Unit 318 Session 1 Rainwater drainage

​Design and installation of advanced rainwater systems

​Design and installation of advanced rainwater systems

​It is so important to design and install an effective rainwater system on properties.
There are many environmental issues that need to be considered and if the system is not suitable the integrity of the property could be affected.

For larger buildings and constructions there are companies that specialise in rainwater systems that allow large roof areas to remove large volumes of water effectively and efficiently.

Open Ended

What do you think is the main consideration when designing rainwater systems for commercial premises or larger buildings?

​We must ensure the system can manage to volume of rain water that will be captured by the roof, in commercial premises or larger buildings this can be an issue.

Many domestic rainwater systems in the UK rely on soakaways to manage rain water however this may not be adequate for commercial premises or larger buildings.

​A rainwater soakaway is essentially a hole in the ground that collects rainwater from driveways, patios and roof gutters etc. As the hole fills up, the water gradually seeps through the soil beneath and into the water table.

​The purpose of a soakaway is to temporarily store rainwater that could otherwise lead to surface flooding or the overwhelming of sewer systems.

Domestic soakaways are usually found under flower beds, lawns or driveways and patios.

​When carrying out the survey of the property, it would be wise to establish the type of drainage for the premises for the property and plan your system around whether it is a combined, separate or partially separate system and whether the local authority has any preference for connections.

If a partially separate system is installed, check for any soakaways on existing remote downpipes.

​Calculating the size of a gutter

To assess the suitability of a gutter system to drain the roof on a building efficiently, the following factors need to be taken in to account:


1. The effective roof area to be drained
2. The local rainfall intensity
3. The flow characteristics of the gutter system
4. The number and position of downpipes.

​BS EN 12056-3 measures rainfall intensity in litres per second per metre squared (l/s/m2) for a two-minute storm event.

The standard also has maps that give details of the rainfall intensity in areas around the country

​The size of the roof directly relates to the size of the guttering required, so if the size of the roof increases so does the guttering and discharge pipework.

This principle also applies to the angle of the roof as the angle affects the flow of rainwater and therefore the speed at which the rainwater enters the gutter.

The rainwater system must therefore be accurately designed to adequately manage the predicted rainfall.

​The angle of the roof is a vital consideration when designing a gutter system.
The steeper the roof angle the faster the rain will flow off the roof, the shallower the roof incline the slower the rain will flow off the roof.

BS EN 12056-3 gives the following formula to work out the effective roof area.

​Please note this will only give the area of one side of the roof.

Fill in the Blanks

​The next stage is to include the factor for the correct roof pitch which can be selected from Table 4.14.

​In an alternative calculation offered by Building Regulations, the length and span are multiplied then multiplied again by a factor for a given roof pitch:

Width × length × (design factor for a given roof angle)

​If the angle of the roof is known then we can calculate using the factor for the pitch as shown in Table 4.14.

​Calculating gutter size

In the calculating roof area using alternative formula example, 36 m2 was calculated as the effective area of the roof which had an angle of 45°, so by using Table 4.15, a gutter size of 100 mm diameter is suitable.

A half round gutter can be chosen along with a 63 mm diameter downpipe