Home drainage basics: slopes, traps and venting

Drainage works by gravity, so slopes, traps and venting are the basis of any successful network. We review the rules as set out in the International Plumbing Code (IPC) of the International Code Council (ICC), remembering that the binding reference in the Kingdom is the Saudi Plumbing Code SBC 701, which covers sanitary drainage, venting and more1.

1) Slope of horizontal drain pipes

IPC section 704.1 requires horizontal drainage piping to have a slope of not less than2:

An exception in the code: piping upstream of a grease interceptor has a slope of not less than 1/4 in per foot (2%)2. Try the slope calculator.

2) The trap and its water seal

A trap is the bend that holds some water under every basin or floor drain. This water forms a seal that stops sewer gases and smells from entering the house. IPC 1002.4 requires each trap's liquid seal to be between 2 and 4 in (51–102 mm), or deeper for special designs relating to accessible fixtures3.

Why do smells appear?

• A dry trap: in unused floor drains (a closed room, a guest bathroom) the water evaporates. The simple fix: pour water in regularly, or use approved trap seal primers. See plumbing problems.
• Siphoning: a large flow of water in an unvented pipe can "suck out" the trap water.
• Leaking joints or a non-compliant trap.

3) Venting

A drainage network needs vent pipes open to the outside air. They balance pressure in the pipes as water flows, protecting trap seals from siphoning and letting gases escape outside. Vent design, sizes and routes are set by the code (the venting chapters of SBC 701 and the IPC) and must be designed by a specialist.

4) The drainage network from inside to outside

1. Fixtures (basin, WC, floor drain), each with a trap holding the water seal3.
2. Horizontal branch pipes at specified slopes2.
3. The vertical stack and the vent pipe reaching outside air.
4. Cleanouts and inspection chambers: points for inspection and clearing blockages; they must stay accessible, not covered by tiles or decoration (practical advice).
5. External connection to the public sewer or cesspit depending on the site.

See also plumbing problems.

5) Tips when renovating a bathroom or kitchen

1. When moving a WC or basin, confirm the required slope can be achieved to the connection point, allowing for the new floor level.
2. Never remove an existing vent pipe during renovation.
3. Provide cleanouts in accessible places.
4. Test the network (water or air test per the code) before backfilling and tiling.
5. Document pipe routes with photos before covering.