Root barriers are often used in green roofs to provide some protection to the waterproofing from invasive stolons, rhizomes and from woody roots from trees and shrubs. The most common root barriers used are thin polyethylene sheets, laid over the waterproofing membrane. These may not be required if the waterproof membrane is certified as root-resistant. Thicker, welded root barriers will be needed for green roofs involving trees, bamboo or other vigorous, spreading grasses.
It is important to check the compatibility of the product with bitumen and polystyrene, especially where there is direct contact with a bitumen-based waterproofing or polystyrene insulation. The root barrier must also be resistant to the humic acids produced when plants decompose.
Separation sheets are sometimes installed between the waterproofing and root barrier to provide additional protection and separate materials that are not compatible. These are typically HDPE sheets laid directly on the waterproofing.
Protection mats or boards are used to protect the waterproof membrane from damage following installation. The most common materials used are water-permeable, hard wearing and dense synthetic fibres, polyester and polypropylene.
Protection matting is installed directly on the waterproofing layer (for root-resistant membranes) or atop the installed root barrier layer, providing further (uncertified) protection against root penetration and doubling as a separation sheet. The protection matting may provide some noise-absorbing capability. It can add to water retention on the roof, although the amount varies (from 3 l/m2 to 12 l/ m2) and is only really useful on slopes below 15 degrees. A range of different products are available, reflecting the required functions, and apart from the material itself vary according to thickness (3 mm to 20 mm), fibre density and mass (320 g/m2 to 1500 g/m2).
Protection mat sheeting should be installed with overlaps of 100 mm and some products have rubber backing that requires gluing in situ. The product should extend 150 mm above the finished surface of all upstands (such as vent pipes, chimneys and other roof penetrations) to ensure complete roof protection.