How Torch-On (SBS Modified Bitumen) Roofing Works
A base sheet, a granulated cap sheet and a propane torch. What each layer does, where it fails, and why it is professional work.
- Professional
- 1 to 3 days for a typical small flat roof
- Updated
The short answer
Torch-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.
Torch-on roofing is a modified bitumen system. Bitumen is the asphalt that has waterproofed roofs for well over a century. Modified means polymers have been mixed in, usually SBS (styrene-butadiene-styrene rubber) or APP (atactic polypropylene plastic), so the asphalt stays flexible in the cold and does not run in the heat. The polymers are carried on a reinforcement mat of polyester or fibreglass, and the whole sheet is sold in rolls about 1 metre wide.
The membrane goes on in two plies. A base sheet is fastened or adhered to the deck, and a cap sheet with a mineral granule surface is torched over it. The flame softens the asphalt on the underside of the roll so it flows into the layer below and cools into one fused, waterproof membrane. Laps are welded the same way, and a small bead of melted asphalt, called bleed-out, squeezes out of every lap as the visible sign that the weld took.
Torch-on is used on flat and low slope roofs, decks over living space, balconies, parapet roofs and commercial buildings. Its strength is toughness and repairability. Its weakness is that the quality depends entirely on the person holding the torch, and the fire risk is real. This guide explains how the system is built. Each step is covered in its own guide, and the order matters.
Tools
- Propane torch with hose and regulator
- Hook blade utility knife
- Torch-on trowel or lap trowel
- Chalk line and tape measure
- Roofing hammer and broom
- Class ABC fire extinguishers, at least two on the roof
Materials
- Modified bitumen base sheet (nailed, self-adhered or torch-applied type)
- SBS or APP granulated cap sheet
- Asphalt primer for concrete or metal
- Cant strips and cover board where the system needs them
- Roofing nails and plates for the base layer
- Flashing membrane and metal edge flashings
Safety first
- Open flame is the hazard. Torch work needs trained operators, fire extinguishers on the roof within reach of every torch, and a planned fire watch after the last flame is out. The RCABC Roofing Practices Manual calls for a fire watch of at least one hour after all torches are out, and NFPA 241 and the CERTA torch safety program call for at least two hours. Follow the strictest requirement that applies to the job, including the insurer's.
- Never point a torch at wood, a cant strip that is not noncombustible, a gap in the deck, a vent or an eave where flame can reach the structure. Use self-adhered membranes or heat-welding methods in those areas.
- Propane cylinders stay upright, secured and out of the sun, with hoses and fittings inspected before every use.
- WorkSafeBC fall protection rules apply at 3 metres (about 10 feet) and wherever a fall of any height could cause serious injury. Parapets, skylights and roof hatches are all fall hazards on a flat roof.
Step by Step
Step 1: Confirm the deck, slope and drainage
A torch-on roof needs a firm, dry, smooth deck and a slope that drains. The common target is at least 1/4 inch per foot (2 percent) toward drains or scuppers, and many manufacturers warn that water left standing for more than 48 hours after rain shortens membrane life. Flat is a description, not a spec. See how to plan flat roof drainage.
Wet plywood traps moisture under the membrane and causes blisters later. Decks are tested and dried before work, and the full checklist is in how to prep a deck for torch-on.
Step 2: Protect combustibles and plan the fire safety
Before any torching, walk the roof edge and look for places flame can get into the structure: gaps between deck boards, open soffit vents, penetrations, parapet cavities and cant strips made of wood or fibre. Those areas get a self-adhered membrane, a heat-welding method or a noncombustible shield instead of direct flame.
Fire extinguishers are placed on the roof, torches and cylinders are set up, and someone is assigned to the fire watch that follows the work. This is part of the job, as much as the membrane is.
Watch out: Most torch-on fires start hours after the crew leaves, from heat trapped in a wall or roof cavity. The fire watch is what catches them.
Step 3: Install the base sheet
The base sheet is the first ply. On wood decks it is commonly nailed with a cap-headed fastener or applied as a self-adhered sheet, so the torch is not heating the plywood. Over concrete or noncombustible cover board it can be torched. Laps are typically 3 inches on the sides and 6 inches at the ends, staggered, but confirm on the manufacturer's instructions.
See how to install a torch-on base sheet for the layout and the three attachment methods.
Step 4: Flash the walls, curbs and drains before the cap
Flashings at parapets, drains and penetrations are built in two plies as well. The first ply of flashing goes up the wall over a cant strip before the field cap sheet, and the second ply goes over the field. That lapping order is what sheds water. The details are in parapet flashing, drains and scuppers and penetrations.
Step 5: Torch the cap sheet over the base
The cap sheet is aligned dry first, rolled back and torched as it is unrolled. The operator heats the roll and the base together until the asphalt becomes a glossy, flowing surface, then presses the roll down. The goal is full adhesion with no voids, and a bleed-out of about 1/4 inch at the laps. Overheating burns the polymer and thins the sheet. Underheating leaves the cap unbonded.
The technique is covered in how to torch a cap sheet, and the lap details in side and end laps.
Step 6: Seal laps, terminate edges and check the work
After the cap is down, every side lap and end lap is run with a trowel while the asphalt is still soft. Any open edge is resealed. Terminations at walls are secured with a termination bar or covered by counter flashing or metal coping.
Probing the laps with a blunt tool once the membrane has cooled shows any open seams. They are fixed before the crew leaves, because a lap that is open in October is a leak in November.
Step 7: Keep a fire watch after the last flame is out
The last step of torching is not finishing the last roll. It is a fire watch. The RCABC practice manual describes a responsible person checking the work area for at least one hour after all torches are out, and industry programs such as CERTA call for two hours with attention to the underside of the roof and flashings. Check current requirements and your insurer's conditions.
The watcher looks at edges, penetrations, parapet tops and any cavity that could hold heat. If anything smoulders, there is water or an extinguisher within reach.
SBS Versus APP
SBS is rubberized. It stretches and recovers, handles cold and building movement well, and melts at a lower temperature, so it is the usual choice for cold climates and for the Fraser Valley. APP is plasticized, stiffer and more heat stable, and is common in hotter regions and in some commercial applications. The two are not mixed in one system unless the manufacturer says so. Confirm the type and its torching temperature on the product data sheet.
Where Torch-On Fits
- Flat and low slope roofs over houses, garages and additions, where shingles cannot be used.
- Decks and balconies over living space, usually under a walking surface or pavers.
- Parapet roofs and commercial buildings, where a thick, repairable membrane handles foot traffic from mechanical service.
- Not for wood-framed details you cannot shield, such as tight eaves or open soffits, where self-adhered or single-ply systems are the safer choice.
Fraser Valley Notes
Long wet winters limit the number of days a deck is dry enough to torch. Moisture on the deck, dew on the base sheet or frost on a concrete slab stops the bond from forming and leaves blisters. Ponding and debris under fir and cedar trees age a membrane faster than sun does, so keep drains and scuppers clear.
Common Mistakes
- Torching directly onto wood or into gaps. It starts hidden fires that can smoulder for hours. Use shielded, self-adhered or noncombustible details at the edges.
- Skipping the fire watch. The fire is often found after the crew has left for the day.
- Torching a wet or dirty deck. Trapped moisture becomes blisters and delamination within a season or two.
- Overheating the cap sheet. Burnt polymer and thin spots reduce the sheet's flexibility and lifespan, and the granules fall off.
- Underheating the laps. An open lap with no bleed-out looks fine on the day and leaks in the first rain.
- Ignoring drainage. A flat roof with standing water ages faster.
When to Call a Professional
Torch-on is not a homeowner method. It needs trained operators, insurance that covers hot work, fire extinguishers and a fire watch. If a roofer is proposing torch-on, ask how they shield combustibles, who does the fire watch and for how long, and what happens at the eaves and penetrations.
If you have a torch-on roof with a leak, bubbling or open seams, see how to repair a torch-on roof. Small patches are sometimes done with cold-applied products, but flame repairs are for a trained crew.
Fraser Roofing does free roof inspections across the Fraser Valley, from Hope to Abbotsford. See our Flat Roofing and Torch On service or the towns we cover.
How Torch-On (SBS) Roofing Works: Questions
What is the difference between SBS and APP modified bitumen?
SBS modified bitumen has rubber polymers that let it stretch and recover, so it stays flexible in cold weather and tolerates building movement. APP has plastic polymers that make it stiffer and more heat stable. SBS is the usual choice in cool, wet climates like coastal British Columbia. The two should not be mixed in one system unless the manufacturer allows it.
How long does torch-on roofing last?
A well built two-ply SBS roof on a dry, sloped deck commonly lasts about 20 to 30 years. The range depends on drainage, exposure, foot traffic and maintenance. Standing water, blocked drains, open laps and heavy moss or debris cut years off. Small splits and lifted edges repaired early keep a torch-on roof serviceable much longer.
Why is torch-on roofing considered dangerous?
Because an open propane flame is used close to wood decks, eaves, vents and gaps, and heat can travel into a wall or roof cavity and smoulder for hours. Risks are managed by shielding combustibles, using self-adhered or heat-welded methods in sensitive areas, keeping extinguishers on the roof and doing a fire watch after the last torch is out.
Can you torch-on over an old roof?
Sometimes, but only where the substrate allows it. A torch cannot be applied directly over wood, old felt, foam or a wet roof without protection, and old layers that trap moisture cause blisters. Most jobs strip to the deck, repair it and add a cover board or a mechanically fastened base sheet. Follow the manufacturer's instructions for the specific product.
Is torch-on better than EPDM for a flat roof?
It depends on the roof. Torch-on is thicker, resists foot traffic and punctures, and is easy to patch, but it needs trained crews and fire precautions. EPDM is installed without flame, is quick on simple roofs and stays flexible in the cold, but its seams and flashings are the weak point. Consider traffic, details, budget and maintenance.
Related Guides
All Torch-On Roofing guides|The full How-To Library
This guide is general education. Always follow the product manufacturer's installation instructions, the BC Building Code and WorkSafeBC requirements, which take priority over anything written here.
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