Solution : Multiply roof length by the horizontal width (not the rafter length).
Consult your national annex (Belgian NBN annex) for a map of rainfall zones. For Brussels: nbn en 12056-3
The NBN EN 12056-3 standard is crucial for ensuring that drainage systems in buildings are designed and installed to function properly, safely, and efficiently. A well-designed and installed drainage system is essential for: Solution : Multiply roof length by the horizontal
Warehouse in Antwerp. Roof: Flat, 80m × 40m = 3200 m² (0.32 ha). Rainfall (Antwerp, Belgium): T=5 years = 320 L/(s·ha); T=100 years = 650 L/(s·ha). Roof type: Inverted (insulation above waterproofing), gravel ballast. A well-designed and installed drainage system is essential
The standard is not merely a suggestion for pipe diameters; it is a safety protocol. Improperly drained water can lead to "ponding" on flat roofs, which adds immense structural weight, or "backflow" in gutters, which can rot wooden eaves and cause interior water damage. The Calculation Methodology
Convert to mm/min: 1 L/(s·ha) = 0.006 mm/min. So 300 L/(s·ha) = 1.8 mm/min.