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Swish Building Products - Rainwater / Roofline Systems & Ventilation

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.

Roofline Systems & Ventilation

The Swish roofline range features all the necessary components for eaves and gable assemblies, including fascias, bargeboards, eaves soffits, as well as over fascia ventilation and protection. Matching PVC mouldings cater for all joints, corners and abutments.


All fascia profiles are available in a wide range of sizes and feature a groove for the engagement of soffit boards by simple push-fit. They can also be used in ‘open eaves’ situations where there is no soffit.

Fixing Methods

Jumbo and 16mm boards do not require a supporting backboard. They provide support at the eaves for the roof covering at all roof pitches.

Boards must be fixed to rafter ends at a maximum of 600mm centres using 2 x 65mm Trimtop nails.

Fix gutter brackets directly to Jumbo and 16mm boards using A4 stainless steel screws which should penetrate the inner face of the board.

Coloured Roofline Profiles

16mm square fascia boards are available in Mahogany, Light Oak, Irish Oak, Rosewood, Black, Anthracite Grey, Cream and White woodgrain effect finishes. Anthracite, Anthracite Grey, Hazy Grey and Light Grey profiles with a self-colour skin are also available.

A range of matching ventilated soffits, flat board soffits, corners, trims and fixings are also available.

Fixing centres for coloured profiles must not exceed 400mm.


Swish roofline systems allow ventilation of the roof void in accordance with Building Regulations for England, Wales, Scotland and Northern Ireland. Swish ventilators meet the recommendations of the 1989 edition of BS 5250:1975.

Through Soffit Ventilation

Ventilated soffit boards (Polo) are available in widths from 150mm to 605mm and feature a single or double row of slots for a 10mm or 25mm continuous air path respectively. A range of PVC strip ventilators in 10mm and 25mm air gap configuration can be used in conjunction with Swish Gee Pee™ or other soffit boards.

Over Fascia Ventilator

Over Fascia Ventilator C504 allows a continuous 10mm air path to be incorporated into new build applications in order to retain a clean appearance to the soffit line. Over Fascia Ventilator C505 allows a continuous 25mm air path.

Over Fascia Protection

Manufactured in 2.5m lengths from 100% recycled PVC, Jumbotec (C502) is designed for eaves protection. In refurbishment applications, it supports the felt behind the fascia and eliminates problems arising from deterioration of traditional roofing felts. A Vented Eaves Protector (C404) is available where both eaves protection and ventilation are required in one unit. A separate Bird Comb Attachment (C405) may also be incorporated into the assembly to stop small birds and larger insects entering under the eaves tiles.

Click the PDF images to view the technical information in full

Roofline Systems & Ventilation

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

Email Swish

Tel: 01827 317200
Fax: 01827 317201

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