Swish Rainwater systems are designed for application in most domestic properties throughout the UK. The following information is given to help specifiers determine the basic requirements

of a gutter system. Swish Technical Services can provide further information and guidance as

required. Tel: 01827 317238

 

To determine the elements of the gutter system the designer must calculate the likely quantity of rainwater run-off from the roof as follows:

 Decide on the local rainfall intensity that the system must cope with.

 Calculate the effective roof area (m2) to be drained.

To meet these requirements, the designer will need to:

Choose a gutter with sufficient flow capacity (litres per second)

 Decide on the system layout including the fall of gutter and the number and position of outlets required to maximise the flow.

 

BS EN 12056 suggests likely rainfall intensities for different areas of the UK, which may be experienced as unusual events of 2 minutes duration, once every 1, 5, 50 and 500 years. The

volume of water involved in these events increases as they get rarer, but because of their relative infrequency, it is suggested that domestic gutter systems should be designed for a storm event that is likely to occur once a year. The intensity of such an event will vary across the UK, but it is sensible to design a system for a minimum intensity of 75mm/hour per m2 or a flow rate of 0.022 litres per second.

 

Effective Roof Area The area of roof that drains into any one gutter (effective roof area : m2) can be calculated in two ways :  (H/2) + W x L  L x W x Pitch Factor A selection of pitch factors is shown below: Roof Pitch Pitch Factor 10 deg 1.088 20 deg 1.182 30 deg 1.288 40 deg 1.419 For other pitch factors please contact Swish Technical Services.

In general terms the most efficient section of gutter runs from the outlet, for a distance of 50 times the maximum height of water the gutter can hold when level. In the case of Swish Round gutter this is approximately 2.4m (ie. 48mm x 50). The flow capacities shown for Swish gutter systems have been calculated in accordance with BS EN 12056:2000 and are based on the following:

It is at once obvious from the flow capacity table that different gutter shapes have different

flow and area drainage capabilities. The roof area to be drained should be compared with the maximum area that a gutter type is able to drain (Table B: right column).

If the figure in the table is too low the designer has the following options:

 Select a system with a higher capacity

 Increase the fall on the gutter. With a longer gutter this has its limitations, as water coming

off the tiles is more likely to overshoot the gutter at the lower end. With a shallow fascia

this may not even be an option. In addition the greater volume of water in a long gutter

will tend to reduce the speed of flow.

 Move the outlet point to a central position to effectively double the gutter drainage capacity.

For further guidance please consult Swish Technical Services.

 

 If an end outlet is located around a corner flow rates should be reduced by 15%.

 Larger valleys may require a dedicated outlet at the corner.

 Where the roof is likely to discharge water rapidly eg. long slopes or where low friction

roof coverings are employed, a larger gutter may be required to avoid water overshooting

the gutter.