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Rainwater Calculations

Rainwater calculations request form

Use this form to request a rainwater calculation from Fixfast, which will provide you with a drainage scheme in accordance with the UK design standard BS EN 12056:3-2000.

Please note we only provide calculations for flat roofing outlets.

Project Information

Applicant Details

Drainage Area Details

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.

For more information about these options please follow the links below:
Vertical outlets
Parapet outlets
Sump outlets

If still in concept design stage and alternative sizes could be specified please tick any options possible.

All vertical surfaces immediately adjoining the drainage area and running off onto it will need to be considered.

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