Rainwater Downpipe Diameter and Flow Rate Calculation

According to EN 12056-3 roof drainage design standard, the flow rate of a rainwater downpipe is calculated, or the required pipe diameter is selected based on the design discharge. The Wyly–Eaton equation is used for hydraulic capacity determination. Where; \( Q_{RWP} \) : Rainwater downpipe capacity (L/s) \( k_b \) : Pipe roughness (mm) — typically taken as 0.25 mm \( d_i \) : Internal pipe diameter (mm) \( f \) : Pipe filling ratio (dimensionless) These are the governing parameters.

Wyly–Eaton equation (Equation 1): \begin{equation} Q_{RWP} = 2.5 \times 10^{-4} \; \frac{d_i^{8/3} \, f^{5/3}}{k_b^{1/6}} \end{equation}