Choosing the right size of Gutters and Downpipes

As extraordinary weather patterns become ever more regular with intense rain storms causing major challenges for rainwater disposal. This guide provides a basic method for calculating flow requirements or 'flow capacity'.

Step 1: Geographical Location and Rainfall Intensity Maps

British Standard BS EN 12056-3: 2000 contains maps showing rainfall intensity in litres/second per m2 for 1, 5, 50 and 500 year storms of 2 minute duration.
(All external gutters designed for 1 year event).

Step 2: Calculating Catchment Area

CA = (P+H/2) x L

CA = Catchment area in square metres
P = Horizontal distance between eaves and ridge
H = Height of roof
L = Length of eaves

Step 3: Frequency and Positioning of outlets/downpipes

Calculate the number of outlets per run.

It is the roof area, the pitch of the roof and the number of 'drops' (downpipes coming down from outlets in the guttering around the building) determine the flow - we will need to know these to calculate the flow capacity for you.

Step 4: Calculate Flow Requirements

Overall rainfall : Catchment area (CA) x Rainfall intensity (RI) = Overall Rainfall (OR)

Flow rate per outlet : Overall rainfall (OR) ÷ Number of outlets = Flow rate per outlet
Choose gutter/outlets according to published flow rate capacities.

flowcalc-picsNote: Depending on building type, a safety factor should be allowed for the sizing of internal gutters.


For accurate project specific advice on rainwater flow calculations contact our friendly, knowledgeable team on 0800 644 44 26.


Download a 'list' pricelist (prices before discounts), installation guide, technical sheets and case studies on our Downloads Page .


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