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Rainwater Design & Installation

Whether it is a new build project or the refurbishment of existing buildings, the rainwater system is a vital building component, without which the structure would not pass building regulation requirements. However, as Graham Smith, Sales Director of Brett Martin Plumbing & Drainage here explains, the rainwater system can often be an afterthought to the roofing construction and materials. Here he brings the importance of the rainwater system to the fore and explains his step by step process for those designing and installing efficient, compliant and aesthetically pleasing rainwater systems.

According to the BRE (BRE Repairing and Replacing Rainwater Goods 1997) a common problem for contractors being called back to a job to repair damaged or badly performing rainwater systems is the prevalence of wrongly sized gutters and inadequately spaced fascia brackets and rainwater pipes.

Such faults are often driven by price, with the need to keep costs down forcing the selection of smaller (and therefore cheaper) gutters and fewer downpipes. It is possible to bypass this problem by designing the rainwater system correctly without unnecessary cost in the first place, by ensuring the components of the rainwater system are sized and positioned correctly. For example, a larger gutter capacity may demand a larger, more expensive downpipe but, it may also result in fewer downpipes and, subsequently, less underground drainage, producing a dramatically lower overall cost. 

There are already methodologies for selecting rainwater components in place. The BS 6367: Code of practice for the Drainage of Roofs and Paved Areas provides offers advice on the size and position on the building of the gutter and downpipes for the most effective rainwater installations. It is important to remember that no two installations are the same, because there are a number of factors affecting their performance.

For the contractor, assessing the weather conditions, roof size, pitch and design, should help to ensure the right size and number of components are purchased to achieve the right performance.

Although it may not always seem the case, especially during the wetter winter months, overall it is fair to say that some areas of the UK experience higher volumes of rainfall than others. The geographic location of the building is the first criteria to asses. This will help to determine how much rainfall the roof will have to deal with in the course of the year. The Building Regulations approved document H: Drainage and waste disposal, provides a guide to rainfall intensities for the design of gutter and rainfall pipes.

Secondly it is the pitch and size of the roof area to be drained that has a knock-on effect on the flow of water into the gutter. Ascertaining the roof’s size and the pitch is the next step to ensuring the correct components are selected.

Once the gutter size has been selected according to the size and pitch of the roof, the number of downpipes will be a simple calculation. 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.

The gutter support spacing should not normally exceed 900mm. Roofs with a pitch over 35°, with smooth surfaces or which are geographically located such that they may be subject to heavy snow loading, are the exception. In these instances the support spacings should not exceed 600mm (snow boards are also then recommended, to avoid the risk of sliding snow). It should be remembered that there was a time, before roof insulation regulations, when snow pretty much melted on contact with domestic roofs due to high heat loss. Nowadays, snow can build up to a critical point and, if the roof is steep and smooth, a large amount of snow can slide off in one go.

Finally the all important overall aesthetic of the building should be considered. A correctly specified system should complement and enhance the building, not create a distraction or appear to be mis-placed. 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.

Rainwater products manufactured in plastic are immensely appealing to the installer because they are lighter, making them much easier and quicker to install, especially when working at height. They are also overcoming the challenge of aesthetics, providing a cost – effective option over traditional materials. Innovations in manufacturing techniques mean that the subtle detailing of a real cast iron rainwater system can be sourced at a fraction of the cost.

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 are highly efficient but suitable for smaller buildings.  For larger roofs a deeper system can offer an increase of up to 50% in some cases for flow capacity, potentially reducing the number of downpipe and drainage runs required, speeding up installation and reducing overall costs.

A correctly specified and installed rainwater system will stand the test of time, ensuring that the installer is not called back to rectify a fault as a result of wrongly sized or too few components. It should not be an after thought to the roof construction itself. By following these simple steps, the contractor 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.