Flow Rate (Q)

• The rate of flow of rainwater to be drained away from a roof under steady state conditions shall be calculated from equation 1:

$Q = r*A*C$

where;
$Q$ is the rate of flow of water, in litres per second (l/s);
$r$ is the rainfall intensity, in litres per second per squre metre $[l/(s*m ^2]$;
$A$ is the effective roof area, in square metres $(m^2)$;
$C$ is a runoff coefficient (taken as 1.0 unless national and local regulations and practice state otherwise), dimensionless.

Reference

1. BS EN 12056-3:2000