By The Roofing Brief Team. Last reviewed: October 2026.
Torch down roofing costs about $3.50 to $7.00 per square foot installed, or $4,400 to $15,000 for a typical flat roof, and the rolls run $100 to $200 each for material, per HomeGuide. Also called torch-on roofing, it is modified bitumen membrane that a crew melts onto the deck with a propane torch, fusing the seams into one waterproof sheet. A two-layer system is expected to last about 15 years and a three-layer system 20 to 25 years, per This Old House (updated June 2026), and the building code sets the minimum design slope at 1/4:12.
Decision table: is torch down roofing right for your roof?
| If… | Then | Threshold or rule | Source |
|---|---|---|---|
| The roof slope is under 1/4:12 (2%) | Fix drainage first (tapered insulation or reframing) | Modified bitumen needs a design slope of at least 1/4 unit in 12 | IBC 1507.11.1; IRC R905.11.1 |
| The contractor cannot show CERTA training and a written fire-watch plan | Do not allow open-flame work | CERTA requires a minimum 2-hour fire watch after torches shut down | NRCA CERTA manual |
| The building is occupied or the deck is combustible and you want no flame | Self-adhered or cold-applied SBS | Self-adhering SBS $9.25 to $14.50 per sq ft vs APP $12.50 to $16.25 in TOH’s comparison | This Old House (via Solvista Roofing) |
| You are in a hot, high-sun climate | APP cap sheet | APP membranes meet ASTM D6222 or D6223 | IBC 1507.11.2 list; GAF data sheet |
| You are in a cold climate or the deck moves | SBS cap sheet | SBS membranes meet ASTM D6162, D6163 or D6164 | IBC 1507.11.2 list; IKO |
| You plan to own the building about 15 years | 2-layer system | $110 to $160 per square in materials; about 15 years | HomeGuide; This Old House |
| You want 20 to 25 years | 3-layer system with granulated cap | $170 to $550 per square in materials; lasts 20% to 30% longer | HomeGuide |
What is torch-down roofing?
Torch-down roofing is 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 waterproof layer for flat and low-slope roofs; the IBC and IRC both set a 1/4:12 minimum design slope for modified bitumen, while asphalt shingles need at least 2:12 under IRC R905.2.2.
The membrane is asphalt reinforced with a polyester or fiberglass mat and blended with a polymer, either APP (a plastic) or SBS (a rubber), per IKO. “Torch-down” describes the application method, not a separate material: the IBC lists the qualifying membranes by standard (ASTM D6162, D6163, D6164, D6222, D6223, D6298 or D6509). 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. IKO describes the 3-layer system as a base sheet, a smooth cap sheet and a granule cap sheet, while the 2-layer version stops at the smooth cap. HomeGuide prices granulated cap sheets for 3-layer roofs at $40 to $90 per roll on their own.
How much does a torch-down roof cost?
Torch-down roofing costs $3.50 to $7.00 per square foot installed, or $350 to $700 per roofing square (100 square feet), per HomeGuide, with a typical flat-roof job landing between $4,400 and $15,000. This Old House’s Homewyse-based figures run higher at the top end: $4,901 to $13,494 for a 1,000 sq ft roof and $9,662 to $26,752 for 2,000 sq ft. 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. HomeGuide puts labor at $160 to $400 per square against $100 to $200 per square for materials, so installed labor often equals or exceeds the material cost.
Torch-down membrane is sold by the roll, and a standard roll covers about one roofing square. GAF, for example, lists its Ruberoid EnergyCap Torch Granule FR cap sheet at 1 square per roll (106.7 gross square feet, about 106 pounds) on its product data sheet. Side and end laps eat part of that gross area, so plan on roughly one roll per square for each ply, plus waste. A 2-layer roof needs a base sheet roll and a cap sheet roll for every square.
| System | Materials per square | Installed per sq ft | Expected lifespan | Source |
|---|---|---|---|---|
| 2-layer torch-down | $110 to $160 | Low end of $3.50 to $7.00 | 15 years | HomeGuide; This Old House |
| 3-layer torch-down | $170 to $550 | High end of $3.50 to $7.00 | 20 to 25 years | HomeGuide; This Old House |
| Installed, per roll (1 square) | $100 to $200 material | $350 to $700 per roll | N/A | HomeGuide |
Add-ons push the number up: a flat roof tear-off runs $1 to $2 per square foot per HomeGuide’s flat roof guide (its torch-down guide puts removal at $90 to $100 per square), permits run $150 to $500, and rotted decking, 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.
Torch down roofing price per roll, by product
| Product | Polymer / standard | Roll area | Roll weight | Thickness | Source |
|---|---|---|---|---|---|
| GAF Ruberoid EnergyCap Torch Granule FR | APP, ASTM D6222 Type I Grade G | 106.7 gross sq ft (1 square) | 106 lb | 0.160 in (4 mm) | GAF data sheet |
| Johns Manville DynaWeld Cap FR | SBS | 95.8 sq ft coverage | 106 lb | See sheet | JM RS-4387 |
| IKO Torchflex TP-250-Cap | SBS | 86 sq ft (78 sq ft net coverage) | See sheet | 4.0 mm (158 mil) | IKO data sheet |
Worked example for a 1,000 sq ft roof (10 squares) with an assumed 10% waste allowance: each ply needs 1,100 sq ft of net coverage. With a 1-square roll such as the GAF cap, that is 11 rolls per ply; with IKO’s 78 sq ft net coverage it is 1,100 / 78 = 14.1, so 15 rolls. At HomeGuide’s $100 to $200 per roll, 11 rolls cost $1,100 to $2,200 per ply before labor, which is why the roll’s net coverage, not its gross size, belongs on the quote.
How long does torch-down roofing last?
Torch-down roofing lasts 15 to 25 years, and the number of plies is the biggest factor. This Old House puts a two-layer system at about 15 years and a three-layer system at 20 to 25 years, and HomeGuide says 3-layer roofs last 20% to 30% longer on average. HomeGuide’s flat roof guide gives modified bitumen in general 10 to 20 years, and manufacturer guarantees run longer on multi-ply systems: Johns Manville offers up to 30 years on DynaWeld Cap FR in most 2 to 5 ply JM SBS systems.
Real lifespan depends on drainage, sun exposure and maintenance more than the brochure number. Standing water is the main killer, which is why IKO warns that cover boards must be laid flat to prevent ponding and the code sets a 1/4:12 minimum slope. Roofs that drain cleanly and get their laps and flashings checked each year 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 matters; IKO notes the ceramic coated granules on Torchflex TP-250-Cap are there to protect against ultraviolet radiation. 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. IKO recommends installing in dry conditions because water can be trapped between layers. The sequence below follows IKO’s system description and NRCA’s CERTA safety rules.
- Prep the deck. Tear off the old roof if needed ($1 to $2 per sq ft per HomeGuide), replace any rotted decking, and confirm a design slope of at least 1/4:12 to the drains.
- Install insulation and cover board. Fasten with screws and plates or fully adhere, then lay the cover board flat so it does not create ponding spots, per IKO.
- Fasten the base sheet. Mechanically fasten or torch a base sheet as the first ply. This base sheet is the torch down underlayment: the plies above bond to it, not to bare decking. IBC 1507.11.2.1 also permits a base sheet meeting ASTM D1970 or ASTM D4601 under a modified bitumen cap.
- Stage fire protection. CERTA requires at least two 4A60BC fire extinguishers within 10 feet of torch work, kept no closer than 10 feet to any propane cylinder.
- Torch the cap sheet. Unroll each cap sheet, heat the underside until the asphalt flows, then roll it flat so bitumen bleeds out at the seam, which shows a full weld. IKO’s TP-250-Cap carries a 3.5 inch selvage for the side lap.
- Flash penetrations and walls. Flash curbs, pipes and wall junctions as the field membrane goes down.
- Fire watch. After all torches shut down, CERTA requires a minimum 2-hour fire watch, checking the underside for smoldering where possible as well as the top side.
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. IKO advises hiring a crew trained under NRCA’s Certified Roofing Torch Applicator (CERTA) program, and NRCA requires authorized CERTA trainers to renew every 3 years.
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 a plastic, stiffer and almost always torch-applied, and APP sheets meet ASTM D6222 or D6223. SBS (styrene-butadiene-styrene) is a rubber with a lower melting point per IKO, flexible in cold, and it can be torched, self-adhered, cold-processed or mopped; SBS sheets meet ASTM D6162, D6163 or D6164.
| 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 |
| ASTM standard | D6222, D6223 | D6162, D6163, D6164 |
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 This Old House’s material comparison (sourced to Solvista Roofing), self-adhering SBS ran $9.25 to $14.50 per square foot, standard SBS $11.00 to $15.00 and APP $12.50 to $16.25, so going flame-free does not have to cost more.
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. HomeGuide’s November 2025 flat roof guide puts the alternatives side by side.
| System | Cost per sq ft (installed) | Typical lifespan | Install method | Source |
|---|---|---|---|---|
| Torch-down mod-bit | $3.50 to $7.00 | 15 to 25 yrs | Propane torch weld | HomeGuide; This Old House |
| Modified bitumen (all methods) | $4.00 to $7.50 | 10 to 20 yrs | Torch, self-adhered, cold or mopped | HomeGuide flat roof guide |
| TPO membrane | $4 to $10 | 10 to 25 yrs | Hot-air welded seams | HomeGuide flat roof guide |
| EPDM rubber | $4 to $10 | 15 to 20 yrs | Adhered or mechanically attached | HomeGuide flat roof guide |
| PVC membrane | $5 to $12 | 20 to 30 yrs | Hot-air welded seams | HomeGuide flat roof guide |
| Built-up (BUR) | $3.50 to $7.50 | 20 to 30 yrs | Hot-mopped asphalt plies | HomeGuide flat roof guide |
Torch-down tends to win where the owner wants a rugged, foot-traffic-friendly surface and has a CERTA-trained crew. Where fire risk, occupancy or insurance rules make an open torch a problem, a hot-air-welded TPO or PVC ($4 to $12 per sq ft per HomeGuide) or a self-adhered SBS removes the flame.
Pros and cons of torch-down roofing
Torch-down roofing trades an involved, flame-based install for a jointless, walkable flat roof. It resists water intrusion well because it is heat-welded into one sheet, and granulated cap sheets about 4 mm thick (0.160 in on the GAF Ruberoid cap, 158 mil on IKO’s TP-250-Cap) hold up to foot traffic better than slick single-ply membranes.
- Pros: jointless heat-welded waterproofing, multi-ply build of 2 or 3 sheets, walkable granulated surface, SBS flexibility in cold, and an installed price of $3.50 to $7.00 per sq ft that undercuts TPO’s $4 to $10 range at the top end.
- Cons: open-flame fire risk that requires a 2-hour fire watch under CERTA, fumes and odor, not a DIY project, a 10 to 20 year general mod-bit lifespan per HomeGuide, and vulnerability to ponding water and puncture from hail or falling branches.
The minimum slope matters. Modified bitumen needs a design slope of at least 1/4:12 (2%) under IBC 1507.11.1 and IRC R905.11.1 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 fits a low-slope residential or small commercial roof at 1/4:12 or steeper when you want a walkable surface for $3.50 to $7.00 per square foot and can hire a CERTA-trained crew. It is a poor choice for dead-flat roofs that pond, and for any building where open-flame work is banned or uninsurable.
For an occupied building, ask the contractor for their CERTA cards and written fire-watch plan (at least 2 hours after the last torch shuts down), whether a hot-work permit and inspection are required in your jurisdiction, and whether a self-adhered SBS or a hot-air-welded single-ply would remove the flame risk. Get those answers in writing before you sign.
Frequently asked questions
What is torch-down roofing?
Torch-down roofing is a flat-roof system made of modified bitumen membrane (APP or SBS) 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 built for low-slope roofs of at least 1/4:12.
How much does torch-down roofing cost?
Torch-down roofing costs about $3.50 to $7.00 per square foot installed, or $350 to $700 per 100-square-foot roofing square, per HomeGuide. A typical flat-roof job runs $4,400 to $15,000. Rolls cost $100 to $200 each for material, and tear-off adds $1 to $2 per square foot.
How long does torch-down roofing last?
Torch-down roofing lasts 15 to 25 years. This Old House puts a two-layer system at about 15 years and a three-layer system with a granulated cap sheet at 20 to 25 years. Good drainage at 1/4:12 or more and annual lap and flashing checks 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 usually finishes with a granulated cap sheet about 4 mm thick (0.160 in on GAF’s Ruberoid EnergyCap Torch Granule FR), which grips better than slick single-ply membranes like TPO or PVC. Still, limit foot traffic to inspection and maintenance to avoid wearing the granules or puncturing the membrane.
What is the difference between APP and SBS modified bitumen?
APP modified bitumen uses a plastic polymer that is rigid and almost always torch-applied, and it meets ASTM D6222 or D6223. SBS uses a rubber polymer that stays flexible in cold and can be torched, cold-adhered, self-adhered or mopped, and it meets ASTM D6162, D6163 or D6164. SBS suits cold or flame-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, NRCA’s CERTA program requires at least two 4A60BC extinguishers within 10 feet and a 2-hour fire watch after torching, and an improperly heated seam fails to bond or scorches the sheet. Hire a CERTA-trained crew.
What roof slope does torch-down roofing need?
Modified bitumen, including torch-down, needs a design slope of at least 1/4 unit vertical in 12 horizontal (2%) under IBC 1507.11.1 and IRC R905.11.1. On a dead-flat roof the membrane will pond, degrade and fail early, so tapered insulation or reframing should fix the drainage before the membrane goes down.