How to Prep a Deck for Torch-On Roofing: Substrate, Slope, Fire Safety and Weather
Most torch-on leaks and nearly all torch-on fires begin with the deck. Dry, sound, sloped, shielded and clean before a flame comes out.
- Professional
- Half a day to 2 days before torching
- Updated
The short answer
Prepping 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.
A torch-on membrane is only as good as what it is glued to. Modified bitumen bonds to a clean, dry, firm and reasonably smooth substrate, and it fails in predictable ways when it does not: blisters where moisture was trapped, wrinkles over uneven plywood, open laps over deflecting joints, and ponds where the deck was flat. The deck also decides how safely the job can be torched, because flame on bare wood or into a gap is how roof fires start.
Most torch-on roofs on wood-framed buildings are built over plywood, often with insulation or a cover board in between. The prep sequence is the same each time: assess and repair, set the slope, protect the combustibles, cover the deck, prime where needed, and wait for weather. This guide covers that sequence. Membrane installation starts in the base sheet guide.
If you are still choosing a system, compare it with EPDM in how to choose a flat roof membrane. Torch-on is not a method for homeowners to try themselves.
Tools
- Moisture meter for wood
- 4 foot level and tape measure
- Circular saw and pry bar
- Screw gun or roofing nailer
- Stiff push broom and leaf blower
- Paint roller for primer
- Class ABC fire extinguishers
- Noncombustible torch shield or flashing shield
Materials
- Exterior grade plywood for deck repairs
- Mechanically fastened base sheet or cover board approved by the membrane manufacturer
- Cant strips of noncombustible or manufacturer approved material
- Asphalt primer for concrete, metal and masonry
- Self-adhered membrane for edges and gaps
- Insulation or tapered insulation if specified
- Sealant tape or strip for deck joints
Safety first
- The prep stage is where you decide where the torch can and cannot go. Mark every wood edge, gap, vent, cavity and penetration before the torch is lit, and brief everyone on the plan.
- Do not torch over wet plywood or frost. Trapped moisture turns to steam and lifts the membrane, and the crew works on a slippery deck.
- Flat roofs have edges without guardrails. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and wherever a fall of any height could cause serious injury, and open skylights, hatches and parapet gaps count.
- Keep fuel cylinders and torches off the roof until prep is complete and the fire plan is agreed.
Step by Step
Step 1: Inspect the deck for rot, deflection and moisture
Walk the deck slowly and feel for bounce or softness. Probe stained areas and any spot around drains, walls and penetrations with a screwdriver. Soft or delaminating plywood is replaced, cut back to framing and fastened to the rafters or joists.
A pin-type moisture meter is the quickest check for wood. Many membrane manufacturers and roofing associations set a maximum moisture content for wood decks, commonly in the low to mid teens as a percentage, but confirm the number on the manufacturer's installation instructions. A deck that reads high, even without rot, needs to dry before it is covered.
See how to inspect a roof deck for rot for what to look for on the underside.
Step 2: Repair the deck and close every gap
Replace damaged panels with exterior grade plywood the same thickness as the neighbours so the surface is flush. Fasten the sheets to the framing at the spacing in the building code and the membrane manufacturer's instructions, with panel edges supported.
Gaps between sheets, around penetrations and at the edges are a fire route as well as a leak route. Close them with backing, blocking or a self-adhered membrane strip before any torch is used nearby. A torch must never be aimed into a gap, because flame and molten asphalt travel into wall and roof cavities.
Watch out: A deck that looks fine on top can have rot underneath at a drain or a wall. Check both sides where the roof has leaked before.
Step 3: Confirm the slope and plan the drainage
A level or a water test shows where water will pond. Standing water accelerates aging in every membrane, and it also weakens laps and flashings. A common target for modified bitumen is at least 1/4 inch per foot (2 percent), though a shallower slope is tolerated by some manufacturers. Confirm the slope on the product's installation instructions.
If the slope is not there, build it with tapered insulation, sloped framing or crickets between drains. See how to install tapered insulation and crickets and how to plan flat roof drainage.
Step 4: Install the cover board or mechanically fastened base layer
On wood decks, the standard approach is not to torch the cap directly on plywood. A mechanically fastened base sheet, a self-adhered base sheet or a cover board sits on the deck, and the torched layer goes over it. That protects the deck from direct flame, hides deck joints and gives a smooth surface.
If there is insulation, it is fastened or adhered per the manufacturer and covered with a cover board suitable for torch application, such as a fibre-reinforced or perlite based panel approved for modified bitumen. Joints in the cover board are staggered from the joints in the deck.
Step 5: Prime concrete, metal and masonry surfaces
Concrete slabs, parapet masonry, and metal drip edges do not bond to hot asphalt by themselves. Sweep the surface clean, let it dry, and roll on asphalt primer at the rate on the can. Primer must be fully dry to the touch, with no wet sheen, before torching, because solvent vapour near a flame is a flash hazard.
Concrete that has not cured or is holding water will not take a torched membrane. A simple test is to tape a clear plastic sheet to the slab overnight and look for condensation beneath it.
Tip: Prime on a dry day and plan torching for after the primer has cured, not on the same afternoon when the vapour is still strong.
Step 6: Shield every combustible and fire-sensitive detail
Walk the perimeter and every penetration with a fire plan in mind. Wood blocking, fascia, soffit vents, combustible wall sheathing, and open cavities are all places flame could enter the structure. Those details are built with noncombustible shields, self-adhered membranes or heat-welded methods and are never torched directly.
Cant strips, where the membrane turns up a wall, should be noncombustible or of a manufacturer approved type. Wood cants are a known fire source where the torch is held near them. See how to flash parapets and upstands and how to flash pipes and penetrations.
Watch out: Never point a torch at wood, at a cant strip that is combustible, or into a gap. If flame cannot be kept off it, use a different method.
Step 7: Clean the deck and check the weather
Sweep off dust, sand, needles and debris, then blow the deck clean. Dirt under a base sheet prevents adhesion and shows as bumps through the cap. Remove nails that stand proud, because they puncture the sheet.
Check the temperature and forecast. Many modified bitumen products can be torch applied in cold weather, but the sheets must be stored warm so they unroll without cracking, and primer and adhesives have their own limits, commonly around 5 degrees Celsius or higher. Wind affects the flame and the operator's control. Rain, frost and dew on the deck mean stopping.
Step 8: Stage the equipment and agree the fire watch
Put at least two fully charged extinguishers on the roof, in reach of every torch, and confirm that the crew know how to use them. Nominate a person for the fire watch and work out how long it will run and from what time, using current industry guidance. 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. Insurers and job specifications may require more, so plan for the strictest one that applies.
Cylinders are secured upright, regulators and hoses are checked for damage, and the roof access for emergency crews is planned.
Decks Other Than Plywood
Concrete decks are common on parapet roofs and over garages. They must be cured, dry and primed, and any trapped moisture has to escape before the membrane goes down, often through a vented base sheet. Steel decks need a cover board or insulation layer rated for torching.
Existing roofs are a different case. Torching over old felt, gravel or foam is not allowed by most manufacturers, and trapped moisture in old layers produces blisters. The safe default is a full tear-off to the deck.
Fraser Valley Notes
A dry week in the wet season is hard to find, and plywood left uncovered in November takes on moisture fast. Many roofers dry in the deck with a temporary membrane, then torch when the forecast allows. Cold mornings and heavy dew are a problem even on dry days, so check the deck surface, not the sky.
Common Mistakes
- Torching directly onto plywood. It scorches the deck, risks fire, and leaves an uneven bond.
- Covering a wet deck. Moisture trapped under the membrane expands in the sun and forms blisters that split the sheet.
- Skipping the slope check. A flat roof with ponds cuts the life of the membrane and every lap.
- Leaving gaps open at edges and penetrations. Flame and molten asphalt run into the cavity below, which is how hidden fires start.
- Unprimed concrete or metal. The membrane releases from the surface at the first cold snap.
- Leaving nails and debris. Raised nail heads and grit telegraph through the cap and puncture the base sheet.
When to Call a Professional
Deck rot, structural slope changes, tapered insulation and combustible details are all decisions that belong to a roofer and, where framing is involved, an engineer or a building inspector. If the roof has been leaking for years, assume the deck is damaged until proven otherwise.
Torching is a professional method with real fire consequences. Ask a roofer how they will shield the edges and penetrations, how long the fire watch lasts, and what substrate goes under the torched layer.
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 to Prep a Deck for Torch-On Roofing: Questions
Can you torch-on roofing directly onto plywood?
Direct torching on plywood is avoided on most wood decks because the flame can scorch or ignite the wood and the heat travels through joints into the building. The usual approach is a nailed or self-adhered base sheet, or a cover board, with the torched cap sheet going over it. Follow the manufacturer's instructions for the exact substrate.
What slope does a torch-on roof need?
A common target is at least 1/4 inch of fall per foot, or about 2 percent, toward drains or scuppers, so water does not sit. Some manufacturers accept less, but ponding that lasts more than about 48 hours after rain shortens the membrane's life. Check the product's data sheet and the drainage plan for the building.
How dry does a roof deck need to be for torch-on?
It needs to be dry on the surface and have a low moisture content. Wood decks are often measured with a pin meter, and many specifications want the reading in the low to mid teens as a percentage or lower. Rain, dew or frost on the surface stops the sheet from bonding and will cause blisters later.
Do you need a cover board under torch-on?
Over insulation, a cover board is normally required, because foam insulation melts or burns under torch heat and the board provides a firm, flat surface. Over plywood, many systems use a mechanically fastened or self-adhered base sheet for the same reason. The specific product and the building's fire risk decide which one is required.
Can you torch-on in cold weather?
Torch-applied membranes are often installed in cold weather because the flame does the work, but sheets need to be stored warm so they unroll without cracking, and the deck must be dry and free of frost. Primers and adhesives have their own minimum temperatures, commonly about 5 degrees Celsius. Confirm each limit on the product data sheets.
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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