What Is Torch-Down Roofing?

What torch-down roofing is, in plain words

Picture a heavy roll of roofing about the thickness of a thick rug. The roll is asphalt that has been mixed with a polymer, a kind of plastic or rubber, and pressed around a fabric mat. That product is called modified bitumen. "Modified" means the polymer was added; "bitumen" is the roofing word for asphalt.

On a torch-down job, the roofer unrolls the sheet, heats its underside with a wide propane torch until the asphalt melts, and presses it down.

As it cools, it bonds to the layer below and to the edge of the next sheet.

The seams are melted together the same way, so the finished roof acts like one waterproof skin.

The technical version: the 2021 International Residential Code groups these products under modified bitumen roofing in section R905.11 ¹. The code says nothing about the torch itself.

It only says the roof must follow that section and the maker's installation instructions. The torch is a way of applying the sheet, not a separate kind of roof.

How torch-down roofing keeps water out

A low-slope roof cannot rely on gravity and overlapping shingles the way a steep roof does. Water moves slowly and sometimes sits. So the roof has to be a continuous sheet with sealed seams, not a set of overlapping pieces.

Modified bitumen does that in layers, called plies. A base sheet goes down first, and a cap sheet goes on top. The code allows a base sheet that meets ASTM D1970, ASTM D4601 or IBC section 1507.11.2 under a modified bitumen cap sheet ¹.

Each seam overlaps the one next to it, and the melted asphalt fuses the overlap. The polymer in the asphalt lets the sheet stretch and recover as the roof heats and cools, which plain asphalt does poorly.

The ASTM standards say the sheets are "intended for use in the fabrication of multiple ply roofing and waterproofing membranes" ^{2 3}. In other words, the standard expects more than one ply.

SBS vs APP: the two main types of sheet

Modified bitumen sheets are named for the polymer mixed into the asphalt. The two you will see most are SBS and APP. The code lists both among the accepted membrane standards ¹.

Both ASTM standards say they are material specifications only. They set minimum properties for the sheet and leave roof design and application method to others ^{2 3}. Ask which standard is printed on the rolls, not just "SBS" or "APP".

Sources: [1] [2] [3]

Worked example: checking a porch roof against the slope rule

A homeowner has a porch roof that is 12 feet deep from the wall to the gutter edge. The roofer measures a 2-inch drop across that depth. Is it steep enough for modified bitumen?

Illustrative numbers. This roof would need tapered insulation or framing changes to reach the minimum before any modified bitumen goes on. Slope rules for every low-slope covering are on our low-slope roof page.

Where you will see torch-down on a house and on paper

• On the house: flat or nearly flat areas such as porch roofs, garage roofs, dormers, additions, row-house roofs and decks over living space.
• On a roofing quote: a line such as "2-ply SBS modified bitumen, torch-applied" or "APP cap sheet over base sheet". The number of plies and the application method should be written out.
• On a product data sheet: the ASTM standard number, such as D6164 or D6222, and whether the cap sheet is granule-surfaced or smooth ^{2 3}.
• On a permit: the building permit for the roof, and in many places a separate fire permit for hot work. One New Jersey borough, for example, requires a fire permit before any roof torch work ⁸.
• On an insurance or job safety form: notes on fire watch, extinguishers and who on the crew holds torch training.

Benefits of torch-down roofing

• Seams are fused, not glued, so the finished roof acts as one continuous sheet over a surface where water moves slowly.
• Two plies give a backup layer; if the cap sheet is scuffed, the base sheet still stands between water and the deck.
• The sheets are made to named ASTM standards, so you can check what is going on your roof ^{2 3}.
• Granule-surfaced cap sheets shield the asphalt from sunlight.
• No hot asphalt kettle is needed on site, unlike a built-up roof mopped in hot asphalt.

Limitations and trade-offs

• It brings an open flame onto your house. Fires can smolder under the surface after the torch is off ⁵.
• It is not allowed below ¼ inch per foot of slope under the 2021 IRC ¹. Dead-flat roofs need to be built up first.
• Torching right at flashings, edges and roof penetrations is where most torch fires start, so those spots often need a flameless product instead ⁷.
• Open flame on a wood deck is a known risk. A Canadian roofing contractors' guideline says it "should also be avoided" on wood substrates ⁷.
• Quality depends heavily on the crew. Too little heat leaves weak seams; too much can damage the sheet or start a fire.

What drives the cost of a torch-down roof

We do not publish prices for torch-down roofing until we have enough sourced, dated quotes. These are the things that move the price; our roof cost calculator and cost guides help you size the job.

• Number of plies: one ply over a base sheet costs less than a full two-ply system.
• Tear-off: the 2021 IRC bars a recover over a roof that is water-soaked, deteriorated or already has two layers ¹. Removing old layers adds labor and disposal.
• Slope fixes: tapered insulation or framing changes to reach ¼ inch per foot add material and time.
• Flameless details: self-adhered or cold-applied sheets at flashings and edges cost more than torching but lower fire risk ⁷.
• Fire safety: a hot work permit where required, a fire watch after torching and extra extinguishers all take crew time ^{4 8}.
• Access and size: a small porch roof has a high setup cost per square foot; a big garage roof spreads it out.

What a professional torch-down install looks like

This is what you should expect to see, not a how-to. Torch work is hot work under the fire code and belongs to a licensed roofer with trained torch applicators. Do not attempt it yourself.

1. The roofer pulls the building permit and, where your fire department requires one, a hot work or fire permit ⁸.
2. The crew checks the roof for fire risks: wood decks, gaps at walls and pipes, and concealed attic spaces under the work ⁵.
3. Old roofing is removed if the code bars a recover, and the deck is checked for rot and slope ¹.
4. Flashings, edges and penetrations are often done first with flameless products, carried about 3 feet into the field of the roof ⁷.
5. The base sheet and cap sheet go down following the manufacturer's instructions, which the code makes binding ¹.
6. Extinguishers stay close to the torching. The CERTA program calls for two 4A60BC-rated extinguishers within 10 feet ⁵.
7. A fire watch stays after the last torch is shut off. The model fire code text we read sets at least 30 minutes; CERTA training teaches two hours ^{4 5}.

Looking after a torch-down roof

• Keep drains, scuppers and gutters clear so water does not sit on the membrane. See roof ponding for what standing water does.
• Look from a window or ladder at the roof edge, not by walking on it, after big storms: check for lifted seams, blisters and bare spots where granules are gone.
• Keep foot traffic, furniture and grills off the surface unless the roof was built with walkway pads.
• Watch the flashings where the roof meets walls, chimneys and pipes. That is where small leaks usually start.
• Ask a roofer to inspect after any repair that involved a torch, and ask how they handled the fire watch.

Risks and warning signs

• Fire during install: the biggest risk, and the reason insurers pushed for torch training in 2003 after rising torch-related losses ⁵.
• Fire drawn into joints: flame can be pulled into gaps between materials at walls, edges and pipes ⁷.
• Blisters: raised bubbles under the cap sheet that can split and let water in.
• Open seams: edges of sheets that lift because they were under-heated or have aged.
• Ponding: standing water on a roof below the ¼-inch-per-foot minimum or with blocked drains ¹.
• Leaks at flashings: the joint between the membrane and a wall or pipe is the weakest point on most low-slope roofs.

Codes and standards that apply

These are the model codes and standards. States and cities adopt them on their own schedule and can amend them, so your building and fire departments decide what applies.

The fire code text above is the 2015 Seattle Fire Code, which is based on the IFC; section numbers and times can differ in your jurisdiction's edition. Hot work permits are set in each fire code's permit section ⁴.

Sources: [1] [4] [2] [3]

Torch-down vs other ways to apply modified bitumen

All four are the same code family, modified bitumen under R905.11 ¹. For how modified bitumen compares with other low-slope coverings such as metal or single-ply membranes, see our guide to low-slope roofs; for steep roofs, see asphalt shingles.

Sources: [1] [7]

Common misconceptions

Myth Torch-down is its own roofing material.

Reality It is modified bitumen applied with a torch. The same sheets can be glued, mopped or self-adhered ¹.

Myth A torch-down roof can go on a dead-flat roof.

Reality The 2021 IRC asks for at least ¼ inch per foot of slope for modified bitumen ¹.

Myth Once the torch is off, the fire risk is over.

Reality Fires can smolder under the surface, which is why a fire watch follows the work ^{4 5}.

Myth SBS and APP are interchangeable.

Reality They use different polymers and are made to different ASTM standards, D6164 and D6222 ^{2 3}.

Myth Any roofer with a torch can do it.

Reality Many fire departments require a permit, and the industry's CERTA program trains crews in torch safety ^{8 5 6}. Ask who on the crew is trained.

Local rules for torch-down roofing

Two sets of local rules apply. The building department enforces the roof itself, usually through its adopted edition of the IRC, sometimes with amendments. The fire department or fire marshal enforces the hot work side through its adopted fire code.

Local fire rules can be stricter than the model code. In Roselle Park, New Jersey, for example, the fire official requires a fire permit for roof torch work, bans torching in high wind, and requires the work area to be checked after completion for smoldering fires ⁸.

Other places may differ. Before work starts, ask your roofer which permits they will pull and whether your fire department has its own rules for roof torching.

When torch-down is the right choice

• Ask whether the roof is SBS or APP, and which ASTM standard the rolls meet.
• Ask how many plies, and whether the base sheet is torched, mopped or self-adhered.
• Ask how flashings, edges and pipes will be done, and whether any of it is flameless.
• Ask who on the crew holds torch safety training and how long the fire watch will last.
• Ask which building and fire permits they will pull.

Keep going

• Roof replacementWhen a low-slope roof needs a full tear-off
• Roof repairFixing blisters, open seams and flashing leaks
• Roof inspectionHave a pro check a membrane roof without walking it yourself
• Recover or tear-off rulesWhen a recover is allowed and when it is not

Related terms

Low-Slope Roof

A low-slope roof, often called a flat roof, is a roof with only a slight fall, usually less than 2 inches of rise per foot, so it needs a sealed covering such as a membrane instead of overlapping shingles.

Roof Ponding

Roof ponding is water that collects in low spots on a roof and is still there more than 48 hours after the rain stops, in weather that would normally dry it, which is the line the roofing trade uses to tell a real drainage problem from a puddle that is just slow to dry.

Reroofing

Reroofing is the work of putting a new roof covering on a house that already has one, either by tearing the old covering off down to the deck (a replacement) or by laying the new covering over the old one (a recover, often called an overlay).

Roofing System

A roofing system is all the parts of a roof that work together to keep water and weather out of a house.

Roof Flashing

Roof flashing is thin, water-shedding material, usually bent metal, installed at the joints of a roof where shingles alone cannot keep water out: along walls, around chimneys and pipes, in valleys and at the roof edges.

Roof Underlayment

Roof underlayment is a sheet laid over the whole roof deck before the shingles or other roof covering go on.

Asphalt Shingles

Asphalt shingles are roofing pieces made from a fiberglass mat coated with asphalt and topped with mineral granules, laid in overlapping rows so water runs off the roof.

Metal Roofing

Metal roofing is a roof covering made of metal sheets, panels or shingles, most often steel, aluminum, copper or zinc, fastened to the roof deck to shed water, snow and sun.