Torch-on Felt
Thermal insulation for torch-on bituminous waterproofing systems
Torch-on felt is one of the most widely used waterproofing systems on UK flat roofs, valued for its robust, seamless finish and straightforward application. The system involves heating bituminous felt sheets with a propane torch, fusing each layer directly to the substrate or the layer below to create a continuous, watertight membrane. When correctly specified with the right insulation, a torch-on felt roof delivers excellent long-term durability with minimal maintenance.
Gradient's PIR insulation boards are engineered to perform within torch-on felt build-ups, providing the compressive strength, dimensional stability, and surface consistency that the system demands — while meeting the thermal performance requirements of current UK building regulations.
Why insulation specification matters
In a warm roof assembly, the insulation board sits directly beneath the waterproofing membrane and must withstand the heat generated during torch application without softening, shifting, or releasing gases that could affect adhesion. A board that performs poorly under these conditions risks compromising the bond between membrane layers — leading to delamination, blistering, or moisture ingress over time.
Key benefits of Gradient insulation for torch-on felt
High compressive strength — With compressive strength of ≥150 kPa, Gradient boards resist deformation under foot traffic during installation and throughout the life of the roof.
Consistent surface finish — Close manufacturing tolerances ensure a flat, uniform substrate, supporting even adhesion of the bituminous membrane across the entire roof area.
Dimensional stability — Low thermal movement prevents the insulation from expanding or contracting in a way that stresses the felt layers above, helping to maintain waterproofing integrity over the long term.
Wide range of thicknesses — Gradient boards are available in multiple thicknesses, giving specifiers the flexibility to achieve the required U-value within the constraints of the build-up depth.
Tapered insulation options — Where roof drainage is a concern, Gradient's bespoke tapered insulation service integrates the required falls directly into the insulation layer, eliminating the need for screed and simplifying the overall build-up.
Suitable products for application:
Gradient's Powerdeck U is specifically designed for warm flat roofs with bituminous waterproofing systems, making it a natural fit for torch-on felt applications. Its glass tissue facers and high compressive strength provide the mechanical performance and surface quality needed for a reliable, long-lasting installation.
For roofs subject to higher loads or frequent maintenance access, Topcover overlay boards can be laid over the insulation to provide additional protection against point loads before the felt is applied.
Powerdeck U
Powerdeck® U is a rigid PIR insulation board for use in warm flat roofs with bituminous torch-on felt waterproofing systems. Its stability, compressive strength of >150 kPa as noted in our Technical...