In this article we will breakdown the application process of torch-on underlays. But firstly, what is a torch-on underlay? A torch-on underlay is a roofing membrane that has been manufactured to be used as part of a BUR roofing system. Its application can be used within a refurbishment or a new build application.

See the step-by-step application process below:

Step 1: Prepare the substrate.

Making sure the substrate surface is clean and tidy and free of debris.

Make sure the deck is dry, and then apply quick-drying primer.

Step 2: Application

Roll the underlay and cut to the required lengths.

Line up the membrane so that there is a 75mm side and head.

Roll the underlay again to the centre from both ends.

Step 3: Applying the underlay

Direct the flame to the underlay at the point where the underlay forms a V to the substrate. Move the torch across the width of the roll. This will release the fusible film allowing a free flow of bitumen. As the membrane is pressure rolled out, you should be able to see a bleed of bitumen at both salvage edges, a bitumen bleed of 4/6mm must be achieved. Do not to over torch the underlay, this can cause footprints and marking.

Step 4: Application of detailing

Several details can be formed using the Underlay. At detailing check for any risk of flame areas, if risk of flame is found a Self-Adhesive Underlay must be applied.

Step 5: Locate the capsheet

Capsheet must be stagged to the underlay.

Step 6: Apply the Capsheet

Follow the same instructions for applying the underlay.  

Make sure you can see a 4/6mm bleed of bitumen on both side and headlaps.

Step 7: Application of detailing:

Several details can be formed using the Capsheet. At detailing check for any risk of flame areas, if risk of flame is found a Self-Adhesive Underlay must be applied.

Below are some Wonderbuilds roofing underlays which are applied using the torch-on process:

WONDERBUILDS 2.5KG T -20C SBS FILM UNDERLAY

WONDERBUILDS 3KG T -20C FILM SBS UNDERLAY

WONDERBUILDS 2.5KG T -20C SBS SAND UNDERLAY

WONDERBUILDS 3KG T SAND SBS UNDERLAY

Below is a roofing system that includes the Wonderbuilds Self-Adhesive Vapour Control Layer (SA VB 600). This system has a 25 Year Guarantee and is safe to torch. It also includes a BROOF (t4) capsheet and underlay.

The Wonderbuilds Film/Film Underlays and Film/Sand Underlays are waterproofing membranes with glass-fibre reinforcement. They are coated with SBS modified bitumen, which makes them high-quality. What sets these underlays apart is that they have high tensile strength and low melting points. These underlays should be used within built-up roofing systems.

Click here to see the full range of Wonderbuilds underlays, along with technical data for each product.

Wonderbuilds offers a flat roofing system where the torch-on application method is used to apply the underlays. This system has 15 years guarantee.

System components:

1. 5KG Capsheet
2. 3KG Film Underlay/Sand Underlay
3. Bitumen Faced PIR Insulation
4. PU Insulation Adhesive
5. VB600 Self-Adhesive VCL
6. SA Primer

Detailing:

1. 5KG Capsheet
2. 3KG Film Underlay/Sand Underlay

In conclusion, Torch-On Underlays are used for all BUR systems. Always check the manufacturer’s guidelines for best results.