Roof Felting Materials, Components and Specifications
Core material families
Candidates must know the main waterproofing families used in roof felting: bitumen-based membranes (oxidised and polymer-modified, e.g. SBS and APP), reinforcement carriers (polyester, glass fibre, composite), and self-adhesive versus torch-applied versus hot-bitumen fixing methods. Understand the role of cap sheets versus underlays, vapour control layers, and torch-on versus cold-applied systems.
Performance concepts
Know what tensile strength, elongation, low-temperature flexibility, heat resistance, and tear resistance mean and why they matter for a given roof. Recognise that polyester carriers give high elongation and tear resistance, while glass fibre gives dimensional stability and heat resistance. Specification questions often hinge on matching membrane to substrate movement, UV exposure and drainage.
Traps to watch
- Assuming 'thicker equals better' - carrier type and polymer modifier often matter more than mass alone.
- Confusing cap sheet (UV-exposed top layer) with underlay (protected lower layer).
- Mixing SBS (elastomeric, flexible at low temperature) with APP (plastomeric, better heat resistance).
- Ignoring compatibility between primer, adhesive and membrane.
Worked rule of thumb
When a question describes a roof with high thermal movement and cold winters, favour SBS-modified polyester-reinforced membranes; when it describes high solar gain and hot service temperatures, favour APP or glass-fibre-reinforced systems. Match the modifier to the climate, the carrier to the movement.