How to select the rigth size gutters with a flow calculation

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 it has become ever more important to ensure you have the calculating right flow requirements or ‘flow capacity’ required by your property to select the right size of gutter and downpipe to adequately protect your property from rainwater.

The rainwater system, comprising gutters or hoppers and downpipes, is essential because it offers protection to your property from ingress by water to the roof, walls or foundations which can cause serious damage to the integrity of the property if it is not efficiently and effectively directing rainwater away from the building and safely into the subsurface drainage system below the building.

This guide describes what goes into a flow calculation:

Step 1: Identify the Geographical Location and Rainfall Intensity

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: Calculate the 'Catchment Area' of the roof

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.

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 need these details from you 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.

How to select the rigth size gutters with a flow calculation

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

We can do the flow calculation for you.

Don’t worry, you are not expected to work this out by yourself! The British Standard Geographical Location and Rainfall Intensity Maps needed in calculations are specialist maps and so are obviously expensive to buy and keep updated.

We offer a free ‘Flow Calculation’ service. Just give us a call on 0800 644 44 28 and let us have the details above to do the calculation for you.