Drainage

Sites for green roofs should be assessed for drainage. Check whether the site has primary and/or secondary drainage systems. 

Primary roof drainage systems may use:

  • box gutters (for near-flat roofs) or eaves gutters (for pitched roofs)
  • simple waterspouts (also known as scuppers)
  • outlets or box drains built into the roof

    Design of drainage systems must be matched to the roof construction and catchment area

    Design of drainage systems must be matched to the roof construction and catchment area

These are collector drains that are designed to flow when only partly full. Primary drainage systems are not designed to remove all of the water that falls on a roof during exceptionally heavy rain. A green roof may require a separately plumbed secondary drainage system, also known as the overflow relief system. For flat or nearly flat roofs, primary drains are located at the lowest point of the roof: flow of water into them is promoted by positive drainage.

Secondary (overflow) drains are located at a higher point on the roof. These are designed to operate in a worst-case scenario where the primary drains are completely blocked and water builds up on the roof due to a torrential downpour of rain and/or a failure of the irrigation system to shut off. Overflow drains remove accumulated water to a depth that the roof can carry without becoming unstable, and ensure that the roof weight loading capacity is not exceeded. For roofs with a very low parapet, overflow drainage may be achieved simply by flow over the roof edges, if accumulation of water to this height fits within the roof’s design weight loading. The need for overflow relief will be established by looking at existing performance of the drainage in conjunction with the historical data on rainfall intensity.

Removal of water from any roof surface is assisted by some degree of pitch or slope. Even roofs that look flat have a gentle fall to promote movement of water into the roof drains, to prevent ponding. ‘Ponding’ refers to water that remains on a roof for extended periods after the end of the most recent rain event. Recurrent ponding can cause lasting downward deflection of the roof structure, which over time may reduce the efficiency of drainage and cause the roof to become unstable. A pitch of at least two per cent reduces the risk of ponding, and a steeper pitch means the roof will drain more quickly. Strengthening the roof construction to reduce deflection may be needed.

Accumulation of water on a roof can cause downward deflection that reduces the efficiency of drainage systems.

Accumulation of water on a roof can cause downward deflection that reduces the efficiency of drainage systems.

When assessing the site and planning for the design of a drainage system, consider:

  • the amount of rainfall that lands directly on the site, and any that drains onto it from adjacent roofs or walls
  • length of rainfall event – estimated from historical records and forecasts of future extreme rainfall events under a warmer climate
  • the speed at which rainfall will collect at the drains (determined primarily by roof pitch)
  • the planned capacity of drains, including the drain dimensions and diameter of gutters and drainpipes