How to Torch a Cap Sheet: Heating, Bleed-Out, Laps and Fire Watch
The skill is in the flame: heat the asphalt until it flows, not until it burns, and read the bleed-out as you roll.
- Professional
- Half a day to 2 days depending on size
- Updated
The short answer
To torch a cap sheet, unroll it dry over the base, align it, reroll both ends to the middle, then heat the base and the underside of the roll together until the asphalt turns glossy and forms a small rolling bead, and press the roll into it. A bleed-out bead of roughly 1/4 inch shows at the edges. Laps are about 3 inches and 6 inches. A fire watch follows.
Torching a cap sheet looks simple from the ground. A roofer walks backward with a torch in one hand, the roll advances, and a clean line of black asphalt squeezes out of the edge. The skill is in seeing what is happening under the flame. The goal is to soften the asphalt on both the underside of the roll and the surface of the base to the point where they flow together, and then to press them together while they are still fluid.
Too little heat and the sheet sits on top without bonding. Too much and the polymer burns, the asphalt smokes and thins, the granules fall away, and the membrane loses flexibility. Both failures look fine the day they are made and show up in the first winter as open laps and blisters. Conditions matter: cold rolls stiffen, wind bends the flame, and a damp base gives poor adhesion.
This guide covers the field cap. Laps are treated in side and end laps, and flashings in parapets and penetrations. The equipment is covered in how to use a roofing torch safely.
Tools
- Propane torch with hose and regulator
- Hook blade utility knife
- Torch-on trowel with rounded tip
- Chalk line
- Roller or weighted tamper for laps
- Class ABC fire extinguishers
- Noncombustible torch shield for edges
Materials
- Granulated SBS or APP cap sheet
- Matching base sheet already installed and inspected
- Propane cylinders
- Granules or mineral surfacing for end laps if the product requires it
- Flashing membrane for upstands and penetrations
Safety first
- Never aim the flame at wood, a gap, a cant strip that is combustible or the inside of an edge detail. Use a shield or a self-adhered method.
- Extinguishers within reach of every torch operator, and a fire watch after the last flame is out. The RCABC and CRCA documents describe at least one hour, and industry programs often call for two.
- Keep the hose and cylinder clear of the roll path and hot asphalt. Hoses are inspected before every use, cylinders are upright and secured, and leaks are checked with soapy water, never a flame.
- Hot asphalt burns skin and ignites if overheated. Wear long sleeves, gloves and safety footwear, and never work alone on a torch roof.
Step by Step
Step 1: Check the base, the weather and the fire plan
Sweep the base and look for wrinkles, water and debris. A damp base must dry before it is capped, and rolls that have been stored in the cold should be brought to a workable temperature so they unroll without cracking.
Put extinguishers on the roof, confirm the wind is manageable, and brief everyone on where the torch must not go. Assign the person who will do the fire watch.
Step 2: Dry-lay the first roll and snap a line
Start at the lowest edge so laps face downslope. Offset the cap sheet side laps from the base sheet side laps by at least about 12 inches so no seams line up between plies. The roll's selvage edge, the smooth strip without granules, is where the side lap goes.
Let the roll relax flat for the time the manufacturer gives. A sheet that still has a memory of the roll will wrinkle as soon as it is heated.
Step 3: Reroll the roll to the middle
Once aligned, reroll the sheet back from one end to the middle, leaving the roll positioned on the line. The roll is then unrolled slowly as the torch heats the underside and the base. The end of the roll is tacked first to hold the alignment, and then the roll is advanced.
Keep the roll straight and avoid twisting it, because a crooked start cannot be corrected part way down.
Step 4: Heat the base and the roll together
The flame is held at an angle so it heats both surfaces at once, at a distance specified in the torch training, usually a few inches. The operator moves the flame side to side across the full width of the roll, not in one spot. The asphalt changes from dull to glossy, and as the roll is pushed forward a small wave, or bead, of molten asphalt forms ahead of it.
Read the asphalt, not the flame. A dull or matte bead means under-heating. Smoke, bubbling and a thin, runny look mean overheating. If a granulated surface begins to slough off, the flame is too close or too slow. Some SBS products have a plastic film on the underside that burns off as a visual cue; follow the manufacturer's instructions for the specific sheet.
Watch out: Do not hold the flame on one spot or aim it at the edge of the roof. A stationary flame burns through the sheet and starts fires in wood below.
Step 5: Roll the sheet into the asphalt and read the bleed-out
As the sheet is pressed down, the melted asphalt squeezes out at the edges. This bleed-out is the quality signal: a continuous, even bead about 1/4 inch wide along the side lap, with a range of about 1/8 to 1/2 inch depending on the manufacturer, shows the layers have fused. Gaps in the bead mean cold spots.
Walk the roll with your weight behind it, pushing out air. Step on the bead edge to flatten it or trowel it afterward. Keep moving forward, and keep the flame ahead of the roll, not on the finished sheet.
Step 6: Stagger and heat the laps
At each side lap, heat the top of the lower sheet's selvage and the underside of the overlapping sheet, then press the lap down. End laps get extra heat, and on granulated caps the granules at the end lap are embedded into the hot asphalt, or scraped off, so the top sheet can seal into a smooth surface. Stagger end laps at least 12 inches and more between rows.
See how to torch side and end laps for the lap-specific technique.
Step 7: Tie in flashings and finish the edges
The upstand and flashing membranes go on in the order that lets the field sheet lap over or under each piece as the lapping rule requires. Follow the details in the manufacturer's drawings and the flashing guides. Keep flame off the wall, off cants that are combustible, and off anything that could hide smouldering.
Terminate at edges with the metal and termination details specified for the system, and seal exposed edges with mastic where the manufacturer directs.
Step 8: Check, trowel and keep the fire watch
Before the crew leaves, run a trowel or a blunt probe along every lap. An open edge is easy to fix while the asphalt is still warm. After the last torch is out, a responsible person stays on the roof and watches edges, flashings and the underside, for at least one hour under the RCABC guidance and at least two hours under NFPA 241 and the CERTA program. Follow the current requirement that applies to the job.
Do not leave the site until the roof has been checked and the last hot spot has cooled. Most torch-on fires are found after the crew has left.
Signs You Are Overheating or Underheating
- Smoke, bubbling and a thin, runny bead: overheating, burnt polymer, weak membrane.
- Dull, matte asphalt with no bead at the edge: underheating, poor bond.
- Granules flaking off in clumps: the flame is too close or lingering.
- Wrinkles ahead of the roll: the roll was not relaxed, or heating is uneven across the width.
Fraser Valley Notes
Wind off the Fraser River and cold, damp mornings change how a torch behaves. Wind bends the flame and cools the sheet. Damp base sheets give weak adhesion, and morning dew can hide on a dark surface. Plan the day so the last roll is down and inspected well before dark, because a fire watch after dusk is harder and the asphalt cools too quickly.
Common Mistakes
- Overheating the roll. Burnt polymer, thin spots and lost granules reduce flexibility and lifespan.
- Underheating. The sheet sits on the base without fusing, and the first freeze lifts it.
- Holding the flame in one place. It burns through the sheet and heats whatever is under it, starting hidden fires.
- Aiming at edges and gaps. Flame at a fascia or a joint is how a roof fire starts.
- Lining up laps between plies. Side laps on the cap over side laps on the base create a thick, weak line.
- Skipping the fire watch. Smouldering is discovered after everyone has gone home.
When to Call a Professional
Torching a cap sheet is the most skill-dependent step in roofing, and it is also the step with the highest fire consequence. Training, insurance, shielding and a fire watch are part of the job. If a roof is being torched, expect extinguishers on the roof, cylinders secured, and a person assigned to watch after.
If you already have a torch-on roof with bubbles, open laps or bare base sheet, see how to repair a torch-on roof for what a repair involves and when it stops making sense.
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 Torch a Cap Sheet: Questions
How do you know when a torch-on cap sheet is heated enough?
The asphalt on the underside of the roll and the top of the base turns glossy and forms a small rolling wave ahead of the roll, and a continuous bead of about 1/4 inch squeezes out at the edges as the sheet is pressed down. Dull asphalt means too little heat. Smoke and a thin, runny look mean too much.
What is bleed-out in torch-on roofing?
Bleed-out is the bead of melted asphalt that squeezes out along the edge of a lap when the sheet is pressed into hot asphalt. It is the visual sign of a proper bond. A common target is about 1/4 inch, within a range of about 1/8 to 1/2 inch depending on the manufacturer. No bead means a cold or unbonded lap.
How long is the fire watch after torching a roof?
Industry guidance commonly calls for at least one hour after the last torch is out, as in the RCABC practice manual, and NFPA 241 and the CERTA torch safety program call for at least two hours, with checks of the roof surface, flashings and underside. Insurers and job specifications may require more. The person on watch keeps extinguishers at hand and stays until the roof has cooled.
Can you torch a cap sheet in the rain?
No. Rain, standing water and heavy dew stop the asphalt from bonding and cause blisters and open laps. The flame can also turn trapped water to steam under the sheet. Wait for a dry deck and base, and stop work when rain arrives. A short dry window in the wet season is better used for prep than for a poor bond.
Why are the edges of my torch-on cap sheet lifting?
Lifting edges usually mean the lap was not heated enough to bond, the base was dirty or damp, or the roof has moved with temperature changes. A missing bleed-out bead at install is the warning sign. Small lifted edges can be reheated and troweled down, but large open areas need a repair by a roofer.
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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