SANS 10400-P covers drainage, the pipes that carry waste water away from a building, and on-site systems like septic tanks where there is no municipal sewer. This guide sets out the rules that affect estimating: drain sizes and falls, and how big a septic tank and french drain must be.
Drain size
SANS 10400-P 4.15 says the nominal diameter of a drain may not in any case be less than 100 mm.
Falls and drain load
A drain must slope enough to carry waste away without blocking, and table 14 links the slope to the load the drain can carry, measured in fixture units. Table 10 gives the units: a washbasin or bidet is 1, a bath, sink or shower 2, a toilet 8, and a sanitary group 12.
| Fall | 100 mm drain, max units | 150 mm drain, max units |
|---|---|---|
| 1 in 10 | 9,000 | 27,000 |
| 1 in 20 | 6,400 | 19,000 |
| 1 in 40 | 4,500 | 13,500 |
| 1 in 60 | 3,650 | 11,000 |
| 1 in 80 | 3,150 (special permission) | 9,500 |
| 1 in 100 | 2,800 (special permission) | 8,400 |
Falls flatter than 1 in 120 for a 100 mm drain, or 1 in 200 for a 150 mm drain, are not permitted; some flatter falls need the local authority’s special permission.
Worked example: a 12 m drain at 1 in 40 falls 12 ÷ 40 = 0.3 m, and a 100 mm drain at that fall can carry up to 4,500 fixture units, far more than one house. The drain fall calculator works out the fall and checks the load.
Septic tanks
Where there is no sewer, a septic tank treats the sewage and a french drain lets the treated effluent soak into the ground. SANS 10400-P 4.8.4 sets a minimum of 1.5 m³ (1,500 litres) for a tank serving a single house, and annex C table C.1 gives the size by bedrooms and how often the tank will be desludged. For example, a 3-bedroom house desludged every 1 to 3 years needs 1,500 L, rising to 2,800 L if it is desludged every 8 years. The tank must be covered by at least 150 mm of soil and have an access opening for emptying.
French drains
The french drain is sized from the daily sewage flow and how fast the ground absorbs water, measured by the percolation test in 4.25. Table 1 gives the flow: 750 L a day for a 2-bedroom house, 900 for 3, 1,100 for 4, 1,400 for 5 and 1,600 for 6, plus 15% for stormwater and contingencies. Table 3 turns the percolation time into a rate of application, from 108 litres per m² of drain side for fast-draining ground down to 33 litres at 30 minutes. Over 30 minutes, no french drain may be built (4.8.9).
Clause 4.8.10 gives the trench length: L = ((flow ÷ rate) − 2 × width × depth) ÷ depth. The standard’s own worked example divides by twice the depth, which gives about half the length; BouWise uses the clause’s formula, the longer result. French drains must be at least 3 m from any building or boundary, no deeper than 1.8 m, and parallel drains at least twice their depth apart. The septic tank and french drain calculator applies all of these rules.
Counting fixture units
Add up the units for everything that drains into a pipe. A house with two toilets (16 units), two washbasins (2), a bath (2), a shower (2) and a kitchen sink (2) comes to 24 units. That is far below what a 100 mm drain carries at any permitted fall, which is why house drains are rarely bigger than 100 mm: the minimum size and a sound fall are what matter.
This guide is general information for estimating. It is not engineering, legal or tax advice. Check SANS 10400 (the National Building Regulations) and your local municipality’s building control officer for your project.