Built-up Felt
Built-up felt roofing is one of the most established flat roof waterproofing methods in the UK, offering a proven, multi-layer approach to weatherproofing that has been trusted on commercial and residential buildings for decades. When correctly specified, a built-up felt system delivers robust, long-term performance and thermal insulation is central to achieving that. Its multi-layer bituminous construction is inherently elastomeric, meaning the system can accommodate structural movement and minor settlement without compromising waterproofing integrity
Gradient's PIR insulation boards are designed to work seamlessly within built-up felt build-ups, providing the high thermal performance and dimensional stability required to meet current building regulations and support a durable, watertight roof assembly.
Why insulation matters in a built-up felt system
In a warm roof build-up, insulation sits above the structural deck and below the waterproofing layers. Choosing the right board is critical: it must withstand foot traffic during installation, resist compressive loads, maintain dimensional stability, and provide a flat, consistent surface for the felt to adhere to.
Poor insulation specification is one of the most common causes of premature felt failure. Boards that compress, distort, or expand under temperature fluctuation can compromise the integrity of the waterproofing membrane above, leading to blistering, cracking, or delamination. Gradient's products are engineered to eliminate these risks.
Key benefits of Gradient insulation for built-up felt
High compressive strength — Gradient boards offer compressive strength of ≥150 kPa, ensuring they retain their shape and surface profile under the weight of multiple felt layers, ballast, and maintenance traffic.
Dimensional stability — Minimal thermal movement keeps the felt membrane flat and free from stress, reducing the risk of splits or separation at joints.
Compatibility with cold-applied and hot-melt adhesives — Gradient boards are compatible with the adhesive systems commonly used in BUF applications, supporting reliable, bonded installations.
Tapered options available — Where falls to drainage outlets are required, Gradient's bespoke tapered insulation service allows the drainage gradient to be built into the insulation layer itself, eliminating screed and keeping the overall build-up slim.
Suitable products for application:
Gradient's PIR insulation boards are designed to work in step with these demands. Powerdeck F is the ideal specification for built-up felt applications, offering high compressive strength that stands up to pedestrian maintenance traffic without risk of deformation. Available across a wide range of thicknesses, it supports a variety of roof build-ups — giving specifiers the flexibility to meet thermal performance targets while keeping the overall construction depth in check.
Powerdeck F
Powerdeck® F is compatible with multiple waterproofing systems, benefiting from a compressive strength of >150 kPa and dimensional stability. This ensures the flat surface finish required for...