Rain gutter

A rain gutter or surface water collection channel is a component of water discharge system for a building. Water from a pitched roof flows down into a valley gutter, a parapet gutter or an eaves gutter. An eaves gutter is also known as an eavestrough (especially in Canada), eaves channel, dripster, guttering or simply as a gutter. The word gutter derives from Latin gutta(noun), meaning "a drop, spot or mark".

Guttering in its earliest form consisted of lined wooden or stone troughs. Lead was a popular liner and is still used in pitched valley gutters. Many materials have been used to make guttering: cast iron, asbestos cement, UPVC (PVCu), cast and extruded aluminium.

Gutters prevent water ingress into the fabric of the building by channelling the rainwater away from the exterior of the walls and their foundations. Water running down the walls causes dampness in the affected rooms and provides a favourable environment for growth of mould, and wet rot in timber.

A rain gutter may be a:

A roof must be designed with a suitable fall to allow the rainwater to discharge. The water drains into a gutter that is fed into a downpipe. A flat roof will have a watertight surface with a fall of 1 in 60, or 1 in the case of lead. They can drain internally or to an eaves gutter, which has a minimum 1 in 360 fall towards the downpipe. The pitch of a pitched roof is determined by the construction material of the covering. For slate this will be at 25%, for machine made tiles it will be 35%. Water falls towards a parapet gutter, a valley gutter or an eaves gutter. When two pitched roofs meet at an angle, they also form a pitched valley gutter: the join is sealed with valley flashing. Parapet gutters and valley gutters discharge into internal rainwater pipes or directly into external down pipes at the end of the run.

A parapet gutter at the base of a sloping roof and a parapet wall, outflowing to a downpipe

A simpler parapet gutter.

A valley gutter between two parallel roof surfaces.

The capacity of the gutter is a significant design consideration. The area of the roof is calculated (metres) and this is multiplied by rainfall (litres/sec/metres²) which is assumed to be 0.0208. This gives a required discharge outfall capacity. (litres/sec) . Rainfall intensity, the amount of water likely to generated in a two-minute rainstorm is more important that average rainfall, the British Standards Institute notes that an indicative storm in Essex, (annual rainfall 500mm per annum) delivers 0.022 l/s/m²- while one in Cumbria (annual rainfall 1800mm per annum) delivers 0.014 l/s/m².

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