Fasteners and components for the building envelope

Category 1 is the minimum accepted by BS EN 12056-3:3000, and assumes the greatest flow to be allowed for is during the peak storm expected in one average year. This is generally used for totally flat roof areas, where it is assumed that the design caters for excess flow that may occur at other times; by alternative run-off, by being able to structurally withstand the weight of standing water, or if the flow is attenuated such that the theoretical peak flow is impossible in practice.

Category 2 is used as standard wherever it cannot be assumed that flow rates beyond the calculated level are acceptable, and therefore only exceptional events should be beyond the scope of the calculation. The basis for this is the peak flow to be expected within 1.5 times the building's expected lifespan, corresponding to a 50% chance that flow beyond this will ever occur within the design life of the building.

Category 3 is for "high importance" buildings where flow rates beyond those calculated for are likely to have unacceptable consequences. The basis for this is the peak flow to be expected within 4.5 times the building's expected lifespan, corresponding to a 20% chance of excess flow for the entire design life of the building.