Internal Gutter Design Features

All internal gutters must have upstands that are hooked or returned. Gutters that return under the eaves are not recommended as this design makes removal for replacement more difficult.

To prevent permanent deflection of the gutter, support for the sole of an internal gutter should be provided by either a plywood lining or by close ribbed sheets of roof cladding, separated by a layer of roofing underlay. Internal gutter support must be strong enough to support the weight of water when at capacity, and if over 300 mm wide, be able to support foot traffic.

Internal box gutters must have a minimum depth of 50 mm at their lowest point, including freeboard. A width to height ratio of 2:1 plus freeboard gives maximum flow as it minimises wet surface area for a given cross-sectional area.

A sharp direction change in flow of an internal gutter will affect discharge capacity. Where two buildings meet at an angle, each gutter must be drained separately, or a specific discharge capacity calculation must be applied.

Internal gutters should have an expansion joint at the stop-end.

Outflows from internal gutters may be scuppers or weirs.


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Scupper outlets should be avoided where possible. They are difficult to weatherproof, and they can inhibit expansion. At the outlet end, a weir overflow should discharge into a sump or rainwater head.

Revision Category: 
4 - Clause Inserted
Revision Detail: 

New clause.