Runoff Coefficient

What is the runoff coefficient in drainage calculation?

DIN 1986-100 Table 9: runoff coefficients for different roof build-ups
DIN 1986-100 Table 9: runoff coefficients for different roof build-ups

The runoff coefficient in the drainage calculation depends on the roof build-up and mathematically accounts for the delayed inflow to the primary outlet caused by gravel or vegetation. Because the runoff coefficient mathematically shifts inflow from the primary outlet to the emergency outlet, the primary drainage is designed with a lower and the emergency drainage with a higher discharge capacity.

The inflow "calculated away" from the primary drainage must be added back to the emergency drainage, because the retarding effect is no longer present at the latest once the roof build-up is saturated. Considerably more water then flows to the primary drainage than it can take up, so the emergency drainage must additionally discharge this differential flow in l/s.

This calculated effect occasionally causes surprise when the emergency drainage, especially with green roofs, has to discharge considerably more than the primary drainage. In such cases particularly high-performance parapet emergency drainage systems with siphonic flow are frequently used.

Examples of the runoff coefficient

In accordance with DIN 1986-100 Section 14.2.3 Runoff coefficients, Table 9 "Runoff coefficients C for determining the rainwater runoff", the following peak runoff coefficients (Cs) are listed for "water-impermeable surfaces, e.g. roof areas".

Smooth surface Cs = 1.0

For flat roofs (pitch up to 3° or about 5 %) made of metal, glass, fibre cement or with waterproofing membranes.

Gravel ballast Cs = 0.8

For flat roofs (pitch up to 3° or about 5 %) with gravel ballast.

Green roof Cs = 0.7 to 0.2

For flat roofs as vegetated roof areas

Extensive planting (> 5°): Cs = 0.7
Intensive planting, from 30 cm build-up thickness (≤ 5°): Cs = 0.2
Extensive planting, from 10 cm build-up thickness (≤ 5°): Cs = 0.4
Extensive planting, below 10 cm build-up thickness (≤ 5°): Cs = 0.5

Deviating from this, providers of green roofs may specify their own runoff coefficients for their respective green roof build-ups, which can then be entered as a "manual runoff coefficient" in the drainage calculation.