DS 432 article covers the SIZING — rain and waste water flow, slope and filling degree. Here we look at the system structure itself: the water trap that keeps sewer gas out, the vent that prevents it being siphoned dry, the soil stacks and branch pipes that carry the water away, and the installation of the plumbing fixtures it all hangs from.
Introduction
A drainage system has two tasks at once: to safely remove wastewater and rainwater — and to keep the foul, health-hazardous and explosive gases from the sewer out of the building. The sizing (pipe sizes, slope, fill level) is described in 'DS 432:2020 — drainage, rainwater and anti-siphon trap'. This article is about how the system is built up inside so it works in practice.
Water trap — the barrier against sewer gas
Under every single drainage object — basin, kitchen sink, floor drain, toilet — sits a water trap. It is simply a U- or P-shaped bend, where a water seal always remains after draining. The water seal (seal water) seals the pipe, so air cannot rise from the sewer and into the room. Without a water trap, the entire dwelling would have a direct connection to the sewer network with odour, bacteria and risk of sewer gases.
The water lock can fail in two ways, and both are a classic plumber's and isolator's troubleshooting case:
- 01Drying out: a drain that is not used (e.g. a floor drain in a rarely used basement) slowly evaporates its water seal away. Then the path opens for sewer gas. The solution is regular water refilling, a supplied water connection, or a membrane trap/dry trap that closes mechanically when water is not flowing.
- 02Siphonage and extraction: when a large body of water (e.g. a WC flush) rushes down through the stack, it can create a vacuum that sucks neighbouring traps empty — or positive pressure that forces air bubbles up through them with gurgling. This is exactly what ventilation must prevent.
Ventilation — pressure equalisation that protects the water seals
When water falls through a vertical stack, it pushes air ahead of it and pulls air behind. Without a path for that air, pressure fluctuations occur, which suck or blow the traps empty. Ventilation gives the pressure a place to equalise, so the water seals stay in place.
- 01Main ventilation over roof: the stack is run continuously up and out through the roof (vent cap). It is the primary, always-functioning solution — air can both enter and exit, and sewer gas is led high above the roof where it does not disturb.
- 02Branch venting: separate vent lines to branch pipes with many or distant objects, so the pressure balancing occurs close to the exposed object.
- 03Vacuum/ventilation valve (air intake valve): a diaphragm valve that ONLY lets air IN at negative pressure and closes tightly again so sewer gas does not escape. Used where a roof vent cannot be done — but it cannot relieve excess pressure and must therefore not replace all ventilation in a system; at least one proper roof vent should normally be retained. Placement and use follow DS 432/DS/EN 12056-2.
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