You are here » Brett Martin Home » Plumbing & Drainage » News & Events » Rainwater System Design

Right as Rain - Step by Step Rainwater System design Advice

Cast Iron Gutter System, Six Heritage Colours

They are a vital part of any building’s design, but as Graham Smith, Sales Director of Brett Martin Plumbing & Drainage argues, every rainwater system is unique. Here he offers his step by step advice for those designing efficient, compliant, cost effective and aesthetic rainwater systems.

One of the most common problems faced by contractors who are called back to sites to repair damaged or poorly performing rainwater systems is the frequency of undersized gutters together with inadequately spaced rainwater pipes, according to the BRE, (BRE Repairing and Replacing Rainwater Goods 1997).

This is likely to have been price driven, with specifiers cutting corners to reduce costs. However, it is possible to minimise the cost of components by ensuring they are sized and positioned correctly. For example, a larger gutter capacity will demand a larger, more expensive downpipe but by installing larger downpipes, fewer components are needed, resulting in a lower overall cost. 

The BS 6367: Code of practice for the Drainage of Roofs and Paved Areas provides a methodology for selecting the components of a rainwater system, their size and position on the building for the most effective installations. Needless to say it is important for the specifier to remember that no two rainwater installations are the same because they are governed by a number of factors.

Following a simple route: weather conditions, roof size, pitch and design, should help the specifier achieve the right system for the job.

The amount of rainfall a roof will see is determined by its geographic location. The Building Regulations approved document H: Drainage and waste disposal, provides a guide to rainfall intensities for the design of gutter and rainfall pipes.

The flow and volume of water into a gutter depends on the area of the surface being drained and the angle of the roof’s pitch. Ascertaining the roof size in terms of the drained area is the next step to designing a successful and cost effective rainwater system. The following table provides a means of allowing for the pitch of the roof by working out an effective area:

Calculation of drained area

Type of surface

Effective design area

Flat roof

Plan area of relevant portion

Pitched roof at 30º

Plan area of portion x 1.29

Pitched roof at 45º

Plan area of portion x 1.50

Pitched roof at 60º

Plan area of portion x 1.87

Pitched roof over 70º or any wall

Elevational area x 0.5

(Drainage and waste disposal approved document H)

Getting the gutter size right will determine the size and number of down pipes. The following table shows the largest effective area that should be drained into a gutter size:

Gutter sizes and outlet sizes

Max. effective roof area (m2)

Gutter size (mm dia.)

Outlet size (mm dia.)

Flow capacity (litres/sec)

























(Drainage and waste disposal approved document H)

Gutters should be installed with a fall towards the nearest outlet, generally a downpipe. These should then discharge into a drain or gully. The size of this pipe should be at least the size of the outlet from the gutter and an outlet that serves more than one gutter should have an area at least as large as the largest of the contributing outlets.

It is important to remember that gutter support spacing should normally not exceed 900mm. However roofs with a pitch exceeding 35°, smooth surfaces, or those which may be subject to heavy snow loading due to geographical location, need support spacings that do not exceed 600mm. In such areas and circumstances, it is recommended to install snowboards as a precaution against sliding snow.

By investigating systems from various manufacturers it is possible for the specifier to select products that deliver added value in terms of aesthetics, cost and labour savings. For example, visually compact systems such as Brett Martin’s 106mm Prostyle System, are highly efficient but suitable for smaller buildings.  For larger roofs the 115mm Deepstyle System offers an increase of 50% in flow capacity, compared to a Roundstyle system, potentially reducing the number of downpipe and drainage runs required, speeding up installation and reducing overall costs.

Finally the specifier needs to consider the overall aesthetic of the building. A correctly specified system should complement and enhance the building. When traditional aesthetics in external detailing are required, drainage can play an important part. However specifiers need to be aware of the cost - both in materials and labour - of selecting real cast iron.

Products rendered in plastic are overcoming this challenge, with a price tag more inline with modern construction methods. Brett Martin’s Cast Iron Style range features three styles of domestic gutter (106mm Profiled Ogee, 112mm Half Round and 115mm Deep gutter profile, all utilising 68mm round downpipes and the new 65mm square downpipe range). Meanwhile for larger scale commercial projects there is a high capacity system with a choice of half round gutter and round and rectangular downpipes.

There are a number of factors that need to be considered in the specification of a successful and long-lasting rainwater system. By following the simple process of weather and geography, roof size, pitch and design, the specifier will be able to determine the best solution for the roof in question.

For further information on the Rainwater and Cascade ranges visit Rainwater Systems and Cascade Cast Iron Style Systems.