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Swish Building Products - Rainwater Systems

Rainwater Run-off - Rainfall Intensity - Flow Capacity - Ventilation - Fascias - Soffit

Rainwater Systems

Swish rainwater systems are designed for application in most domestic properties throughout the UK. In addition, Swish manufactures soil and underground systems.

Specification & Installation

The following information (see table in pdf) is given to help specifiers determine the basic requirements of a gutter system. Swish Technical Services can provide further guidance as required on Tel: 01827 317238.

Rainwater Run-off

The rainwater run-off from a roof during a storm event can be calculated by multiplying the rainwater intensity by the effective roof area.

Rainfall Intensity - 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 year. It is sensible to design a system for a minimum intensity of 75mm/hour per m² or a flow rate of 0.021 litres per second.

Effective Roof Area - The area of roof (m²) that drains into any one gutter can be calculated in two ways (see diagram):

  • (H/2) + W x L
  • L x W x Pitch Factor

A selection of pitch factors is shown below (see pdf). For other pitch factors please contact Swish Technical Services.

Flow Capacity

Capacities shown above for Swish gutter systems have been independently assessed on a flow rig in accordance with BS EN 12056:2000. They are based on a ‘short’ run of gutter (i.e. 50 x height of water the gutter is capable of containing), the gutter being set straight and level (i.e. a fall of up to 3mm/m) with a storm event running at 75mm per hour per m² or 0.021 litres per second. Each capacity has been reduced by a ‘safety factor’ of 10%.

System Layout

The roof area to be drained should be compared with the maximum area that a gutter type is able to drain. If the figure is too low the designer may:

  • Select a system with a higher capacity
  • Increase the fall on the gutter: with a long 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 significantly increase the gutter drainage capacity
  • Reduce the flow rate by 15% if the run has a corner in it
  • Require a dedicated outlet at the corner for larger valleys

Where the roof discharges water rapidly or where low friction roof coverings are employed, a wider gutter may be required to avoid overshooting the gutter.

Click the PDF image to view the technical information in full

Roofline Systems & Ventilation

Request Swish Info

Swish Building Products
Pioneer House
Lichfield Road Industrial Estate
B79 7TF

Email Swish

Tel: 01827 317200
Fax: 01827 317201

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