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Aluminium rainwater systems roof drainage design |
| Design basis Flow capacities of Alutec gutters are compared in the Flow Capacity Table calculated with the gutters being fixed nominally level. Most metal gutters are installed level for aesthetic purposes. However, if installed to a fall of 1:600 the flow capacity will be marginally improved. Factors to be considered when designing an eaves drainage system. 1. Rainfall Intensity design rate (l/s/m2). 2. Effective Roof Area (ERA) to be drained (m2). 3. Gutter flow capacity (l/s). 4. Size, number and position of outlets. 5. Frictional resistances in gutter layout. |
Rainfall intensity (design rate) Building Regulations 2000 Document H3 recommend a general design rate of 0.021 l/s/m2 (which is virtually the same as the traditional design rate of 75mm/hour) for eaves gutters where if overflowing occurs, water overspill will discharge clear of the building without risk of water ingress. If required, alternative rainfall intensity design rates can also be considered. Meteorological data published in the National Annex to EN12056-3 and Building Regulations 2000 Document H3, show varying rainfall intensities through out the U.K. depending on geographical location. The Flow Capacity Table indicates maximum flow, the gutter being full to the brim, however EN12056-3 states the design rate run-off from the roof should not exceed 90% of the gutter capacity. Also shown are the differences between outlet capacities when positioned centrally or at the end of a gutter run. |
Effective roof area Effective roof area can be determined by calculation as set out in EN12056-3. The following example shows a basic calculation method that can be used as a guide in establishing the effective roof area (ERA). |
Gutter capacity Assuming the recommended rainfall intensity of 0.021 l/s/m2 is acceptable, determine if the gutter outlet is to be positioned centrally, or end of the gutter run. Refer to the Flow Capacity Table and find the nearest roof area m2 in either the ‘central’ or ‘end outlet’ options to determine the size/type of gutter/rainwater pipes required. Should a different rainfall intensity design rate be required, multiply the alternative design rate by the ERA to establish the required gutter capacity (l/s). Then refer to the Gutter Flow Capacity Table and select the nearest gutter flow capacity (l/s). Ensure that appropriate proportional allowances for central or end of gutter outlets are made. Example:- Alternative design rate 0.025 l/s/m2 x 140m2 = 3.5 l/s 3.5 l/s into central outlet = Ogee No. 46 with 63mm outlet  |
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