Reinforced bitumen sheets for roofing are classified by BS 747: 2000 according to their base fibre and primary function. Classes are subdivided into types according to their surface finish and their intended use within roofing. Class 1 bitumen sheets, manufactured from organic fibres incorporating a layer of jute hessian, are only recommended for use under tiles and slates; they may be aluminium faced on the underside to provide a heat-reflecting surface. However, where the risk of condensation is high, sarking sheets, which are waterproof but vapour permeable, may be appropriate.
Only Class 3 and Class 5 felts are tear-resistant and durable and therefore appropriate for use in built-up roofing systems (Table 6.1). Class 3 sheets have a glass-fibre base. The Type 3B sheets have a fine surface finish and are suitable for the lower layers of a built-up roofing system, while Type 3E has a granular finish for the exposed layer. Type 3G sheets are perforated for the first layer where partial bonding and/or venting is required. Class 5 sheets have a high performance polyester base. Type 5U sheets are suitable as the underlayer for a built-up roofing system, while Type 5B sheets form the lower layers and the exposed layer, if additional surface treatment is to be applied. Type 5E sheets are suitable for the exposed finish layer including flashings and vertical surfaces. Class 4 (Type 4A) reinforced bitumen sheets incorporating jute fibre are sheathing felts used as underlay for mastic asphalt roofing and flooring on a timber base. However, on solid substrates such as concrete, glass fibre tissue, which does not compress under load, is the more appropriate separating layer.
Built-up roofing systems using the Class 3 glass-fibre based products are standard, but the Class 5 polyester-based products have greater strength and durability at a higher initial cost. A typical three-layer system is illustrated in Figure 6.2. All oxidised bitumen sheets, when exposed to ultraviolet light and ozone, gradually age harden and become less resistant to fatigue failure.
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