System Specification - Gutters

Kayflow guttering systems are suitable for all projects, from domestic and  light commercial properties to buildings with large roof areas. 

This guide sets out the basic methods for calculating the rainwater run-off. This run-off figure (in litres per second) can then be matched to the appropriate gutter performance in order to work out which system should be installed.

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As well as normal levels of rainfall, a domestic or light commercial eaves gutter system must be able to cope with an intense rainfall event, lasting at least two minutes, that could happen once a year anywhere in the UK. The British Standard BS12056 shows how to work out the amount of rainwater (in litres per second) that could run off a roof.

Rainwater runoff = rainfall intensity (litres per second per m²) X the effective roof area (m²)

  1. For the rainfall intensity value either:
    a. Look up the rainfall intensity for a major annual storm event in the area of the UK where the building is located (as shown in BS EN 12056-3:2000)
    b. Or calculations can be based on 0.021 litres per second per m² of roof area. This is nearly the highest intensity shown in the BS Standard and so should cover most areas.
  2. Rainwater-Runoff-Calculation.jpgFor the effective roof area one of two formulas may be used in reference with Fig.1.
    a. (W+H/2) x L
    W = width of one pitch of the roof
    H = vertical height from the eaves to the top of the pitch.
    L = the length of the roof on one side of the building.
    b. L x W x Pitch factor (see table opposite)

A roof has dimensions L = 2m, H = 3m and W = 4m.

  • The rainfall intensity is 0.021 l/s/m2
  • The Effective Roof Area is (4+1.5) x 2 = 11 m2
  • So the Rainwater runoff is 0.021 x 11 = 0.231 litres per second

See Gutter Performance tab to verify that at this flow rate the Half Round gutter would easily cope with the volume of water.

Kayflow systems have different flow rates and capabilities.  The following flow rates are for installations with an end outlet and laid level gutter (up to 3mm fall per metre).

The roof area to be drained should always be compared with the maximum area that a gutter type is able to drain. If the figure is too low the options are to select a system with a higher capacity or increase the fall on the gutter. A third option is to move the outlet point to a central position to effectively double the gutter drainage capacity.

TypeHeightWidthFlow RateArea
Round* 50mm 114mm 0.9 l/s 43m²
Square 57mm 117mm 1.6 l/s 76m²
Deepflow* 70mm 114mm 1.8 l/s 86m²
Ogee* 70/80mm 120mm 2.2 l/s 105m²
Superdeep170 108mm 170mm 4.3l/s 205m²

All flow rates are for end outlet with the gutter laid level and have been independently established through physical testing to BS12056-3:2000.
*Standard and Cast Effect versions have the same performance characteristics.

Kayflow rainwater systems carry:

  • Kitemark KM508760, BS EN 607:2004 (Eaves Gutters & Fittings),
  • BS EN 12200-1:2000 (Plastic Piping Systems)
  • BS EN 1462:2004 (Brackets for Eaves Gutters)
  • Manufactured under ISO 9001 Quality Management System.


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