A rain gutter, eavestrough or surface water collection channel is a component of a water discharge system for a building. It is necessary to prevent water dripping or flowing off roofs in an uncontrolled manner for several reasons: to prevent it damaging the walls, drenching persons standing below or entering the building, and to direct the water to a suitable disposal site where it will not damage the foundations of the building. In the case of a flat roof, removal of water is clearly essential to prevent water ingress and to prevent a build-up of excessive weight.
Why do I need rain gutters?
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:
Roof integral trough along the lower edge of the roof slope which is fashioned from the roof covering and flashing materials.
Discrete trough of metal, or other material that is suspended beyond the roof edge and below the projected slope of the roof.
Wall integral structure beneath the roof edge, traditionally constructed of masonry, fashioned as the crowning element of a wall.
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.