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The Alumasc range of calculation tools on this website are intended to provide guidance only. Accuracy for the results is not guaranteed and should not replace professional calculations where they are required. Alumasc Exterior Building Products Ltd shall not be liable for any direct or consequential loss incurred as a result of the use of these calculators.
Flat Roof Calculator
The Alumasc Flat Roof Calculator program contains detailed algorithms to perform calculations to BSEN12056-3:2000. By entering simple information relating to building dimensions and geographical location, the software will provide a list of Alumasc/Harmer roof outlets that will provide adequate drainage capacity.
Calculations are undertaken using two categories of rainfall risk:
Roofs with a pitch of 1:80 or less have a minimal risk of generating deep water depth during rainfall events, due to the extensive time period which elapses between the rain falling, and it running to the low point. In this case BSEN12056-3:2000 recommends the use of rainfall intensity evaluated using Category 1 as defined in annex NB of BSEN12056-3:2000. This relates to a storm return period of 1 year. This replaces the 75mm figure used in BS6367:1983, the previous and now obsolete standard. Figures for Category 1 rainfall vary from 0.022 l/(s.m²) (79mm/hr) in the South East of England to 0.016 l/(s.m²) (58mm/hr) in Scotland.
Roofs with a pitch of 1:79 or greater have a significant risk of generating deep water depth during rainfall events, and must be designed for large rainfall events. In this case BSEN12056-3:2000 recommends the use of rainfall evaluated using Category 2 as defined in annex NB of BSEN12056-3:2000. This relates to a storm return period of 90 years (60 years x 1.5 factor of safety). Figures for Category 2 rainfall vary from 0.066 l/(s.m²) (238mm/hr) in the South East of England to 0.044 l/(s.m²) (158mm/hr) in Scotland.
All Alumasc Harmer Roof outlets have been independently physically tested at a test facility located at the University of Salford. Flow rates are evaluated at a water depth of 35mm as per BSEN120576-3:2000.
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