The Thermal and Rainwater Runoff Performance of an Extensive ...

Although less versatile than intensive green roofs, they are cheaper in terms of constructionand maintenance due to the reduced substrate depth (typically less than 200 mm) [3].3.5 Green Roof ComponentsThe four main components of a green roof system (the root barrier, drainage layer, substrateand vegetation layer) are discussed in more detail as follows.3.51 Root BarrierThe root barrier is an essential part of a green roof system because it prevents the roots ofvigorous plant species penetrating through to the waterproof membrane. The type of rootbarrier used will depend upon the system and the type of plants used and is therefore usuallyspecified by the green roof supplier or waterproofing supplier.3.52 Drainage LayerThe purpose of the drainage layer is, in combination with the substrate, to control the waterretention properties of the green roof [3]. The key benefits of the drainage layer include:• Reduction of water ponding on the roof. This is particularly important on flat roofconstructions as prolonged water ponding can result in structural fatigue.• Providing the correct drainage conditions to allow desired vegetation to survive andflourish. If a green roof is too free draining then plants may die back during dryperiods. Conversely, if the drainage is insufficient then waterlogging may occurwhich can cause plants to rot.• The control of the amount and timing of rainwater runoff. This contributes to themanagement of stormwater runoff and helps to meet planning and regulatoryconstraints.Niall Carroll Page 25The Thermal and Rainwater Runoff Performance of an Extensive Green Roof System