Modified Bitumen Flat Roofing: A Complete Guide

Modified bitumen roofing — often called torch-on felt or torch-on membrane — is a modern evolution of traditional hot-pour bitumen systems. It's widely used on commercial flat roofs across the UK and offers significantly better performance than traditional mineral felt.

What is Modified Bitumen?

Modified bitumen membranes are made from bitumen modified with either APP (Atactic Polypropylene) or SBS (Styrene-Butadiene-Styrene) polymers. These additives improve the membrane's flexibility, UV resistance, and temperature performance compared to unmodified bitumen.

APP vs SBS Modified Bitumen

APP-modified membranes are harder and more UV-resistant. They're typically torch-applied. SBS-modified membranes are softer and more flexible, especially in cold temperatures — they can be torch-applied, cold-adhesive bonded, or self-adhesive. SBS is generally preferred in the UK climate.

Installation

Torch-applied membranes are installed by heating the underside of the roll with a propane torch while unrolling it onto the substrate. The heat activates the bitumen and creates a bond. This requires trained operatives — incorrect torch technique can cause fires or inadequate bonding.

Lifespan and Performance

A quality 3-layer modified bitumen system can last 20–25 years. It's durable, resistant to foot traffic, and suitable for green roof build-ups. Its main weakness versus modern single-ply is the lap joints, which require careful installation to ensure watertightness.