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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A scupper is formed where an internal gutter discharges horizontally through the side or end wall of a gutter through a restricted opening. If a scupper is the same dimension as the gutter, standard calculations for internal gutter sizing may be used. A scupper in the side of a gutter counts as a right-angle bend when using the Gutter Drainage Calculator. When scuppers have a restricted opening, the size of the opening, not the size of the gutter, determines the effective size of the gutter and its maximum catchment capacity. Scupper apertures are vulnerable to blockage and it is recommended that they are fitted with an overflow to alert the building inhabitants of a problem.

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Revision Category: 
2 - Editing and rearrangement
Revision Detail: 

Added clarifying text regarding scuppers.