Drain
Installing a drainage system with the correct fall
Falls, filling ratios, joints and backflow valve according to DS 432:2020
A drainage installation must carry away both wastewater and rainwater without blockage or backing up. This requires the right fall, correct filling ratios and — in low-lying installations — protection against backflowing sewage. The guide covers the design basis, falls, joints, venting, backwater valves and leak testing.
Determine the design basis
Determine the wastewater flow (qS,d) from the fittings schedule/discharge units and the rainwater flow with q_R,d = A · φ · i, where φ is the runoff coefficient (impermeable surfaces/roof = 1.0; slabs with gravel/grass joints = 0.8; gravel = 0.6; grass/garden = 0.1). The rainfall intensity i depends on the local municipal rainfall series × climate factor — not a fixed standard value.
φ roof/impermeable surfaces1,0φ grass/garden0,1Choose and set out the correct fall
Set out a fall that ensures self-cleansing flow. As a guide, allow at least approx. 10–20 ‰ — verify against DS 432 and the project. Too little fall causes deposits; too much fall can leave solids behind when the water runs away from them too quickly.
Minimum slope~10–20 ‰ (guideline)Observe the fill levels
Size the pipes so that the indicative filling ratios are maintained: vented wastewater ≤ 0.5 (0.7 for ≥ 150/160 mm downstream of a manhole), combined sewer ≤ 0.7 and rainwater ≤ 1.0. The filling ratio ensures that the pipe can be vented and does not siphon traps empty.
Wastage is not a constant≤ 0.5Common line≤ 0.7Rainwater≤ 1.0Assemble the drain pipes correctly
Assemble PP drain pipes (e.g. Ø50/Ø75/Ø110) with socket and rubber ring so that the seal faces the direction of flow. Clean the pipe end and socket, and push the joint fully home. Follow the manufacturer's requirements for sound insulation where there are acoustic requirements.
Fit pipe supports and accommodate expansion
Fit correct pipe supports that both hold and guide the pipe, and allow room for thermal expansion — plastic expands noticeably with temperature changes from hot waste water. Without room for expansion, joints can be pulled apart or build up stresses.
Ensure venting of soil stacks
Run the stack vents correctly above the roof (or use an approved air admittance valve where permitted). The venting equalises the pressure so traps are not siphoned empty and odours are kept out of the home.
Protect low-lying installations with a backflow valve
Fit low-lying basement installations with a backflow valve (or pump sump) that prevents waste water from backing up during cloudbursts. DS 432:2020 raised the design rainfall intensity because of climate change, which means backflow valves are required more often.
Leak test and document
Leak test the installation and document it in a record. Check connections to the sewer/shared system against the drainage plan and the municipality's permit. Note: connection to the public sewer is a separate authorisation track (drainage contractor + KLS).
Drainage work requires a drainage contractor's authorisation — a plumbing authorisation alone is not enough for connection to the public sewer.
Source
Based on the worktree article «DS 432:2020» + the template «Checklist: drainage installation» and the calculations (rainwater flow, runoff coefficients, fall ‰, filling ratios) and the product range (PP drain pipes, backwater valve). DS 432:2020.
Videos from NDLA
Video guide· Norwegian speech
Bunnledninger for avløp
Source
NDLA
«Bunnledninger for avløp» · Photographer: Kjell Brørs
License: CC BY-SA 4.0 · Original at NDLA ↗
Shown unaltered in the original language with the source's own subtitles. The video shows Norwegian practice — the method is often the same, but rules and standards may differ from the Danish ones.