7.2 Pitched roofs

7.2.22 Drainage

Roof drainage shall adequately carry rainwater to a suitable outlet.

Drainage should be:

  • provided where roofs are greater than 6m2; however, consideration should be given to providing drainage to smaller roofs such as dormer, porch roofs and balconies (see Clause 7.1.4)
  • of a sufficient size to accommodate normal rainfall, and sized to cope with concentrated flows, ie, where there are dormer roofs
  • designed and fitted to prevent erosion of the lower surface, where water from a large roof surface discharges onto another surface
  • fixed in accordance with the design, using the correct type of fittings for internal and external angles, outlets etc to ensure efficient drainage of the roof
  • supported and jointed in accordance with the manufacturer’s recommendations
  • insulated when passing through a home, in accordance with Part 8.0 ‘Internal services and low or zero carbon technologies’
  • installed ensuring gutters are provided with stop ends, and are laid with a sufficient fall towards the outlet, unless designed to be flat.

The discharge of rainwater in gutters and pipes from one roof to another should be avoided, where practicable. Small quantities of rainwater discharge, from one roof to another, are acceptable, if the total quantity discharged does not exceed the normal drainage characteristics of the roof below.

Where gutters are behind parapet walls, an overflow should be provided:

  • sized for effective flow rate and positioned to prevent water from entering the building
  • of higher capacity than the combined capacity of the other outlets
  • positioned to discharge safely away from the building
  • be visible when in operation.

Where a downpipe discharges above ground level, or above a drainage gully, the downpipe should be fitted with shoes.

Last updated: 2nd January 2024

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