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.
Rainwater Calculator
The Alumasc Rainwater Calculator program contains detailed algorithms to perform calculations to BSEN12056-3:2000. By entering simple information relating to building dimensions, geographical location and proposed gutter style, the software will provide a list of gutter and downpipe combinations from the Alumasc range that will provide adequate drainage capacity.
All calculations are undertaken using rainfall data evaluated using Category 1 as defined in annex NB of BSEN12056-3:2000. This relates to a storm return period of 1 year (i.e. the gutter will overflow once every year). This is the standard rainfall level for all true external gutters which can overflow without water entering the building and 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.
The gutters used in the Alumasc calculator are all eaves gutters and so no freeboard is allowed, as per the requirements of BSEN12056:3-2000. The calculator will automatically allow for the effect of corners on hipped roofs, and allows for the effects of friction in gutters longer than 50 times the gutter depth.