How to Install a Roof Scupper and Collector Box: Sizing, Flashing, Conductor Heads and Overflows
A scupper is an opening in the parapet that lets water leave the roof. Set it flush with the membrane, flash it properly, and give it somewhere to go.
- Professional
- 3 to 6 hours per scupper
- Updated
The short answer
A scupper is a metal-lined opening through a parapet wall that lets roof water exit to a downspout or grade. Set the scupper bottom flush with the finished roof surface, flash its flange into the membrane, fasten a collector box or conductor head outside to catch water, and install a separate overflow scupper about 2 inches higher. Size the opening from the roof area and design rainfall.
A scupper is an opening through a parapet or wall at roof level that lets water leave the roof. It is one of the simplest drainage devices and one of the most reliable, because it has no pipe to block inside the building. Water runs to the opening, passes through and falls to a downspout or the ground outside. On a building with a parapet, scuppers are commonly used as the primary drains or as overflows beside interior drains.
Simple does not mean forgiving. A scupper has to sit at the level of the roof surface, not above it, or water will pond behind it. Its flange has to be sealed into the membrane, since it is a hole through the wall and a path for water into the building. And it has to be big enough: an undersized scupper is the usual reason a flat roof fills during a storm.
This guide covers through-wall scuppers and the collector box below them. For interior drains, see how to install a flat roof drain. For the larger plan, see how to plan flat roof drainage.
Tools
- Tape measure and level
- Reciprocating saw or circular saw with appropriate blade for the wall material
- Drill and impact driver
- Hot-air welder or torch, depending on the membrane
- Sheet metal snips, brake and hand seamer
- Caulking gun
- Chalk line
Materials
- Scupper with flange, sized for the opening, in clad or coated metal to match the membrane
- Collector box or conductor head
- Downspout and brackets
- Wood blocking or framing to support the opening
- Membrane flashing and patches
- Sealant approved for the membrane and metal
- Stainless or compatible fasteners
Safety first
- Scuppers are on the edge of the roof. Tie off before leaning over the parapet, and keep tools and offcuts from falling to the ground below.
- Cutting through a wall can hit wiring, pipes or structure. Check the wall before cutting and protect the area below.
- Metal flanges and cut edges are sharp. Wear cut-resistant gloves and avoid working alone on an exposed edge.
Step by Step
Step 1: Size the scupper and choose the location
Scupper openings are sized by a designer or plumber based on the roof area drained, the local design rainfall and the depth of water allowed to stand at the scupper. Common openings are 4 to 12 inches wide and at least 4 inches high, but the size depends on the design. Do not guess, because an undersized scupper lets water rise and overload the roof.
Place each scupper at a low point in the slope plan, where water will run to it from both sides, and away from structural columns and corners. Confirm with the structure that the wall can take an opening.
Step 2: Frame and cut the opening in the parapet
Mark the bottom of the opening at the finished surface of the membrane, not at the deck or the insulation, so water leaves at the roof's actual surface. Add a bit of extra for the membrane and the flange. The opening's sides and top are framed with blocking so the wall keeps its strength and the scupper has something solid to fasten to.
Cut the opening with the right saw for the wall, whether wood sheathing, steel stud or concrete, and check for wires and pipes first. Clean the cut edges and treat any exposed wood, as the flashing will cover them.
Step 3: Set the scupper flush with the roof surface
The scupper sleeve is slid through the wall with the roof flange flat on the roof side. Its bottom should be at the membrane surface, with a slight slope of about 1/8 to 1/4 inch toward the outside so water does not stay in the sleeve. A scupper set even a 1/2 inch high creates a permanent pond behind it.
Fasten the flange to the blocking with the approved fasteners, without piercing the area where the membrane will be welded or adhered. Sheet metal scuppers with a coating to match the membrane, such as TPO-coated metal, make welding possible. See how to install TPO-coated edge metal.
Step 4: Flash the scupper into the membrane
The membrane is lapped over the scupper flange, sealed to the metal and run into the opening so water leaving the roof travels over the membrane and onto the metal, never behind either. For single-ply, weld the membrane to the clad flange with a hand welder and roller, with laps at least 1-1/2 inches wide, and probe every edge. See how to hot-air weld TPO seams.
The corners of the opening are the weak point. Use pre-formed corners or round patches at each one. On torch-on roofs, a flashing piece is dressed into the scupper, as described in how to flash drains and scuppers in torch-on. Finish the wall side with counter flashing or sealant as the detail shows.
Watch out: Do not rely on sealant alone around a scupper flange. A caulked joint at a point where water is concentrated will fail. The flange must be lapped and fused or adhered.
Step 5: Install the overflow scupper
Where the roof is enclosed by parapets, an overflow is needed in case the primary drain blocks. The overflow scupper is usually set about 2 inches above the primary drain's surface, and its size should handle the full design flow. Building codes specify when it is required, so confirm with the current code.
The overflow should discharge where someone will see it, such as over a walkway or a door, not into a hidden space. The roof structure must carry the water depth at the overflow level, which is why the height is set by the designer.
Step 6: Fasten the collector box and conductor head
A collector box sits under the scupper and funnels water into a downspout. A conductor head is the decorative version. Both should be wider than the scupper so the water does not splash out, and fastened to the wall with sealed fasteners. Seal the joint between the box and the wall, since water running down the wall is one of the most common causes of staining and rot.
Attach the downspout with brackets, and check that it leads water away from the foundation. In freezing spells near Hope and in the canyon, discharge onto the ground can freeze and create an ice hazard, so direct it to a drain where possible.
Step 7: Test the flow
Hose water onto the roof near the scupper and watch. Water should run toward the scupper and through it without ponding behind it. If it stops, the scupper is too high or the slope is wrong.
Look inside the building below the scupper after the test. A drip at the opening means the flange or the wall seal has a gap. Fix it now, not after the first storm.
Scupper Sizing Basics
A scupper acts like a weir: the deeper the water, the faster it flows, so a scupper can be small if the roof is allowed to hold more water, and must be large if it is not. That is why the allowed water depth and the structure's capacity are part of the design. More scuppers along the parapet spread the flow and reduce the depth.
Design rainfall in coastal BC is high and storms can deliver heavy short bursts, so scuppers and overflows are sized for peak intensities, not average rain. The designer or plumber works from the code figures for the location.
Keeping Scuppers Working
- Clear debris: leaves, needles and moss collect at the opening and block it.
- Check the box and downspout: a blocked downspout backs water up into the scupper.
- Inspect the flange: look for lifted edges, cracked sealant and rusting fasteners once a year.
- Watch the wall below: streaks and staining show water is running where it should not.
Common Mistakes
- Scupper set too high. Even 1/2 inch above the membrane creates a permanent pond behind it.
- Undersized opening. Water rises during storms and loads the roof, and the overflow may not be enough.
- Sealant-only flange. A caulked flange at a concentrated water path cracks and leaks.
- Unflashed corners. The corners of the opening are where the membrane is most stressed.
- No overflow. A blocked primary drain can fill a parapeted roof with no warning.
- Collector box that is too small or unsealed. Water splashes onto the wall, stains the cladding and rots the framing.
When to Call a Professional
A scupper is an opening in a structural wall that has to be flashed, sized and sometimes approved against the building and plumbing codes. Cutting into a parapet, flashing it and coordinating an overflow is a job for a roofer and often for a designer.
Call a roofer if water is ponding behind a scupper, if the wall below is stained, if the collector box is pulling away or if you notice water backing up behind the parapet during storms.
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 Scupper and Collector Box: Questions
What is a roof scupper?
A scupper is an opening through a parapet or wall at roof level, usually lined with metal, that lets water leave a flat roof. Water flows through it to a collector box and downspout or straight to the ground. Scuppers are used as primary drains on roofs with parapets and as overflow drains beside interior drains.
How big should a roof scupper be?
Size depends on the roof area it drains, the local design rainfall and the depth of water the roof is allowed to hold. Common openings are 4 to 12 inches wide and at least 4 inches tall, but a designer or plumber should size them under the code. An undersized scupper is a common reason flat roofs fill during storms.
How high should an overflow scupper be?
The usual arrangement is to set the overflow scupper inlet about 2 inches above the primary drain's surface, with a size that can carry the full design flow. The roof structure must be able to hold the water depth at that level. The exact requirement is in the current building and plumbing codes.
What is a collector box on a scupper?
A collector box, or conductor head, is a metal box fastened to the wall under the scupper to catch the water and funnel it into a downspout. It should be wider than the scupper so water does not splash out, sealed to the wall, and connected to a downspout that carries water away from the foundation.
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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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