Outlets & Overflow Weirs

Outlets should be in the bottom of the gutter and they can either be directly into a pipe, or preferably via a box to ensure optimum drainage flow from the gutter.

BS EN 12056-3 defines how to determine the correct box and pipe sizes for a particular situation. Typically the diameter of a downpipe connected directly to the sole of a rectangular gutter should be approximately 75% of the width of the gutter.

Siphonic Rainwater Systems

Where conventional gravity rainwater systems may cause obstructions inside a building because of reduced headroom below pipes under the roofs and/or position of vertical pipes, siphonic systems should be considered. Siphonic pipes are usually smaller in diameter than normal rainwater pipes and the system is driven by pressure differences which allows horizontal pipe runs to be used.

Siphonic systems have to be designed by the manufacturer to suit the roof layout with the collection area and outlet capacity balanced against pressures in the pipe system. Each pipe system has a maximum flow rate and also a minimum limit at which siphonic self cleaning action will commence.

Excess water at one outlet needs to be able to flow along the gutter to other outlets which introduces limits on smaller gutters.

The system needs to be designed so that the minimum velocity in the system at the design rainfall shall prevent deposition in the pipework and ensure rapid commencement of the siphonic effect.

The large difference between average rainfall intensity and peak design rainfall calculated to BS EN 12056 in some parts of the British Isles often requires that two siphonic systems are fitted to valley gutters. One is designed to accept the normal rainfall range and the secondary system (which can be siphonic or gravity) is normally dry except in heavy storms.

Siphonic systems can be designed and supplied by:
Fullflow Te: 0114 247 3655
Geberit UK Te: 01765 602082
Sapaflow Te: 01226 297200