The most common material for residential flat roofs are built up reinforced bitumen membranes (RBM). These sheet materials are based on a carrier which is normally of varying strengths of polyester and coated in hot bitumen so that they fuse together to form a single waterproof layer. Two or three layers of sheet materials are rolled out over the roof.
This waterproofing is always supported by a structural roof deck of some type of timber boarding which in turn is supported on joists. The ceiling, if any, is usually fixed directly to the underside of the joists.
In almost all cases where RBM has been installed, the surface of the waterproofing will need protection, both from ultraviolet light and from fire from external sources with the Building Regulations laying down the degree of protection required in the case of fire. This can be achieved by adding a layer of mineral chippings bedded in compound to the surface of the roof, or alternatively by using a mineral surfaced layer of RBM as the top layer, or ‘Capsheet’. Liquid applied and Single layer systems have this protection built in and do not require additional protection.
If inspection reveals the following, re-roofing is probably inevitable:
- Roof sagging between joists (or puddles getting larger) – this is a sign of condensation or roofing overloading;
- Membrane split in the line of deck boards which is another sign of movement in the deck due to condensation or structural problems;
- Membrane split round perimeter is indicative of excessive movement between walls and roof;
- Roof blisters getting bigger and more widespread which is a sign of leakage and/or condensation.
Most RBM roofs need some protection. If the only access onto the roof is for inspection, use either:
- Stone chippings (12+mm):these should be bonded to the waterproofing in a gritting compound, not bitumen.
- Mineral finish where small slate flakes are bonded to the felt during manufacture. Various colours are
- Reflective paints which although effective when first applied, these soon wear and discolour and need re-painting every 3 to 4 years.