How to Flash Roof Drains and Scuppers With Torch-On Membrane
Drains and scuppers carry all the water on a flat roof. Sumps, clamping rings, target sheets and a lap that never faces the wrong way.
- Professional
- 1 to 3 hours per drain
- Updated
The short answer
To flash a torch-on roof drain, set the drain bowl in a sump slightly below the roof surface, install the base ply and lap it under the clamping ring, torch a target sheet over the bowl, then lap the cap ply over it and clamp. Scuppers are lined with a formed box or membrane that runs through the wall and laps onto the roof. Keep flame off the pipe and wall.
A flat roof is not flat. Every square foot of it is tilted, however slightly, toward a drain or a scupper, and all the rainfall on the roof leaves through those few openings. That makes them the most critical flashings on the building. A drain that leaks puts water into the ceiling under it, and a drain that is set too high, clogged or surrounded by a poor lap makes the roof pond.
A torch-on drain detail has three jobs. The bowl sits low enough that water flows into it, the membrane layers lap under or over the clamping ring so they cannot leak, and the strainer keeps debris out. Scuppers carry water through a parapet wall and have their own detail, a liner or box that turns the membrane through the opening and laps it onto the roof.
Planning comes first. See how to plan flat roof drainage for sizing and overflows, how to install a flat roof drain for the body and plumbing, and how to install a scupper and collector box for the through-wall metal.
Tools
- Propane torch with hose and regulator
- Noncombustible torch shield
- Round-tip trowel
- Hook blade utility knife
- Screwdriver or socket for clamp ring bolts
- Fire extinguishers
- Caulking gun
Materials
- Roof drain with clamping ring and strainer, sized by the plumber or engineer
- Base sheet and cap sheet
- Target sheet or flashing square of membrane
- Asphalt mastic or flashing cement
- Scupper box, liner or pre-formed scupper sleeve
- Sealant approved for membrane and metal
- Stainless fasteners for the ring
Safety first
- Drains and scuppers pierce the deck and wall. Flame can drop into the drain body or cavity. Keep the torch off the pipe, off the sleeve and out of the gap around the drain, and use a shield.
- Use self-adhered or mastic based details near plastic drain bodies and sheathing. Hot asphalt and open flame can deform plastic pipe and ignite combustible material.
- Keep extinguishers at the drain. The fire watch includes the drain bowl and the space under the deck where access allows.
- Do not work above a drain without checking that the pipe beneath is clear of debris and rags that could burn.
Step by Step
Step 1: Confirm the drain is the low point
The drain bowl is set in a sump, a shallow depression in the deck or insulation of about 1/2 to 1 inch, so water flows to it from every direction. Tapered insulation or a cricket feeds water to the drain. See tapered insulation for the plan.
Test the slope with a level or a hose before membrane goes down. If the drain is higher than the field, no amount of good flashing will stop ponding.
Step 2: Install the base ply to the drain bowl
Install the base ply in the normal manner, cutting it in a clean circle or square around the drain so it lies flat on the bowl flange. Apply a bed of asphalt mastic or flashing cement on the flange where the manufacturer directs, and press the base sheet into it.
Keep the flame away from the drain body. Many drain bodies are cast iron, but others are PVC or ABS, and plastic deforms and burns. Use mastic, self-adhered membrane or a shield at the drain.
Step 3: Set a target sheet over the flange
A target sheet is a patch of membrane, usually about 12 to 24 inches square, larger than the ring so it extends out onto the base sheet. It is laid into mastic, pressed flat, and held down so there are no voids. Laps of the target sheet face the drain so water runs over the joint and into the bowl.
Some systems use a prefabricated flashing or lead flashing on the bowl instead of a target sheet. Follow the drain manufacturer's detail.
Step 4: Clamp the ring and cut out the membrane
Lay the cap sheet over the drain area and heat it onto the base without flame on the bowl. Cut the opening in the membrane for the drain pipe, carefully, so the membrane reaches the drain sump but does not cover it. The clamping ring then holds the membrane tight to the flange with stainless bolts tightened evenly in a cross pattern.
Do not pinch the membrane so hard that it cuts. Drain manufacturers give bolt torque.
Watch out: Never torch above a plastic drain body or into the drain pipe. Fuel, rags and debris in the pipe can ignite.
Step 5: Build the scupper lining or box
A scupper is an opening through a parapet wall that lets water flow out to a downspout or the ground. It is lined with a metal box, a formed sleeve or a membrane liner so water never touches the wall. The membrane runs down the floor of the scupper and out to the face of the wall, and the cap ply laps the liner.
Stagger the laps so the roof side laps over the scupper liner, so water runs onto the liner and out. See how to install a scupper and collector box for the metal work.
Step 6: Add the strainer and set overflows
A dome strainer keeps leaves and debris out of the drain. Overflows, commonly a scupper set 2 inches or more above the main drain level, provide a path if the drain blocks. Check the current building code for the overflow requirement on your roof.
Flood-test if the roof specification calls for it, and keep the roof clean through the first wet season.
Step 7: Probe, test and keep the fire watch
Probe the target sheet laps, the membrane edges at the clamping ring and the scupper liner joints. Run water into the drain and look for leaks below. Fix any open lap with a trowel and mastic.
The fire watch should include the space under the drain where accessible. Heat travels down the pipe.
Drain and Scupper Checks
- Sump depth: the drain bowl is lower than the field so water runs to it.
- Laps face the drain: membrane laps run toward the bowl so water crosses them with the lap, not against it.
- Clamp ring torque: bolts tightened evenly in a cross pattern, not over-tightened.
- Overflow: sized and set above the main drain, per the building code.
- Strainer: in place, clear, and checked each autumn.
Fraser Valley Notes
Needles, leaves and moss under firs and cedars block drains every autumn. Clear strainers after the first heavy leaf fall and before the first long rain, and check again in late winter. A blocked drain plus an overflow that is set too high or too small is how ceilings get wet.
Common Mistakes
- Drain set above the field. Water ponds around it, and the laps age quickly.
- Torch flame on a plastic drain body. It melts and can ignite the pipe.
- Laps facing away from the drain. Water runs against the lap and into the seam.
- Overtightening the clamp ring. It cuts the membrane and starts a leak at the flange.
- No overflow. A blocked drain then loads the roof with standing water.
- No target sheet. Field membrane alone is not enough at the flange, where movement and water concentrate.
When to Call a Professional
Drain work combines waterproofing, plumbing and building code, and a leak at a drain is hard to trace. If a drain is ponding, if the body is cracked, or if the overflow is missing, call a roofer and, where plumbing is involved, a plumber.
For leaks that may be at a drain or scupper, 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 Roof Drains and Scuppers With Torch-On: Questions
How do you flash a roof drain on a torch-on roof?
Set the drain bowl in a shallow sump, run the base ply to it, bed a target sheet of membrane over the flange in mastic, lap the cap ply over it, then fasten the clamping ring so the membrane is held under compression. Laps face the drain so water runs over them. Keep the torch off the drain body.
What is a target sheet on a roof drain?
A target sheet is a square or round patch of membrane, bigger than the clamping ring, bedded over the drain flange between the base and cap plies. It adds an extra layer of waterproofing where movement and water are concentrated. Its laps face the drain so water runs over the joint into the bowl.
Why does a flat roof need an overflow?
If the main drain blocks, water has no other route and can load the roof beyond what it is designed to carry. An overflow drain or scupper, set a couple of inches above the main drain level, lets excess water leave safely. Check the current building code for overflow size and height on your roof.
Can you torch around a plastic roof drain?
Direct flame near a plastic drain body is avoided because plastic deforms and can ignite, and the pipe below can carry heat into the building. Use mastic, self-adhered membrane or a shield at the drain and apply heat only to the membrane. Follow the drain and membrane manufacturers' instructions.
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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