The pipe used to convey the surface water collected from the roof to the drain at ground level. In most cases this pipe is run down the outside of the building; normally, no joining medium is used and the spigot simply enters the socket. If the rainwater pipe is run internally, the joints must be made watertight. As with eaves guttering, it is essential to allow for the expansion of the material — otherwise buckling or cracking of the pipes will result. For the sake of appearance, it is essential to ensure that the rainwater pipes are fixed perfectly vertical; for this, it is best to use a plumb bob. The rainwater pipe may terminate at the lower level in a rainwater shoe, discharging into a gully, or, preferably, may he run directly into a back inlet gully. The size of the rainwater pipe should be at least that of the gutter outlet, and where a pipe serves more than one gutter, it should be as large as the combined areas of both outlets.
Rainwater connections to discharge stacks
In some areas with a combined system of drainage, the local authority will permit the eaves and waste water discharge stacks to receive rainwater from roofs. Designing a system in this way can save on the cost of installing separate rainwater pipes. To avoid excessive air pressure fluctuations within the discharge stack, this method of rainwater disposal is not to be recommended for buildings storeys in height, or for the removal of rainwater from roof areas exceeding 4Om per stack. The main disadvantage of designing above ground drainage in this way is the problem of flooding if a blockage occurs in the discharge stack. The rainwater pipe should he connected to the stack via a branch connection; this ensures a free unrestricted flow of air through the ventilating pipe.