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
As a conservative default, i ≈ 0.015 l/(s·m²) = 150 l/(s·ha) (2-year rainfall BEFORE climate factor) is often used. The rainfall intensity is NOT a fixed DS 432 standard value: it is calculated from the local municipality's rainfall series for a chosen return period T (before climate factor ≈ 120/150/230 l/(s·ha) for T=1/2/10 years) TIMES a climate factor (typically ≈ 1.3) — municipalities may require higher values (Source: kloakviden.dk — indicative).
Calculation example — rainwater from a roof
A roof with a horizontally projected area A = 150 m² and a sealed roof covering (runoff coefficient φ = 1.0) gives, before climate factor, q_R,d = A · φ · i = 150 · 1.0 · 0.015 = 2.25 l/s. With a climate factor of 1.3, the design stormwater flow becomes 2.25 · 1.3 ≈ 2.9 l/s. That is the flow that downpipes, branch and stormwater drains must be able to carry — stormwater drains are calculated at a filling ratio of 1.0 (completely full pipe).
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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