Torch-Applied Modified Bitumen

Torch-applied modified bitumen, often called torch-on, is a low-slope roofing method in which rolls of polymer-modified bitumen membrane are bonded by heating their underside with a propane torch until the bitumen melts, then rolling them into place. It is common in Europe, Canada, the Middle East and many other regions, typically as a two-layer system of base sheet and cap sheet.

Technique

Canadian guidance describes holding the flame 50 to 150 mm (2 to 6 in) in front of the roll at the point where it meets the substrate, moving the torch steadily across the full roll width, and unrolling once a small bead of melted bitumen appears. On field sheets a roll puller lets the installer walk backward so they never step on hot membrane. Signs of overheating include melted surface film, sinking granules, bitumen burning away to expose the reinforcement, and waves in the laid sheet.

Laps and staggering

• Side laps of at least 75 mm (3 in) and end laps of at least 150 mm (6 in) for both base and cap sheets.
• Cap sheet side laps offset at least 300 mm (12 in) and end laps at least 450 mm (18 in) from base sheet laps.
• A visible bleed-out at side laps of no more than about 6 mm (1/4 in).
• Granules at granular end laps embedded by heating and pressing with a hot trowel.

Flashings

Base flashings extend at least 100 mm (4 in) onto the deck and at least 200 mm (8 in) up walls, and cap flashings at least 150 mm (6 in) onto the deck. Reinforcing gussets are used at inside and outside corners.

Safety

Torch flames can exceed 1093 °C (2000 °F). Canadian guidance restricts torching to trained workers, requires a fire extinguisher on hand, and warns against torching near combustible materials. Many insurers and jurisdictions require torch safety training and a fire watch after work ends, and some restrict torch application on certain buildings. Where open flame is not acceptable, the same membranes are available for cold-adhesive or self-adhered installation, or can be set in hot-mopped asphalt.

Weather

Torching slows in cold, humid conditions, and rolls should be kept warm and dry before use.