Expert Installing Scuppers on Flat Roofs in Nassau County

A properly sized and installed scupper can move 250 to 400 gallons of water off a flat roof per hour-yet many Nassau County roofs rely on a single, undersized hole punched through a parapet that barely lets a trickle escape. That’s not drainage; it’s a leak waiting to happen. If you’re seeing standing water against your parapet wall after every storm, staining on your exterior brick, or water seeping into the wall core itself, you need scuppers that are correctly located, sized, and installed as part of a complete drainage system-not just an afterthought hole.

This guide walks you through how to install a scupper on a flat roof the right way: from mapping where water actually goes on your roof, to cutting through the parapet without compromising the wall, to tying in waterproofing membrane so the scupper itself doesn’t create the leak it’s supposed to prevent. Whether you’re retrofitting a leaky Baldwin two-family or designing overflow drainage for a new commercial roof in Garden City, the principles and sequence are the same.

Why Scupper Installation Is a System Problem, Not Just a Hole

Most building owners think of a scupper as a simple metal box or pipe through the parapet. That’s part of it. But after 23 years installing scuppers on roofs from Levittown ranch houses to three-story Mineola mixed-use buildings, I can tell you: scupper installation fails when you treat the outlet in isolation. Water doesn’t magically find the scupper. It has to flow there, and that means your roof needs slope, your scupper needs to sit at the low point, and the opening has to be big enough to handle the design storm intensity for Nassau County-which codes assume is about 3.5 inches per hour in a 100-year event.

On a Westbury multifamily I worked on two summers ago, the original builder had installed a 3-inch round scupper dead center in a 40-foot parapet wall. Roof sloped to all four corners. Water ponded in two corners, never made it to the scupper, and eventually found its way into the brick veneer and down the interior CMU. We ended up cutting two new scuppers at actual low points, sizing them at 4×5 inches each, and re-pitching the roof deck in cricket panels so water actually flowed where it needed to go. The scupper isn’t the system-it’s the outlet of the system.

That means before you cut anything, you need to understand:

• Where water ponds on your existing roof after a heavy rain
• How much drainage area each scupper will serve
• Whether you need primary scuppers, overflow scuppers, or both
• What size opening will handle your calculated flow
• How the scupper connects to a downspout or controlled discharge point below

Get any of those wrong, and you’re just adding a metal fixture to a roof that still holds water.

Step One: Map Your Roof and Choose Scupper Locations

Walk your roof after a rainstorm. Bring a camera. Look for where water collects-those are your low points, and that’s where scuppers should go. On a perfectly sloped roof, water flows to the perimeter and exits at predictable spots. On real Nassau County roofs-especially older tar-and-gravel or modified-bitumen installations that have settled or lost crickets-you’ll find ponds in random places: mid-span of a wall, back corners, near HVAC penetrations where deck framing sagged.

Each ponding zone needs its own scupper or a path to an existing scupper. If you’re working with internal drains and adding scuppers as backups, place the scuppers 2 inches above the roof surface so they only activate when drains are clogged or overwhelmed. If scuppers are your primary drainage-common on smaller residential and low-slope commercial roofs without room for internal piping-set them at the membrane surface or within a half-inch above it.

On a Rockville Centre flat-roof addition, the homeowner had a single scupper at one corner. Roof was about 20×30 feet. Problem was, the deck sloped slightly toward the opposite corner-water pooled there, eight feet from the scupper, never drained. We added a second scupper at the actual low point and tied it to a new leader on the side yard. Took one afternoon to install; ended five years of ponding and leak calls.

Spacing and drainage area: For primary scuppers, figure each one can handle about 400 to 600 square feet of roof in moderate-slope conditions (1/4 inch per foot). Steeper pitch, you can stretch that. Flatter or complex roof with multiple direction changes, cut it down. If your roof is 1,200 square feet, two scuppers placed at natural low points will move water faster and more reliably than one oversized unit trying to collect flow from across the entire perimeter.

Choosing Scupper Type and Sizing the Opening

Scuppers come in a few basic forms, and what you pick depends on your parapet construction and aesthetic goals:

• Through-wall scupper boxes: Sheet-metal boxes (usually 24- or 26-gauge galvanized, copper, or aluminum) that replace a section of the parapet. The box sits flush with interior membrane and exterior wall face. Most common on masonry parapets.
• Scupper tubes or sleeves: Round or rectangular pipes cast or cut through the wall, then flashed at both ends. Simple and cheap, but harder to waterproof correctly at the interior membrane connection.
• Conductor head scuppers: Scupper opening feeds directly into a decorative or functional leader head mounted on the exterior wall face, which then connects to a downspout. Clean look, keeps water off the wall below the scupper.
• Overflow scuppers: Secondary outlets set 2 inches or more above the primary drainage system. Required by code on roofs with internal drains to prevent catastrophic ponding if drains clog.

For sizing, the rule of thumb in the plumbing and roofing codes is one square inch of scupper opening per 150 square feet of roof area for primary drainage. That’s conservative-it assumes you’re handling about 4 inches per hour rainfall intensity. So a 600-square-foot roof section needs at least a 4-square-inch scupper opening. In practice, I never go smaller than 3×4 inches (12 square inches) on residential work because leaves, shingle granules, and membrane debris will partially block any opening over time. Bigger is almost always better, as long as you can fit it in the parapet and maintain structural integrity of the wall.

Overflow scuppers should be sized the same as primary scuppers. Some inspectors want them even larger. The idea is that if your primary system fails completely, the overflow has to handle the full design storm by itself.

How to Install a Scupper on a Flat Roof: The Step-by-Step Sequence

Once you know where the scupper goes and what size you need, installation breaks into five stages: cutting the opening, building or inserting the metal scupper body, flashing the interior connection to the roof membrane, sealing the exterior face, and connecting the discharge system below. Do these out of order or skip a detail, and you’ll have a beautiful scupper that leaks at every seam.

Cutting the Parapet Opening

Measure and mark your scupper location on both the interior roof side and exterior wall face so the opening aligns. If you’re working with a brick or block parapet, use a concrete saw or angle grinder with a diamond blade to cut the opening. Cut slightly oversized-about a half-inch larger than your scupper box on all sides-so you have room to fit the metal and apply sealant. On wood-framed parapets (common on older Long Island residential flat roofs), you’ll cut through plywood or board sheathing, then frame the opening with blocking so the scupper box has solid nailing surfaces top and bottom.

Important: if you’re cutting through a load-bearing masonry wall, consult a structural engineer or at minimum install a lintel above the scupper opening to carry the load. I’ve seen DIY scupper installations where the homeowner cut a 6×8-inch hole in a single-wythe CMU parapet with no lintel, and within two years the block above the scupper cracked and dropped. On most residential parapets under 3 feet tall, a simple steel angle lintel or pressure-treated wood header is enough, but don’t guess.

Installing the Scupper Box or Sleeve

Slide your fabricated scupper box into the opening. The interior flange-usually 4 to 6 inches wide-should sit flat on the roof membrane surface or on the substrate if you’re installing the scupper before the final membrane layer (which is often easier and cleaner). The box should extend all the way through the parapet thickness and project about an inch past the exterior wall face so water clears the wall and doesn’t run back down the brick.

Secure the scupper to the wall. On masonry, I use Tapcon screws through pre-drilled holes in the metal flange into the block or brick. On wood framing, exterior-grade screws into blocking. Some contractors rely on sealant alone-don’t. Wind-driven rain and ice expansion will push an unsealed, un-fastened scupper right out of the wall. Use mechanical fasteners, then back them up with sealant.

If you’re using a simple pipe sleeve instead of a formed box, the same rules apply: tight fit in the opening, mechanical attachment at both ends, and a wide flashing flange on the interior side to tie into the membrane.

Flashing the Membrane Connection-The Most Critical Detail

This is where most scupper leaks originate. You have a metal object penetrating your waterproof membrane, and unless you tie that metal into the membrane system with proper flashing, water will wick along the seam and into the parapet core or down the wall cavity.

Here’s the correct sequence for modified bitumen or TPO/EPDM single-ply systems:

Modified bitumen: After the scupper box is in place, apply a base layer of modified bitumen over the interior flange, lapping at least 4 inches onto the roof membrane in all directions. Torch or cold-adhere depending on your system. Then apply your finish cap sheet over that, again lapping well past the flange and up onto any adjacent parapet flashing. The scupper flange is now sandwiched between two membrane layers. Seal the top edge of the flange where the cap sheet terminates with a heavy bead of compatible mastic or lap sealant.

TPO or EPDM: Use factory-fabricated inside and outside corner patches or cut your own from sheet stock. The scupper flange needs a membrane strip heat-welded (TPO) or bonded with splice adhesive (EPDM) that extends 6 inches in all directions. Then your field membrane laps over that and is welded or glued down. Finally, a termination strip or coping flashing covers the top edge of the scupper flange where it meets the membrane. Every seam that touches the scupper should be probed and tested before you call it complete.

On a Hempstead commercial job, the original contractor installed four scuppers with no membrane flashing-just pushed the TPO up to the metal edge and caulked it. Within six months, water was running down the interior face of the parapet every time it rained. We had to cut out every scupper, re-detail the openings with proper TPO inside and outside corners, and re-weld the field sheet. Cost the owner three times what it would have cost to do it right the first time.

Sealing the Exterior Wall Face

On the outside of the building, the gap between the scupper box and the masonry or siding needs to be sealed to keep wind-driven rain and insects out of the wall cavity. Use a high-quality polyurethane or silicone sealant around the entire perimeter of the scupper where it meets the wall. If you’re working with brick, tool the sealant into a neat fillet; if it’s stucco or siding, a clean bead is fine.

Some installations benefit from a metal trim plate or escutcheon on the exterior face-essentially a decorative cover that hides the rough cut edges and provides a finished look. Copper scupper heads with a rectangular trim plate are popular on higher-end residential jobs in Great Neck and Manhasset.

Connecting the Downspout or Leader

A scupper without a controlled discharge just dumps hundreds of gallons of water against your foundation or onto a walkway. You need a leader, downspout, or conductor system that carries water away from the building. Options include:

• Direct-connect downspout: A rectangular or round downspout attaches to the exterior face of the scupper and runs to grade or into an underground drainage system. Most common and code-compliant.
• Conductor head or collector box: A decorative or functional box mounted just below the scupper that funnels water into a smaller downspout. Slows the flow, filters debris, and gives you a place to inspect and clean the system.
• Scupper extension or spout: A short metal chute that directs water away from the wall face and into a gutter or splash block below. Only works if you have clearance and a safe place for water to land.
• Through-wall leader: On buildings with interior chases or room for plumbing, you can run a leader down inside the wall and exit at grade through a separate outlet. Cleaner exterior appearance but more complicated installation.

Whatever you choose, make sure the connection between scupper and leader is sealed and mechanically fastened. I’ve been called to jobs where the downspout was just friction-fit into the scupper and slid off during the first heavy rain, dumping water straight down the wall.

Primary vs. Overflow Scuppers: Why You Need Both on Some Roofs

If your flat roof has internal drains as the primary drainage system-common on commercial buildings and larger multifamily properties-code requires overflow scuppers as a backup. These overflow outlets must be set at least 2 inches above the normal roof surface (the low point where your internal drains sit), so if the drains clog with leaves or ice, water can still escape before it reaches dangerous ponding levels.

The math: 2 inches of water on a 1,200-square-foot roof is 1,500 gallons and weighs about 6 tons. That’s enough to overstress roof joists, cause progressive collapse in timber construction, or blow out parapet walls. Overflow scuppers prevent that catastrophic scenario by giving water a secondary path once it rises above a safe depth.

Install overflow scuppers exactly the same way you’d install primary scuppers-through-wall box, membrane flashing, exterior sealing, connected downspout-but position them higher. I typically set the bottom of the overflow scupper opening 2 to 3 inches above the roof membrane. Some engineers spec 4 inches on critical structures. Mark them clearly (“Overflow” or “Emergency Drain”) so maintenance crews know they’re only supposed to flow during abnormal conditions and can investigate why the primary system failed.

Common Scupper Installation Mistakes and How to Avoid Them

Undersizing the opening. A 2-inch pipe through the parapet looks like it should drain a roof, but it can’t handle the volume during a real storm. Water backs up, ponds, and finds other ways out-usually through your membrane seams or down into the wall. Stick to the 1 square inch per 150 square feet rule minimum, and go bigger when in doubt.

Installing scuppers where water doesn’t actually go. I see this constantly: scupper placed for aesthetic symmetry or because “that’s where the downspout is,” not because water flows there. Map your ponding zones first, then put scuppers at those low points even if it means adding a new downspout on an inconvenient wall face.

Skipping the membrane tie-in. Caulk is not flashing. If you don’t integrate the scupper flange into your roofing membrane system with overlapping, sealed layers, water will migrate along the metal-membrane interface and leak. Every. Single. Time.

No structural support over the opening. Cutting a hole through a masonry parapet without a lintel above it invites cracking and settlement. Even on wood parapets, you need blocking or headers to carry loads around the opening.

Discharging onto the building or adjacent property. Some installers let scuppers just dump water onto a lower roof section, a porch, or even the neighbor’s driveway. That’s a code violation in most cases and a liability problem. Run a leader to grade or an approved drainage point.

When to Call Platinum Flat Roofing for Scupper Installation

If you’re dealing with persistent ponding, wall leaks below your parapet, or you’re planning a flat-roof replacement and need scuppers designed into the system from the start, we’ll evaluate your roof drainage as a whole: slope, existing outlets, membrane condition, and parapet construction. We fabricate custom scupper boxes in copper, galvanized, and aluminum; cut and flash them to match your membrane system; and size every opening based on calculated roof area and Nassau County storm intensity, not guesswork.

We also retrofit overflow scuppers on roofs where internal drains have proven unreliable-common on older buildings where drain lines are undersized or prone to freezing. And if you’re seeing water damage on your exterior brick, stucco, or siding near the roofline, we’ll trace it back to see whether your existing scuppers are too small, improperly flashed, or discharging back onto the wall instead of safely away from it.

Scupper installation isn’t glamorous work, but it’s some of the most important waterproofing you’ll do on a flat roof. Done right, scuppers move water fast, protect your building envelope, and never show up on your maintenance list. Done wrong, they’re just another place water gets in. After two decades of installing and fixing these systems across Nassau County, I can tell you the difference comes down to design, sequence, and details-especially that membrane tie-in that nobody sees once the job is finished but makes all the difference when the next Nor’easter rolls in.