While not strictly mandated, a fall between 1:350 and 1:600 (roughly 1.7–3 mm per metre) is recommended for efficiency.

To ensure a roof drainage system is compliant with EN 12056-3, engineers and architects must consider four critical factors:

The NBN EN 12056-3 standard covers several key aspects of drainage system design and installation, including:

[ Q = k_st \times A_cs \times R_h^2/3 \times \sqrtS ]

The standard requires designers to use statistical data based on the building’s location. In Belgium, this typically involves choosing a "return period" (e.g., a 1-in-100-year storm) depending on the risk level associated with the building. Effective Roof Area (

Failing to provide access invalidates compliance.