How to Install a Torch-On Base Sheet: Nailed, Self-Adhered or Torched
The first ply sets the layout for the whole roof. Which attachment method you use is a fire safety decision first and a waterproofing decision second.
- Professional
- Half a day to 1 day for a small roof
- Updated
The short answer
A torch-on base sheet is the first ply of a two-ply modified bitumen roof. Start at the low point, run rolls perpendicular to the slope or as the manufacturer directs, lap sides about 3 inches and ends about 6 inches, and stagger end laps. On wood decks the base is commonly nailed or self-adhered so no flame touches the wood. Torching is reserved for noncombustible substrates.
The base sheet is the first ply of a two-ply torch-on roof. It gives the cap sheet a smooth, continuous surface, carries part of the waterproofing, and, on a wood deck, keeps the torch off the plywood. A good base layer is flat, lapped consistently and fastened well enough that wind and deck movement do not wrinkle it.
There are three ways to attach it. A mechanically fastened base sheet is nailed through plates or cap fasteners into the deck, which is the standard approach on plywood. A self-adhered base sheet has a sticky underside with a release film and needs no fire at all. A torch-applied base is heated to the substrate, and it belongs only on noncombustible decks such as prepared concrete or a suitable cover board. Which method is allowed depends on the deck, the manufacturer and the fire risk.
Before any base sheet goes on, the deck has to be ready. See how to prep a deck for torch-on. Once it is down, the cap sheet follows in how to torch a cap sheet.
Tools
- Hook blade utility knife
- Roofing hammer or pneumatic nailer
- Chalk line and tape measure
- Push broom
- Weighted roller for self-adhered sheets
- Class ABC fire extinguishers
- Propane torch kit where torching is allowed
Materials
- Base sheet (mechanically fastened, self-adhered or torch-applied type)
- Cap-headed or plate fasteners approved for the deck
- Asphalt primer for concrete or metal
- Cant strips
- Self-adhered membrane for edges and gaps
- Release-film remover and end lap sealant if the product needs them
Safety first
- Do not torch directly on wood, or into gaps, joints or edges. Choose a nailed or self-adhered base on wood decks so the base layer can be installed with no flame at all.
- Keep extinguishers on the roof and stage cylinders away from the work area until torching starts.
- Base sheets are slippery when wet and the rolls are heavy and awkward. Carry them in pairs and keep clear of unprotected edges, as WorkSafeBC fall protection applies at 3 metres (about 10 feet) and wherever a fall could cause serious injury.
- Release films and primers can be slippery. Clear off the film scrap as you go.
Step by Step
Step 1: Choose the attachment method for the deck
On plywood and other wood decks, use a mechanically fastened or self-adhered base sheet. Both avoid open flame on the wood. On concrete or a torch-rated cover board, a torched base is an option, and it gives a full bond to the deck.
Choose from the manufacturer's data sheet and the design specification. Mixing brands or types of base and cap is not allowed unless the manufacturer says the system is approved.
Step 2: Dry-lay and snap layout lines from the low point
Start at the lowest edge, near a drain or eave, so every lap faces downslope and sheds water like a shingle. On a roof with several drains, work from each low point toward the high point.
Snap a chalk line for the first row, allowing for overhang at the edge if the detail calls for it. Unroll the first roll dry, let it relax for the time the manufacturer specifies so it lies flat, and check that it is straight. A crooked first row is carried across the whole roof.
Tip: Store rolls upright and warm. Cold rolls resist unrolling, crack at the edge and never lie flat.
Step 3: Fasten or adhere the first row
For a nailed base, fasten the lap edge at the spacing shown on the manufacturer's instructions, commonly every 6 to 12 inches along the side lap, with additional fasteners in the field. Drive cap-headed or plate fasteners flush, not through the sheet, into sound framing or deck.
For a self-adhered base, fold back half the roll, peel the release film, and press the sheet into place working from the middle outward to push out air. Use a weighted roller on the whole sheet. Self-adhered products have minimum temperatures, often around 5 degrees Celsius, and the substrate must be dry and primed if the product requires it.
Step 4: Lap the sides and stagger the ends
Most modified bitumen base sheets lap about 3 inches on the sides and 6 inches at the ends, with lines printed on the sheet to show the lap. The exact dimensions come from the product's instructions. End laps are staggered at least 12 inches and commonly more, so no four corners meet at one point.
Lay each row so the lap points down slope. Water must flow over each lap, not into it.
Step 5: Seal the base laps
On a nailed base, the lap is commonly sealed with a torch or hot air, or with lap adhesive where flame is not permitted. The torch is held so the flame heats the lap sheet, not the deck, and the heat comes from above. On wood decks, many roofers seal laps with cold adhesive or use a lap that has its own adhesive strip.
Self-adhered laps are pressed with a hand roller along the full length. Check that the release film is fully removed in the lap.
Watch out: If the torch flame can reach the gap under the lap, stop. Flame under a lap heats the plywood and the joint below it.
Step 6: Bring the base up the edges and across the drains
At walls, the base sheet is turned up over a cant strip to the height the details call for, commonly about 4 to 8 inches for the first ply of a two-ply flashing, and secured. At drains, cut the sheet back neatly and follow the drain manufacturer's detail. Gaps around penetrations are closed with self-adhered strips, not left open for the torch.
See how to flash parapets and upstands, drains and scuppers and penetrations.
Step 7: Inspect the base before the cap goes on
A flat, continuous base gives a flat cap. Cut and flatten wrinkles, reseal any lifted laps, and patch punctures with a base sheet piece. Sweep off debris, because grit under the cap sheet leaves bumps and weak spots.
Plan to cap the same section soon after the base goes down. An exposed base sheet is not a finished roof, and sun, traffic and rain damage it, so in the Fraser Valley it should not be left exposed through the wet season.
Which Method When
- Nailed base: the standard on wood decks. No flame touches the deck, and it resists wind uplift when fastened correctly.
- Self-adhered base: no open flame at all, good near edges, eaves and gaps, but it needs a primed, dry surface and mild temperatures.
- Torched base: gives a full bond and is used over concrete and torch-rated cover boards, never directly over wood.
- Hot asphalt or cold adhesive: used in some systems but outside the scope of this guide.
Fraser Valley Notes
Fall and winter installs lean toward self-adhered bases, because a torch works in the cold but a wet deck does not. If a wood deck is wet in the morning, nothing should go down. Dew matters even on clear mornings.
Common Mistakes
- Torching a base sheet directly onto wood. It is a fire risk and does not give a reliable bond over plywood joints.
- Starting at the high side. Laps then face upslope and push water into the seams.
- Stacking end laps. Four corners meeting at one point makes a thick lump and a leak path.
- Leaving the release film in a lap. A self-adhered lap with film in it is open from day one.
- Overdriven or underdriven fasteners. They either cut the sheet or stand proud and puncture the cap later.
- Leaving the base exposed. UV, foot traffic and rain degrade an uncapped base and trap moisture beneath the cap.
When to Call a Professional
The choice of base method, the fire plan at edges and the fastening pattern for wind uplift are decisions a trained roofer makes from the product data and the building. Torching any layer next to wood, insulation or open cavities is professional work and should not be attempted without training and fire protection.
If you are replacing an old torch-on roof, ask whether the old layers are being removed or overlaid, and what base sheet method will be used over the deck.
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 Install a Torch-On Base Sheet: Questions
What is a base sheet in torch-on roofing?
The base sheet is the first ply of a two-ply modified bitumen roof. It is nailed, self-adhered or torched to the deck and gives the cap sheet a smooth surface to bond to. On wood decks it is usually nailed or self-adhered so that no flame touches the wood, and the cap sheet is torched over it.
What laps does a torch-on base sheet need?
Most modified bitumen base sheets lap about 3 inches on the sides and about 6 inches at the ends. End laps are staggered between rows, and every lap faces downslope so water runs over it. The exact lap widths are printed on the sheet or given on the manufacturer's installation instructions, which take priority.
Can you use a self-adhered base sheet under torch-on?
Yes, many manufacturers approve it, and it is often chosen on wood decks because it needs no flame to install. The surface must be dry, clean and primed if the product requires, and the air and surface temperature must be above the product's minimum. The cap sheet is then torched over the self-adhered base.
Do you torch the base sheet or nail it?
It depends on the deck. On plywood, the base is nailed or self-adhered so no flame heats the wood. On concrete or a torch-rated cover board, a torched base gives a full bond. The safest rule is to follow the manufacturer's approved method for the substrate and never aim a torch at wood or into a gap.
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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