Last edited 03 Jan 2018

Rainwater downpipe

A rainwater downpipe is a pipe that is used to direct rainwater away from a building, typically from roof guttering to a drainage system. It is sometimes also referred to as a downspout, drain spout, roof drain pipe or leader.

The Building Regulation’s part H, Drainage and water disposal, requires that adequate provision is made for rainwater to be carried from the roof of buildings. Approved document H suggests that to achieve this, roofs should be designed with a suitable fall towards either a surface water collection channel or gutter that surface water to vertical rainwater downpipes, which in turn connect the discharge to the drainage system.

A downpipe is typically vertical and extends to ground level. They are most commonly found attached to the corners of a building. The discharge from a downpipe can be:

  • Directly connected to a drain discharging into a soakaway.
  • Directly connected to a drain discharging into a surface water sewer.
  • Indirectly connected to a drain via a trapped gully if the drain discharges into a combined sewer.

Traditionally, rainwater downpipes were made using cast iron, although uPVC systems are increasingly common due to ease of installation and low maintenance requirements. As long as adequate size, strength and durability can be assured, they can also be made of aluminium alloy, galvanised steel, stainless steel, and so on.

Downpipes have a tendency to become blocked when debris such as leaves and twigs accumulate. To prevent this, they should be properly maintained, and are usually attached to a building using brackets which can be easily removed for cleaning if necessary.

The size and number of rainwater downpipes required will depend on the intensity of rainfall that is likely and the area of the surface to be drained. A number of online calculators are available to help determine the required size and number of rainwater downpipes.

Rainwater downpipes are most commonly round in section, but may be any shape. They typically have a diameter ranging from 50 - 150 mm, but any size can be used. They may include access panels for inspection or rodding, branch connectors, bends, hoppers and leaf guards at the top, diverters, rainwater harvesting systems, angled shoes at the bottom, and so on.

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