will reduce the water level in a basin trap by 25mm.


7.     In badly designed systems backpressure can also occur which is sufficient to remove water from a trap.


8.     Waste pipes from appliances which discharge into larger pipes avoids siphonage problems because the larger pipes do not normally run full.

For example, a 32mm waste from a wash hand basin is connected to a 100mm diameter Soil and Vent pipe.

9.     Waste pipes from appliances which discharge into pipes of the same diameter have limitations on lengths, number of bends and gradients to minimise siphonage problems.

10. Self-siphonage is not normally a problem for sinks, baths and showers because of the near flat base of each appliance allowing the trap to re-fill should it empty.


11. The horizontal length of soil pipe from a WC is limited to 6m (Building Regulations U.K.).


12. Soil and Vent stacks should have no waste branch close to the connection of the WC.


13. Sometimes it is not possible to prevent pressure fluctuations in pipework in which case separate vent pipes should be installed. It may not be possible to limit the length of branches or provide reasonable gradients in some installations.


14. A velocity of flow of 0.6 to 0.75 m/s should prevent stranding of solid matter in horizontal pipes.


15. Gradients from 1 in 40 to 1 in 110 will normally give adequate flow velocities.


16. A range of 4 lavatory basins, the traps from which discharge into a straight run of 50mm waste pipe not more than 4m long, with a fall of 1-21/2o, will give rise to a need for venting. (reference British Standard No. 5572)


17. It is normal practice to connect a ground floor water closet straight into a manhole. Self-siphonage and induced siphonage will not occur because of the large pipe from a W.C. diameter (100mm) and because the drain is vented.


18. Access points should be sited: 

(a) At a bend or change indirection

(b) At a junction, unless each run can be cleared from an access point.

(c) On or near the head of each drain run.

(d) On long runs

(e) At a change of pipe size.



19.         The soil & vent stack or branch to which at least one WC is connected must have an internal diameter of at least 100mm.

Outlets from wash basins have a 32mm minimum diameter branch pipe and sinks and baths have branch discharge pipes of 40mm diameter.

For large drainage installations pipe can be sized using discharge units and appropriate graphs.


20.         Drains should be laid at a depth of 900mm (minimum) under roads and at least 600mm below fields and gardens.


Drainage - Design Points