COP v3.0:roof-drainage; gutters

5.5 Gutters 

External gutters must be installed with the back lower than the fascia board or cladding.

External and internal gutters must have a cross-sectional area in accordance with Cross-sectional Area of Gutter, and sumps, rainwater heads and downpipes must not restrict the flow from the gutter.

For design purposes, gutters are assumed to be level. It is not recommended to obtain fall by tapering, as it reduces the cross-sectional area of the gutter.

With the limited fall available it is not always possible to ensure that all internal or external gutters will remain dry without ponding and so, to avoid premature corrosion, consideration should be given to using non-ferrous metals. See 5.5.8 Fall.

Unpainted galvanised steel is not guaranteed for spouting and gutters.

Because dirt retains moisture and causes corrosion, ponding voids warranties.

All AZ coated steel spouting and gutters must be maintained to prevent ponding due to the collection of debris or dirt as required in 16 Maintenance .

Where their renewal within 15 years would be difficult, AZ-coated or pre-painted steel must not be used for internal gutters.

All gutters are subject to expansion and, therefore, there is a maximum recommended length before an expansion joint is needed. The maximum length is determined by the metal, its thickness and colour. It is similar to the limit recommendations for roof cladding but should not exceed 12 m. See 7.3.2 Roof Cladding Expansion Provisions.
N.B. A sump or spouting angle provides sufficient movement for expansion.

Where a spouting or gutter can move freely and independently the increase in length should be according to the specifications in A Sliding Washers .

Sliding washers and the spouting or gutter should be of a cross-sectional dimension capable of resisting expansion forces. Copper or aluminium spouting or guttering that has been softened by brazing is not suitable for an extended length.

Outlets are only required at twice the length module because an expansion joint can be either a sump, rainwater head or a saddle flashing. See Gutter Capacity.