Plumbing

Roof drainage

Q

3,00l/s

DN

70

Roof drainage calculates the roof runoff to DIN 1986-100 and EN 12056-3. Area, runoff coefficient and rainfall give the flow, and the downpipe is the smallest suitable HT or SML nominal size.

Total

3,00

l/s

DN

70

HT

Per downpipe
3,00l/s
Capacity
3,82l/s
Utilisation
78%
Qnot
3,00l/s

Verify buried pipes and emergency overflows separately per DIN 1986-100.

Roof

Check values against DIN 1986-100 Table 9.

Take local rainfall values from KOSTRA-DWD.

Pipe

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How it's calculated

Roof drainage follows DIN 1986-100 and EN 12056-3. Discharge is Q = r(5,2) · C · A / 10 000 in l/s, with the horizontal roof area A and the runoff coefficient C. The emergency overflow is Qnot = (r(5,100) − r(5,2) · C) · A / 10 000. The downpipe is sized with Wyly–Eaton: Q = 2.5·10⁻⁴ · kb^(−1/6) · di^(8/3) · f^(5/3), with kb = 0.25 mm and a degree of fill f = 0.33. The smallest HT or SML nominal size whose bore carries the share per downpipe is chosen.

Worked example

100 m², C = 1.0, r(5,2) = 300 l/(s·ha), r(5,100) = 600, one HT downpipe: Q 3.00 l/s, DN 70, capacity 3.82 l/s, emergency overflow 3.00 l/s.

FAQ

What is r(5,2)?
The local rainfall for five minutes and an event once in two years, in litres per second per hectare. It comes from KOSTRA-DWD, not from a national average.
Can I change the runoff coefficient?
Yes. The presets are the guide values of Table 9. A different roof is calculated with its own C.
Why HT and SML?
The downpipe uses the stored bores of those two ranges. Silent-db is not the sizing range here.
Does this finish the buried pipe?
No. Buried pipes and emergency overflows have to be verified separately to DIN 1986-100.