DS 432:2020 is the standard for drainage systems. It raised the design rain intensity due to climate change, which points towards larger pipes and requirements for high-water-level traps in low-lying basement installations. Here are the formulas and the indicative figures you work with.
Introduction
DS 432 sets out how drainage systems are dimensioned — both wastewater from sanitation and rainwater from roofs and surfaces. Following DS 432 is the recognised way to comply with the Building Regulations' drainage requirements (BR18 ch. 4, §§ 69–81).
Rainwater flow
Som konservativ standardværdi regnes ofte med i ≈ 0,015 l/(s·m²) = 150 l/(s·ha) (2-årsregn FØR klimafaktor). Regnintensiteten er IKKE en fast DS 432-normværdi: den udregnes af den lokale kommunes regnrække for en valgt gentagelsesperiode T (før klimafaktor ≈ 120/150/230 l/(s·ha) for T=1/2/10 år) GANGE en klimafaktor (typisk ≈ 1,3) — kommuner kan kræve højere værdier (Kilde: kloakviden.dk — vejledende).
Calculation example — rainwater from a roof
Et tag med et vandret projiceret areal A = 150 m² og tæt tagbelægning (afløbskoefficient φ = 1,0) giver før klimafaktor q_R,d = A · φ · i = 150 · 1,0 · 0,015 = 2,25 l/s. Med en klimafaktor på 1,3 bliver den dimensionerende regnvandsstrøm 2,25 · 1,3 ≈ 2,9 l/s. Det er dét flow, tagnedløb, stik- og regnvandsledning skal kunne føre — regnvandsledninger regnes ved fyldningsgrad 1,0 (helt fyldt rør).
A 110 mm rainwater pipe has good margin to 2.9 l/s, but the exact capacity depends on the slope and is read from DS 432's design tables — select the dimension there, not by estimate. If the municipality uses a higher return period or climate factor, q_R,d increases accordingly, and then a larger dimension or delay/infiltration may become necessary. (Source: DS 432:2020; kloakviden.dk — indicative rainfall intensity and climate factor.)
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