How to Flash Parapets and Upstands With Torch-On Membrane
Walls are where the torch is closest to the building. Cants, heights, two plies and termination, done without putting flame into the wall.
- Professional
- 1 to 3 hours per 50 feet of wall
- Updated
The short answer
A torch-on parapet or upstand flashing is built in two plies over a cant strip. The base flashing ply runs up the wall, commonly at least 8 inches above the finished roof, the cap flashing ply laps over the field, and the top is terminated with a bar, counter flashing or coping. Keep flame off wood, cavities and combustible sheathing, and use self-adhered sheet where needed.
Parapets, curbs, equipment pads and wall-to-roof joints all need the membrane to turn up a vertical surface. That upturn is the upstand, and it carries two jobs. It keeps water that pools or runs against the wall out of the building, and it gives the roof membrane room to move without pulling away from the wall. Most flat roof leaks that are not at laps are at upstands.
Torch-on upstands are built in two plies to match the field: a base flashing sheet up the wall and a granulated cap flashing sheet over it, each lapping onto the roof membrane so water is shed over the joints. The transition at the bottom is cushioned by a cant strip, a 45 degree wedge that keeps the membrane from making a sharp bend that cracks over time.
This is also the place where torch fires start most often. A torch held against a wall sends heat straight into sheathing, insulation and framing. The work is done with shields, with self-adhered membranes and with a careful plan. This guide covers the method. The field roof is in how to torch a cap sheet, and the wall cap in how to install parapet coping metal.
Tools
- Propane torch with hose and regulator
- Noncombustible torch shield
- Round-tip trowel
- Hook blade utility knife
- Chalk line and level
- Fire extinguishers
- Caulking gun
Materials
- Base flashing membrane (self-adhered or torch-applied type)
- Granulated cap flashing sheet
- Noncombustible or manufacturer approved cant strips
- Asphalt primer for masonry or concrete
- Termination bar and fasteners
- Sealant for the termination
- Counter flashing or coping metal
Safety first
- Walls hide cavities. Sheathing, studs, parapet framing and wall insulation can be reached by flame and heat, and fires start inside them. Do not aim a torch at wood, a gap at a corner, or a joint in the wall.
- Use self-adhered membranes or heat-welding methods on wood-framed walls and any wall that cannot be shielded. Noncombustible shields are placed between the flame and the wall.
- Keep extinguishers at the wall where you are working, and plan a longer, careful fire watch that includes the inside of the wall where access allows.
- Parapet tops and low walls are fall hazards. WorkSafeBC requires fall protection at 3 metres (about 10 feet) and where any fall could cause serious injury.
Step by Step
Step 1: Prepare the wall and plan the fire protection
Check the wall for soundness and moisture. Masonry and concrete walls are cleaned, dried and primed with asphalt primer. Wood-framed and sheathed walls are covered with a suitable layer, often a self-adhered membrane or a cover board, so the flame is never directed at wood.
Walk the wall with the fire plan in mind: corners, gaps, penetrations, vents and any cavity that leads inside. Close them, cover them or plan to use heat-welding or self-adhered methods there.
Watch out: If the wall is combustible and cannot be shielded, do not torch. Use a self-adhered flashing membrane instead.
Step 2: Install the cant strip
The cant strip, commonly about 3 to 4 inches on each face, supports the membrane at the change of direction so it does not crack or bridge a void. Perlite, fibreboard and mineral based cants are used in torch applications. A wood cant is combustible and is a known fire source where the torch is held close.
Fasten or adhere the cants tight to the wall and deck, and keep joints staggered from the base sheet joints. See how to prep a deck for torch-on for where noncombustible details are needed.
Step 3: Turn the base sheet up the wall
The first ply, the base flashing, starts on the field about 4 to 6 inches out from the cant and runs up the wall to the termination height. Cut the pieces in lengths that are manageable, about 3 feet or one roll width high for tall walls, and lap side to side about 3 inches with laps facing to shed water.
The base flashing is typically adhered with a self-adhered sheet or mechanically fastened at the top. On concrete or masonry, it may be torched to a primed surface with shielding. Follow the manufacturer's approved method for the wall material.
Step 4: Set the upstand height
Most manufacturers and roofing associations call for flashings to rise at least 8 inches above the finished membrane, with 6 inches as a common minimum in many specs. Confirm the requirement in the manufacturer's detail and the building code. Where snow or ponding water is likely, go higher.
Every inch of height is protection against wind-driven rain and ponding. A short upstand is one of the most common causes of wall leaks on flat roofs.
Step 5: Torch or adhere the cap flashing ply
The cap flashing sheet is cut to reach from the field up the wall, and laps the field cap sheet by at least 4 inches where the manufacturer's detail specifies. It is heated and pressed in with a shield between the flame and the wall, and with the torch held low and moving. Where walls cannot be torched, a self-adhered cap flashing is used.
Stagger the flashing laps away from the field laps and from the base flashing laps by at least 6 inches. At inside corners, fold the membrane and avoid cutting through it.
Step 6: Terminate the top edge
The top of the upstand is where water gets behind the membrane, so it is secured and covered. Options include a termination bar fastened every 6 to 8 inches and sealed with a compatible sealant, a counter flashing set into a reglet, or a coping cap that covers the parapet. Do not leave the top of the flashing relying on sealant alone.
On masonry, see the approach in how to cut a reglet for counter flashing. For parapet caps, see parapet coping.
Step 7: Inspect corners and fire watch
Corners are where flashings fail. Inside corners need a folded or patched piece so there is no open seam, and outside corners need a wrap that does not stretch the sheet. Probe each lap and corner and fix any open spot.
The fire watch after flashing work should include the wall and the top of the parapet. Smouldering behind a flashing is invisible from the roof surface. Industry programs often call for two hours, and at least one hour under RCABC guidance.
Cant Strips and Why They Matter
A sharp 90 degree bend in the membrane cracks as the roof moves, and the crack forms at the lowest point where water collects. A cant strip softens the bend. It also gives the torch a place to heat from without aiming at the wall. The material matters because the cant is the closest thing to the flame: use noncombustible or manufacturer approved cants, not scrap wood.
Fraser Valley Notes
Rain driven sideways against parapets for months makes the top edge the most important part of the wall detail. Where parapets face the prevailing wet wind, specify coping or counter flashing, and do not rely on sealant at the top.
Common Mistakes
- Torching flame at the wall. It heats sheathing and framing and starts fires that appear hours later.
- Wood cant strips. They burn or char under torch heat, so use noncombustible or approved cants.
- Short upstands. Flashing under 6 inches leaks when water ponds or snow melts against the wall.
- Terminating with sealant only. It cracks in a few seasons, so use a bar, counter flashing or coping.
- Stacking flashing laps on field laps. The thickness causes weak spots and leaks.
- Ignoring corners. Inside and outside corners without a patch or wrap are where most wall leaks start.
When to Call a Professional
Parapet and wall details combine fire risk, waterproofing and structure. If the wall is wood-framed, if there is insulation or a cavity behind the flashing, or if the parapet is high, this is work for trained roofers who can choose a method that does not put flame near the building.
If you have a parapet leak, the first places to look are the top edge, the corners and the lowest point of the upstand. See how to find a leak on a flat 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 Flash Parapets and Upstands With Torch-On: Questions
How high should parapet flashing go on a torch-on roof?
Flashing is commonly run at least 8 inches above the finished roof surface, with 6 inches as a minimum in some specifications. Higher is better where water can pond or snow can build up. The manufacturer's detail and the building code set the requirement, so confirm them for your product and location.
Why is a cant strip used at a parapet?
A cant strip is a 45 degree wedge at the wall to roof joint. It turns the membrane gradually instead of bending it at a sharp right angle, which would crack as the roof moves. It also gives a surface to bond to. Use noncombustible or manufacturer approved cants for torch work, not wood.
Can you torch-on flashing against a wood wall?
Direct torching against wood or wood sheathing is avoided because the flame and heat can ignite the wall or smoulder in the cavity. The usual approach is a noncombustible shield, a cover board or a self-adhered flashing membrane on the wall, with the torch used only on the roof side where it is safe. Follow the manufacturer's approved method.
How do you finish the top of a torch-on flashing?
The top is secured with a termination bar fastened every 6 to 8 inches and sealed, or covered by counter flashing set into a reglet or by a coping cap. Sealant alone is not enough, because it cracks within a few seasons and lets water behind the membrane. The top edge is the most important part of a wall flashing.
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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