Torch-On Roofing: How the System Goes On, and How to Do It Safely
9 step by step guides
Torch-on roofing is a two-ply modified bitumen system: a base sheet, then a granulated cap sheet, each rolled out and softened with a propane torch so the asphalt on the underside flows into the layer below. Done properly it gives a tough, seamless, repairable flat roof. It is also the roofing method with the most direct fire risk, which is why every guide in this category treats safety as part of the method, not an add-on.
Start with how torch-on roofing works to see the layers and where SBS and APP differ, then read how to prep a deck for torch-on, because most torch-on failures and nearly all torch-on fires start with the substrate. The sheets themselves are covered in the base sheet guide and how to torch a cap sheet. Laps, parapets, drains and pipes are where these roofs are won or lost, and each has its own guide.
Torch-on work is professional work. It needs trained torch operators, fire extinguishers on the roof, a fire watch after the last flame is out, and a plan for every combustible the flame could reach. These guides explain how it is done so you can understand a quote, check a roofer's method, or judge a repair. They are not an invitation to learn it on your own roof. For the torch itself, see how to use a roofing torch safely.
Torch-On Roofing Guides
- Guide 1How Torch-On (SBS) Roofing WorksTorch-on roofing is a two-ply modified bitumen system. A base sheet goes down first, then a granulated cap sheet is torched over it so the asphalt on the underside melts and fuses the layers into one membrane. Side laps are about 3 inches and end laps about 6 inches. A properly built SBS roof on a sound deck commonly lasts 20 to 30 years. It is professional work because of the open flame.Professional1 to 3 days for a typical small flat roof
- Guide 2How to Prep a Deck for Torch-On RoofingPrepping a deck for torch-on means repairing rot, confirming a slope of at least 1/4 inch per foot to drains, drying the deck, fastening a cover board or base sheet that keeps flame off wood, priming concrete or metal, sealing gaps and shielding every combustible edge. Torching stops if the surface is wet, frosty or dusty. Prep takes longer than the membrane does.ProfessionalHalf a day to 2 days before torching
- Guide 3How to Install a Torch-On Base SheetA torch-on base sheet is the first ply of a two-ply modified bitumen roof. Start at the low point, run rolls perpendicular to the slope or as the manufacturer directs, lap sides about 3 inches and ends about 6 inches, and stagger end laps. On wood decks the base is commonly nailed or self-adhered so no flame touches the wood. Torching is reserved for noncombustible substrates.ProfessionalHalf a day to 1 day for a small roof
- Guide 4How to Torch a Cap SheetTo torch a cap sheet, unroll it dry over the base, align it, reroll both ends to the middle, then heat the base and the underside of the roll together until the asphalt turns glossy and forms a small rolling bead, and press the roll into it. A bleed-out bead of roughly 1/4 inch shows at the edges. Laps are about 3 inches and 6 inches. A fire watch follows.ProfessionalHalf a day to 2 days depending on size
- Guide 5How to Torch Side and End LapsTorch-on side laps are about 3 inches wide and end laps about 6 inches, staggered at least 12 inches from the laps below and beside. Heat the top of the lower sheet and the underside of the overlapping sheet until asphalt flows, press the lap down, and look for a continuous bead of about 1/4 inch. End laps need the granules embedded or removed. Check every lap with a trowel.ProfessionalPart of the membrane installation, about 1 to 2 hours per 100 feet of lap
- Guide 6How to Flash Parapets and Upstands With Torch-OnA torch-on parapet or upstand flashing is built in two plies over a cant strip. The base flashing ply runs up the wall, commonly at least 8 inches above the finished roof, the cap flashing ply laps over the field, and the top is terminated with a bar, counter flashing or coping. Keep flame off wood, cavities and combustible sheathing, and use self-adhered sheet where needed.Professional1 to 3 hours per 50 feet of wall
- Guide 7How to Flash Roof Drains and Scuppers With Torch-OnTo flash a torch-on roof drain, set the drain bowl in a sump slightly below the roof surface, install the base ply and lap it under the clamping ring, torch a target sheet over the bowl, then lap the cap ply over it and clamp. Scuppers are lined with a formed box or membrane that runs through the wall and laps onto the roof. Keep flame off the pipe and wall.Professional1 to 3 hours per drain
- Guide 8How to Flash Pipes and Penetrations on Torch-OnTo flash a pipe on a torch-on roof, set a pre-formed sleeve or a flashing boot over the pipe on the base ply, wrap it with a membrane strip or collar that extends at least 4 inches onto the field, lap the cap ply over it, and seal the top with a clamp or sealant. Never aim the torch at combustible pipe, vent or wall material. Use self-adhered membrane or mastic there.Professional30 minutes to 2 hours per penetration
- Guide 9How to Repair a Torch-On RoofTo repair a torch-on roof, find the leak source first, usually a lap, flashing, split or puncture. Clean and dry the area, cut out blisters or splits, and cover the repair with a patch of cap sheet that extends at least 4 to 6 inches past the damage, bedded in mastic or fused with heat. Flame repairs carry a fire risk and should be done by trained roofers.Professional1 to 4 hours per repair
Also on this site
Torch-On Roofing: Common Questions
What is torch-on roofing?
Torch-on roofing is a modified bitumen roof, usually two plies of SBS sheet, where a propane torch heats the asphalt on the underside of each roll so it melts onto the layer below. The result is a continuous, fused membrane with a granulated cap sheet on top. It is used mainly on flat and low slope roofs, decks and balconies.
Is torch-on roofing dangerous?
It carries a real fire risk because an open flame is used next to roof edges, wood, vents and gaps where heat and flame can travel into the building. Trained crews manage that with shielding, extinguishers on the roof, careful flame control and a fire watch after the last torch is out. It is not a suitable do-it-yourself method.
How long does a torch-on roof last?
A well built two-ply SBS roof commonly lasts about 20 to 30 years, depending on drainage, exposure, foot traffic and maintenance. Ponding water, blocked drains, unsealed flashings and a thin or damaged granule surface shorten that. Keeping drains and scuppers clear and repairing small splits early does most to protect it.
Can torch-on roofing go over an existing roof?
Sometimes, but it depends on what is below. A torch cannot be applied directly over wood, old felt or foam without a protective layer, and trapped moisture in an old roof will cause blisters. Many roofers strip to the deck and install a cover board or mechanically fastened base sheet first. The product manufacturer's instructions decide what is allowed.
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