Torch-down roofing is a flat-roof system built from modified bitumen membrane that a crew melts onto the deck with a propane torch, fusing the seams into one waterproof sheet. It runs about $3.50 to $9.00 per square foot installed ($350 to $900 per roofing square), lasts 15 to 25 years depending on how many layers go down, and is meant for low-slope roofs between roughly 0.5:12 and 2:12 pitch. This guide breaks down what it is, what it costs, how long it lasts, and exactly how it goes on.
What is torch-down roofing?
Torch-down roofing is a type of modified bitumen membrane installed by heating the underside of the roll with an open propane flame until the asphalt melts, then pressing it onto the roof so it bonds and the overlaps weld shut. The result is a continuous, jointless waterproof layer built for flat and low-slope roofs where shingles would leak.
The membrane itself is asphalt reinforced with a polyester or fiberglass mat and blended with polymers that make it flexible and heat-weldable. “Torch-down” describes the application method, not a separate material. It is one way to install a modified bitumen roof, alongside cold-adhesive, self-adhered, and hot-mopped methods.
Most torch-down jobs use two or three plies: a base sheet mechanically fastened or torched to the deck, and one or two cap sheets torched on top. The top cap usually carries embedded mineral granules that shield the asphalt from ultraviolet light and give the surface grip.
How much does a torch-down roof cost?
Torch-down roofing costs roughly $3.50 to $9.00 per square foot installed, or about $350 to $900 per roofing square (100 square feet). A typical flat-roof job lands between $4,400 and $15,000, driven by roof size, layer count, tear-off, and regional labor rates. Two-layer systems sit at the lower end and three-layer systems at the upper end.
Labor is the biggest single line item because torch application demands a trained crew and fire-safety measures, not just material. Material alone runs about $1.50 to $3.00 per square foot; installed labor often equals or exceeds the material cost.
| System | Cost per sq ft (installed) | Cost per square (100 sq ft) | Expected lifespan |
|---|---|---|---|
| 2-layer torch-down | $3.50 – $6.50 | $350 – $650 | 15 years |
| 3-layer torch-down | $4.00 – $9.00 | $400 – $900 | 20 – 25 years |
Add-ons push the number up: tearing off an old roof runs $1 to $2 per square foot, replacing rotted decking adds $2 to $3 per square foot for the affected area, and new flashing, edge metal, or roof drains are billed separately. For how these line items appear on a bid, see how to compare roof repair quotes.
How long does torch-down roofing last?
Torch-down roofing lasts 15 to 25 years, and the number of plies is the biggest factor. A two-layer system typically reaches about 15 years, while a three-layer system with a proper granulated cap sheet commonly reaches 20 to 25 years. Single-ply modified bitumen without a cap sheet may only last 10 to 15 years.
Real lifespan depends on drainage, sun exposure, and maintenance more than the brochure number. Standing water is the main killer: flat roofs with low spots let water pond, which degrades the asphalt and shortens service life. Roofs that drain cleanly and get resealed every few years reach the top of the range.
Ultraviolet exposure is the slow failure mode. The asphalt eventually dries out and cracks from UV and thermal cycling, which is why the granulated cap sheet or a reflective coating matters. For how mod-bit compares against other flat-roof materials over time, see the low-slope roof systems overview.
How is torch-down roofing installed?
Torch-down roofing is installed by rolling out modified bitumen sheets, heating the underside with a handheld propane torch until the asphalt liquefies, then pressing the membrane down so it bonds to the layer below and the seams fuse. A crew works in overlapping courses across the roof, building two or three plies.
- Prep the deck. Tear off the old roof if needed, replace any rotted decking, and confirm the surface is clean, dry, and properly sloped to the drains.
- Fasten the base sheet. Mechanically fasten or torch a base sheet to the deck as the first ply and fire barrier.
- Install flashing and details. Flash penetrations, curbs, and wall junctions before or as the field membrane goes down.
- Torch the cap sheet. Unroll each cap sheet, heat the underside until the asphalt shines and flows, then roll it flat so bitumen squeezes out at the seam, which confirms a full weld.
- Fuse the seams. Overlap each course several inches and reheat the laps so every seam becomes one continuous waterproof bond.
- Fire watch. Keep extinguishers and a fire-watch crew on site during and after torching, since smoldering can start hours later.
This is not a do-it-yourself job. The open flame carries real fire risk, and an improperly heated seam either fails to bond or scorches the membrane. Hire a crew that torches flat roofs regularly rather than a generalist.
APP vs SBS: which modified bitumen for torch-down?
Torch-down membranes come in two polymer blends, APP and SBS, and the difference is temperature behavior. APP (atactic polypropylene) is stiffer, has a higher softening point, and resists UV well once cured, which suits hot, sunny climates and is almost always torch-applied. SBS (styrene-butadiene-styrene) is more flexible and handles cold and building movement better.
| Trait | APP modified bitumen | SBS modified bitumen |
|---|---|---|
| Polymer | Atactic polypropylene (plastic) | Styrene-butadiene-styrene (rubber) |
| Flexibility | Rigid, higher softening point | Flexible, better in cold |
| UV resistance | Excellent once cured | Good, usually needs granule cap |
| Best climate | Hot, high-sun regions | Cold or wide temperature swings |
| Application | Almost always torch-applied | Torch, cold-adhesive, self-adhered, or mopped |
The practical takeaway: APP is the classic torch-down membrane, while SBS gives you the option to skip the flame entirely with a cold-adhered or peel-and-stick install. In cold or fire-sensitive settings, self-adhered SBS often makes more sense than torching, though it may cost more per square foot.
Torch-down vs other flat-roof systems
Torch-down competes mainly with single-ply membranes like TPO and EPDM. Its edge is a tough, multi-ply, walkable surface with granule protection; its drawbacks are the open-flame install and a lifespan on the shorter side of the flat-roof range. Single-ply membranes install without a flame and can outlast mod-bit, but scuff and puncture more easily.
| System | Cost per sq ft (installed) | Typical lifespan | Install method |
|---|---|---|---|
| Torch-down mod-bit | $3.50 – $9.00 | 15 – 25 yrs | Propane torch weld |
| TPO membrane | $4.50 – $9.50 | 20 – 30 yrs | Hot-air welded seams |
| EPDM rubber | $4.00 – $8.50 | 20 – 25 yrs | Adhered or mechanically attached |
| Built-up (BUR) | $4.50 – $9.00 | 15 – 30 yrs | Hot-mopped asphalt plies |
Torch-down tends to win where the owner wants a rugged, foot-traffic-friendly surface and has a contractor experienced with flame work. Where fire risk, occupancy, or insurance rules make an open torch a problem, a hot-air-welded or adhered single-ply is often the better call.
Pros and cons of torch-down roofing
Torch-down roofing trades an involved, flame-based install for a durable, jointless, walkable flat roof. It resists water intrusion well because it is heat-welded into one sheet, and the granulated surface holds up to foot traffic better than slick single-ply membranes.
- Pros: jointless heat-welded waterproofing, multi-ply durability, walkable granulated surface, flexible in temperature swings, and lower material cost than many single-ply systems.
- Cons: open-flame fire risk during install, fumes and odor, not a DIY project, shorter lifespan than premium single-ply, and vulnerable to ponding water and puncture from hail or falling branches.
The minimum slope matters. Torch-down needs some pitch, roughly 0.5:12 to 2:12, so water sheds to the drains. On a dead-flat roof the membrane will pond and fail early, which is why drainage design is as important as the membrane choice.
Is torch-down roofing a good choice?
Torch-down roofing is a solid choice for a low-slope residential or small commercial roof when you want a tough, budget-friendly, walkable surface and can hire a crew experienced with torch work. It is a poor choice for steep roofs, dead-flat roofs that pond, and any situation where open-flame work is banned or uninsurable.
For an occupied building, ask the contractor about their fire-watch protocol, whether a permit and inspection are required for hot work in your jurisdiction, and whether a cold-adhered SBS or a hot-air-welded single-ply would remove the flame risk for a small premium. Those questions vary by state and building type and are worth settling before you sign.
Frequently asked questions
What is torch-down roofing?
Torch-down roofing is a flat-roof system made of modified bitumen membrane that a crew melts onto the deck with a propane torch. Heating the underside of the roll liquefies the asphalt so the sheet bonds to the roof and the overlapping seams fuse into one continuous waterproof layer. It is designed for low-slope roofs where shingles would leak.
How much does torch-down roofing cost?
Torch-down roofing costs about $3.50 to $9.00 per square foot installed, or $350 to $900 per 100-square-foot roofing square. A typical flat-roof job runs $4,400 to $15,000. Two-layer systems sit at the low end and three-layer systems at the high end, with tear-off and decking repair adding to the total.
How long does torch-down roofing last?
Torch-down roofing lasts 15 to 25 years. A two-layer system typically reaches about 15 years and a three-layer system with a granulated cap sheet commonly reaches 20 to 25 years. Good drainage, UV protection, and periodic resealing push it toward the top of the range, while standing water shortens it.
Can you walk on a torch-down roof?
Yes. Torch-down roofing has a granulated surface that grips well even when wet, making it safer to walk on than slick single-ply membranes like TPO or PVC, and it resists scuffing. Still, limit foot traffic to inspection and maintenance to avoid wearing the granules or puncturing the membrane over time.
What is the difference between APP and SBS modified bitumen?
APP modified bitumen uses a plastic polymer that is rigid, has a high softening point, and resists UV well, making it the classic torch-applied membrane for hot climates. SBS uses a rubber polymer that stays flexible in cold and can be torched, cold-adhered, self-adhered, or mopped. SBS suits cold or fire-sensitive jobs; APP suits hot, sunny ones.
Can you install torch-down roofing yourself?
Torch-down roofing is not a DIY project. The install uses an open propane flame to weld the membrane, which carries a real fire risk and can smolder hours later, and an improperly heated seam fails to bond or scorches the sheet. Hire a crew that torches flat roofs regularly and follows fire-watch protocols.
What roof slope does torch-down roofing need?
Torch-down roofing is made for low-slope roofs, roughly 0.5:12 to 2:12 pitch. It needs enough slope to shed water to the drains but is not appropriate for steep roofs. On a dead-flat roof the membrane will pond, degrade, and fail early, so proper drainage design is essential.
Reviewed by The Roofing Brief Team. Last reviewed July 2026.