Drains & Scuppers



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As seen in JLC Magazine


In my business installing waterproof decks, I often get calls to fix decks that are leaking or coming apart. A lot of times these are jobs are familiar – they are ones we lost the bid on because we priced the job so we would have time to get the details right. The worst problems often involve drains and scuppers. We see problems with drip edges on open decks, too, but it takes much longer for damage to occur at the perimeter. Edge flashing just doesn’t see the amount of water that drains and scuppers do. But an enclosed deck that drains to a scupper or floor drain is essentially a big bathtub. The whole deck surface is pitched towards these exit points. If there’s a leak near one, it can quickly dump a lot of water into the framing , leading to some pretty spectacular problems.


Photo (1) above shows significant damage to wood framing from water intrusion at a through-wall scupper. Poor detailing, including a lack of an exterior counterflashing allowed water to leak in from both sides …. [more about this particular disaster: how old was this deck? Is this case where bad lathing near the penetration contributed? wasn’t sure …]

For this repair, the customer insisted on copper, which we will only use if requested, and we disclaim responsibility for it in our contract. Even when it’s installed correctly, it is subject to fail, because the characteristic green tint that makes copper so attractive for many finish applications, will eventually get under the polymer waterproofing and cause it to debond. The green patina forms when acids in rainwater or polluted air comes in contact with exposed copper surfaces. This reaction happens even more quickly in salt air. While the acid and salts are neutralized during the chemical reaction with the copper, the copper oxidizes at the edges of the polymer coating, and begins to separate the waterproof coating from the metal. This allows more moisture in, and oxidation continues to works its way eventually separating the polymer from the metal.

The other two options are stainless steel or bonderized steel. Stainless holds up in salt air or an urban environment, but requires a careful sanding with 80- to 120-grit paper first. We use marine-grade 316 stainless, which holds up better than 304. But 316 has a harder surface, which makes scuffing more challenging; it’s time intensive.

Bonderized steel is a form of galvanized steel that has been put through a phosphate bath, which allows it to bond to a coating. Don’t use shiny galvanized material. It is covered with a thin layer of form oil, which causes polymer waterproofing to peel right off. The bright, shiny stuff can only be used if you age it for six-months in the weather (which isn’t likely) or etch the surface with acid (which is difficult to do safely on site). With a bonderized material, however, there is no other prep work than cleaning off any dirt or dust before applying the primer coat.

Regardless of the material, it’s critical to integrate the scupper assembly with L-flashings and cladding materials. Note in the photo (2) that the L-flashing laps over the scupper flanges, and these laps are bedded in a polyurethane caulk. Prior to applying our primer and waterproof top coating, we sand the interior of the scupper to promote adhesion of the coatings to the metal. This will avoid the coating from debonding as the copper oxidizes as it has in photo (3)

The exterior side has been counter flashed as a best practices method. (4) We have nailed it at the top, but not the bottom. The contractor installing a water resistive barrier can can slip that layer under the shield and trim his paper down over the front of the shield, providing a highly leak resistant barrier.


Leaks at drains often occur because the contractor used the wrong assembly, such as a toilet closet flange (4) or a plastic shower drain (5). Neither type can be adequately flashed to the deck. Instead contractors usually just pump a lot of caulk around them and hope for the best. Eventually they leak.

4. 5.

We replaced both of these drains with commercial deck drain assemblies, like the ones shown in the photos (6 and 7). Here, the wide copper flange can be integrated with layers of waterproofing, and because this flange is never exposed to the elements, it won’t oxidize so the polymer bond stays intact over time.

Care needs to be taken to integrate the flange with metal lath, which is commonly used with cementitious waterproofing systems. The metal lath needs to isolated from the copper to prevent galvanic corrosion. The flange is first coated with primer – a diluted mix of the polymer additive used for the cementitious waterproofing, and the metal lath cut around the flange (7). We bridge between the flange and lath with the Panzer mesh. If bonderized steel L-flashing lap the copper drain flange, the two metals should be separated with a peel-and-stick membrane. Ideally, L-flashings should lap over the drain flange but in retrofit repair work, we will sometimes compromise if the existing flashing

We nail off the flange with use 1 1/4″ coil gun ring-shank stainless steel roofing nails. These provide considerably more holding power than the typical smooth-shank roofing nail (8). And we secure the lath with 1-inch-crown stainless steel lathing staples. Stainless fasteners are hard to find in my area. We order them online as we need them from Fastener USA (www.fastenerusa.com).

After everything has been tied in securely, the top coating can be troweled over the lath and finished to the drain grate (8).

Difficult Details #1: Scuppers
by Robert Thomas
October 19, 2000

Scuppers are a type of true-wall penetration that functions as a drain for water. Usually this is for rain water, but it can also serve for water occurring by hosing off a deck or roof. Often, scuppers go through the wall of a deck that has EIFS on both sides, or through a parapet, which has EIFS on both sides. This article will provide you with some insight how to properly handle this common detail. You can keep this list and use it as a reminder when designing or bidding EIFS projects.

Scupper time 

Scuppers in EIFS walls are very difficult to seal because the wall is often hollow (made of studs and sheathing), and it is difficult to “marry” the complex scupper flashings into the EIFS coatings. Hence, if the scupper leaks, it gets into the EIFS wall construction, and may also get into the soffit below (if there is one). This can create havoc, as often decks on multi-unit condos or apartments are stacked-up for many stories, and if one leaks, well …




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