What is a blue roof and how does it work?
- Read time: 3 minutes
- Date: 13 Jun 2023
- Living Roof
What is a blue roof?
Unlike conventional roofs which allow water to drain off freely, blue roofs temporarily store water during heavy rainfall and storms.
Blue roofs form part of a sustainable drainage system (SuDS) to allow water to disperse into sewers, waterways and river systems at a controlled rate. They are used to slow down the release of water in urban environments to prevent flooding.
How do blue roofs work?
Blue roofs are typically flat and do not have any fall (pitch or slope) that would allow rainwater to run off freely. Instead, rainfall is retained in a special void at either roof or podium level. Water flows freely to the drainage outlets which are fitted with control devices that manage the rate at which the water is drained.
How is the roof deck of a blue roof constructed?
The void for attenuated (stored) water is situated above the waterproofing element. The capacity of the void is carefully calculated in line with the loadbearing structure of the roof. It typically covers the entire area of the roof and has a depth of less than 100mm.
For a warm blue roof, the compressive strength of insulation must exceed the loads for the weight of a fully saturated blue roof including safety factors. XPS insulation with BBA certification for the application must be used for an inverted roof.
Apart from the rainwater outlets or emergency overflows that are required for drainage functionality, the roof design should minimise or eliminate penetrations in the attenuation void.
Can I use any outlet for a blue roof?
Choice of outlet is critical. It should be fitted with a flow rate restrictor and designed to withstand high water pressures. Once fitted, the connection between the spigot and the downpipe must be sealed and secure.
How do I stop water from contaminating insulation?
The outlet and any penetrations should be locally isolated to prevent water from contaminating the insulation. This is achieved by forming a secondary seal between the vapour barrier and underlayer 500mm back from the outlet or penetration.
Should I use a leaf guard on a blue roof?
A leaf guard should always be used to prevent surface debris such as leaves from blocking or restricting the water flow if they are drawn into the outlet.
