How do I know which gutter system my project needs?

How do I know which gutter system my project needs?

Determining the size of the gutter system needed for your project is a crucial first step when considering which guttering products to buy. There are a number of factors that you have to take into consideration. Some calculations are also necessary to make sure the products you purchase are appropriate for your project.

Here are the things you should consider, and the calculations you need to do to figure out which gutter system is right for the job:

Rainfall intensity

This figure will vary from location to location across the UK, as rainfall intensity differs according to where you are and the surrounding landscape. As a guide, 75mm per hour is the intensity of rainfall that we usually work with as the maximum in the UK. This figure is then used for calculating the discharge requirements for gutters and downpipes.

Specific roof specifications and requirements

Each roof will collect a different amount of water. To take full advantage of your gutter system, the specific placement of outlets should be considered. The amount of water that a roof collects is essential in deciding which gutter system you should opt for, and the position of outlets. The key piece of information needed is the effective roof area. This is then matched to the capabilities of the different gutter systems to find the ideal choice for your project.

Effective roof area

Depending on the style of roof, the way we calculate the effective roof area is different.

Most straightforward is a flat roof style. In this instance, the effective roof area is the same as the plan area of the roof.

Sloping roofs require some more calculations. BS EN 12056-3:2000 is the set of regulations that contain the calculation used to determine the effective roof area of more complex sloping roof structures. 

For simple sloping roofs, we use the formula E = (B+C/2) x L where:

E = effective roof area (m2)

B = half roof span (m)

C = ridge to eaves height (m)

L = slope length (m)

Effective area of walls

It is also essential to remember that walls above a roof slope also drain onto the roof below. This adds to the total amount of water that the roof collects. This means that the rainwater management system fitted to the roof below must be able to cope with this extra, accumulated water.

The effective area of the wall is calculated as half the area of the elevation. The formula for this is: E = (L1 x H1)/2.

E = effective area (m2)

L1 = length of elevation (m)

H1 = height of elevation (m) 

Rainwater runoff

We calculate the amount of rainwater runoff (R) in litres per second from a given roof by using the effective roof area we have already calculated. The formula is: R = 0.021 x E.

Gutter flow capacity

Now that we have a good idea of the amount of water that our gutter system will need to handle, we can compare these numbers to the capabilities of the different gutter systems on offer. It is also important to remember that the gutter gradient and the positioning of outlets will determine the overall performance of the gutter system. 

We take 1:600 as the ideal fall for the gutter system to effectively manage rainwater flow. Whether we place an outlet at the centre or at the corner will then decide the flow capacity and maximum roof area of the various gutter systems.

The Brett Martin Deepstyle 170mm industrial gutter system stocked by us at Cotterill Civils has the following flow capacity when the gradient is a fall of 1:600:

Outlet at Centre Outlet at End 
Flow Capacity 

10.24 L/sec 

5.12 L/sec 

Max. Roof Area




The distance between gutter angles and outlets also has an impact on the effective roof area that can be drained. Gutter angles that are closer than 2m to an outlet reduce the effective roof area that can be drained by 10%. If a gutter angle is further than 2m from an outlet, then reduce this area by just 5%.

Managing snowfall

Snowfall presents some unique challenges for gutter systems, and especially so when snowfall is heavy. For this reason, projects based in northern areas of the UK should make special considerations about mitigating the strain that snowfall can place on guttering. Extra support brackets can be installed to the gutter solutions sold by us to deal with some extra snow loads. Another consideration for areas prone to particularly heavy snowfall is snow boards, which can be fitted above the guttering to prevent damage from bearing excessive weight.

A quick note on building regulations

As with all installations, rainwater management solutions should be designed in line with the appropriate regulations that apply in the location of your project. 

BS EN 12056-3:2000 Roof Drainage Layout and Calculation is another document which sets out thorough guidance for designing and installing rainwater management systems.

Give us a call on 0121 351 3230 if you have any questions or would like to consult with one of our experienced sales representatives on the specifics of your project. We’ll be more than happy to help.

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