How to Torch Side and End Laps on a Modified Bitumen Roof
Laps are where torch-on roofs leak. Width, heat, bleed-out and a trowel check decide whether they hold.
- Professional
- Part of the membrane installation, about 1 to 2 hours per 100 feet of lap
- Updated
The short answer
Torch-on side laps are about 3 inches wide and end laps about 6 inches, staggered at least 12 inches from the laps below and beside. Heat the top of the lower sheet and the underside of the overlapping sheet until asphalt flows, press the lap down, and look for a continuous bead of about 1/4 inch. End laps need the granules embedded or removed. Check every lap with a trowel.
Modified bitumen is waterproof across the field of the roof. Where it fails is at the laps, the strips where one roll overlaps the next. A lap that is not fully fused is a slot water can follow, and one that is overheated is thin and brittle. Most torch-on leaks on an otherwise sound roof trace to a lap, particularly the T-joints where three layers meet.
The idea is simple. Heat the surfaces that touch until the asphalt flows, press them together while they are fluid, and confirm the bond by a consistent bleed-out and a trowel pass. The details are not simple: the lap widths, how the granules are handled, which way the lap faces, and how the heat is distributed along the width. This guide covers those details. The field sheet technique is in how to torch a cap sheet.
If you are inspecting an existing roof, how to repair a torch-on roof shows how open laps are fixed.
Tools
- Propane torch with hose and regulator
- Round-tip lap trowel
- Hook blade utility knife
- Chalk line
- Fire extinguishers
- Hand roller for self-adhered laps
Materials
- Granulated cap sheet with selvage edge
- Base sheet with lap lines
- Mineral granules or sand for embedment where the product needs them
- Asphalt mastic for repairs at cold spots
- Flashing membrane at T-laps
Safety first
- Laps run along edges and over joints, so the flame is closest to hidden cavities here. Keep the torch aimed at the sheet, never at a gap, an edge or the wood behind it.
- Extinguishers on the roof, and the fire watch continues after the last lap is done.
- Hot laps are sticky and dangerous underfoot. Avoid stepping on soft asphalt, and keep hoses clear of the lap line.
- Cold laps and wet laps are hazards too. Work only on a dry sheet.
Step by Step
Step 1: Lay out laps so they face downslope and never stack
Laps work like shingles: the upper sheet overlaps the lower so water runs over the edge and not into it. Plan the layout so every lap faces down the slope and away from the prevailing rain where that is possible.
Offset cap sheet side laps from base sheet side laps by about 12 inches or more, and stagger end laps so that no four corners meet at one point. Stacked laps create a thick, weak spot.
Step 2: Set the lap width from the manufacturer
Most SBS cap and base sheets use about 3 inches for side laps and about 6 inches for end laps, and the printed lap lines on the sheet show the edge to align to. Some products differ, and a few call for 4 inch side laps, so check the product data sheet.
Keep the laps straight. A wandering lap makes the next roll wander, and the crooked joints multiply as the roof goes on.
Step 3: Heat the lap surfaces together
The flame is directed so it heats both surfaces of the lap together and moves along the lap at an even pace. Heat across the full width of the lap, not only at the edge, so the asphalt is fluid throughout. Keep the flame off the wood behind any gap, and keep it off the edge of the deck.
Because the selvage is smooth, it heats evenly. Over granules, the heat needs to go further down, which is why end laps need extra attention.
Step 4: Press the lap and read the bleed-out
Step or roll the top sheet down onto the lap as the asphalt flows, pushing out air. A uniform bead of asphalt squeezes out along the edge. The common target is about 1/4 inch, with manufacturers accepting roughly 1/8 to 1/2 inch. A bead that is missing in places is a cold spot and needs reheating.
Do not chase the bead with the flame. If the bead is thin, reheat the lap gently from the edge and press again.
Step 5: Deal with granules at end laps
On a granulated cap sheet, the end lap overlaps a surface covered in mineral granules. The torch is used to soften the asphalt surface so the granules sink in, often aided by a trowel, or the granules are scraped off where the sheet is overlapped so the top sheet bonds to asphalt. Follow the manufacturer's method for the specific product.
Granules left between layers prevent adhesion and leave a channel for water. Trowel them into the surface and cover the resulting asphalt with surface granules where the manufacturer directs, so the joint is protected from UV.
Step 6: Treat T-joints and corners of laps
Where an end lap crosses a side lap, the thickness doubles or triples and the top sheet tends to lift. Cut a neat diagonal at the corner of the lower sheet so the joint lies flat, and heat the area enough to fuse all the layers. Many roofers add a small patch of flashing membrane over the T.
Check these spots twice. A T-joint that looks sealed but is not is the most common lap leak.
Step 7: Trowel and probe every lap
A trowel pass while the asphalt is soft flattens the bead, pushes it into small gaps and shows lifted spots. After it cools, a blunt probe run along the edge finds places that did not bond. Mark them and reheat them before the crew leaves. A lap that fails a probe check is not finished.
Keep the fire watch after the last lap. Laps are close to edges, and heat from lap work can reach cavities that are not visible.
Tip: Use a rounded trowel tip. A pointed trowel cuts the sheet and leaves a channel for water.
Common Lap Defects
- Fishmouths: the edge of the top sheet curls up and leaves a gap. Caused by wrinkles, a misaligned roll or an edge not heated to the lap.
- Cold laps: dull asphalt and no bleed-out. Caused by moving too fast or a cold sheet.
- Burnt laps: a thin, bubbled edge with flaking granules. Caused by too much heat in one place.
- Stacked laps: thick ridges where joints line up. Caused by poor layout.
Fraser Valley Notes
Shrinkage and expansion are slow but real, and cold nights test poorly fused laps. A lap that looks tight in a dry September can be open after the first November freeze. Take the time at install, because repair in the wet season is difficult.
Common Mistakes
- Laps too narrow. A lap of 2 inches or less leaves no margin for movement and leaks at the first bad weld.
- Stacked laps. Four corners meeting at one point cause thick ridges and uneven heating.
- Ignoring the granules at end laps. Stone between layers prevents adhesion and opens a channel.
- No bleed-out. It means the asphalt did not flow, and the lap sits on top of the sheet beneath.
- Overheating the edge. It burns the polymer and thins the lap.
- No probe check. An open lap found in November is a repair in the wet.
When to Call a Professional
Lap work is fine detail work with a live flame next to edges and cavities. Seeing a lap that does not look right, such as a lifted edge or a missing bead, is enough reason to call a roofer to check it before the next rain.
If you inspect a torch-on roof and find open laps, bubbles along the seams or bare base sheet showing at an edge, the repair is covered in how to repair a torch-on roof.
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 Side and End Laps: Questions
How wide are the laps on torch-on roofing?
Most SBS cap and base sheets lap about 3 inches on the sides and about 6 inches on the ends, with lap lines printed on the sheet. Some products call for slightly different widths, so the manufacturer's installation instructions decide. Side laps face downslope, and end laps are staggered at least 12 inches from the laps in the next row.
Why do torch-on roof seams leak?
Usually because the lap was not heated enough to fuse, because the granules were left between layers at an end lap, or because a T-joint was not cut and heated properly. Overheating can also thin the lap. Movement from cold weather then opens the weak spot. A lap with no bleed-out at install is a likely leak later.
What is a fishmouth on a torch-on roof?
A fishmouth is an open, curled edge on a lap where the top sheet has lifted and left a gap shaped like a mouth. It is caused by wrinkles, poor alignment, an edge not fully heated or movement after install. It lets water in and needs to be reheated and pressed down, or patched with a piece of cap sheet.
How do you check torch-on laps?
Look for a continuous bead of asphalt about 1/4 inch wide along the lap edge while the roof is being built, and run a rounded trowel along it while the asphalt is warm. After it cools, probe the edge with a blunt tool to find open spots. Any lap that lifts or has no bead is reheated and pressed down.
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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