Also see "Roof Drainage".
An Introduction to Roof Scuppers
What is a scupper? According to the International Building Code, a scupper is “an opening in a wall or parapet that allows water to drain from a roof”. A scupper is almost always part of a peripheral drainage system, which is a drainage system in which the water drains off the sides of the roof, instead of through the roof itself using drains (an internal drainage system). Scuppers provide a pathway for water to flow through parapet walls or any other raised edge around a roof, such as a gravel stop.
Scuppers mainly come in two forms: channel-type (a 3-sided scupper, open at the top), or thru-wall (a rectangular 4-sided scupper, completely surrounded by the wall). Occasionally, you’ll see a round scupper or a decorative scupper* in some other shape. Thru-wall scuppers are typically lined with sheet metal such as galvanized steel, aluminum, or copper. This metal component is called a “scupper box”. The scupper box is integrated with the roof system (flashed, or “stripped in”) on the interior side of the wall, and usually terminates on the outside of the wall with a faceplate with sealant around it. 3-sided scuppers are also typically lined with metal, but the metal is integrated with the coping or other edge metal. Less commonly, other materials, such as liquid-applied waterproofing resin, are used. Often with single-ply membranes such as PVC, the same membrane material will completely line the scupper hole and terminate on the outside of the wall. The scupper may channel water into a conductor box with a downspout or a gutter, or, with overflow scuppers, it may let the water run down the side of the building.
Scuppers have one of two functions: they are either a part of the primary drainage system, or they are a part of the secondary (overflow) drainage system. The overflow drainage system is intended to function independently of the primary system and to drain water from the roof if the primary drains and scuppers become blocked.
There are different code requirements for scuppers depending on which role they play. Scuppers that are part of the primary drainage system are installed so that the bottom edge of the scupper holes are even with the surface of the roof at the lowest areas of the roof. Scuppers that are part of the overflow drainage system are also installed next to the lowest areas of the roof, typically very close to the corresponding primary drains or primary scuppers. The bottom edges of the overflow scuppers that you see on existing buildings today will almost always be about two inches above the adjacent roof surface. It’s important to note that the language regarding overflow scuppers in recent versions of the International Building Code actually states that “the quantity, size, location and inlet elevation of the scuppers shall be sized to prevent the depth of ponding water from exceeding that for which the roof was designed”. Which is not very helpful if you don’t have access to the structural design specifications for the building. In fact, the only unconditional guideline for scuppers provided by the code is that “scuppers shall not have an opening dimension of less than 4 inches (102 mm).” If you’re looking for technical guidelines for sizing and locating scuppers, FM Global’s “DS 1-54 Roof Loads for New Construction” (Data Sheet) is an excellent resource (see the link below).
Things you should know about roof scuppers:
Sealant Deterioration: The sealant around the flange or faceplate on the exterior wall surface outside of a thru-wall scupper is a maintenance item. It prevents water from getting between the metal scupper box and the wall masonry, and then backing up into the roof system and into the building. Sealant deteriorates over time. It’s important to check the condition of this sealant every year and replace it when it starts to go.
Clogged Drains: Scuppers that serve as primary drainage components typically direct water into a conductor box, downspout, or a gutter. You don’t normally want to see water running down the side of your building or shooting out into the air like a miniature waterfall. But you do if your drains are clogged. That’s how you can tell if your drains are clogged. Your view of the overflow scupper openings should be unobstructed. You want to be able to see if water is coming out of them, so you can tell right away if you need to get your primary drains cleaned.
Ice Build-Up: In colder climates, ice build-up at the scuppers in the winter can be a problem. Due to the fact that the scuppers are in the wall and not over the heated part of the building, they tend to be colder than the rest of the roof. Meltwater from snow on the roof may re-freeze at the scuppers and block water drainage, which could overload the roof. It’s important to have your scuppers checked and cleaned if this situation applies.
Re-Roofing: The original scuppers were designed for the original height of the roof. Energy codes demand thicker roof insulation these days, which means that if you are replacing your roof, the surface of the new roof will probably be higher than the surface of the old roof. This means that your primary thru-wall scuppers will be partially or even completely blocked by the new roof, and your overflow scuppers will no longer be at the proper height. In order to comply with the building code, as well as common sense, you are going to have to enlarge the holes. Additionally, the bottom edge of the overflow scuppers will have to be raised.
1. Building Codes: "1503.4.2 Scuppers" and "(P) 1503.4.1 Secondary (emergency overflow) drains or scuppers" in the 2015 International Building Code. Available on the UpCodes website.
2. Building Codes: "R903.4.1 Secondary (Emergency Overflow) Drains or Scuppers" from the 2015 International Residential Code. Also note exception 2 under subsection 1 under "Section R908 Reroofing". Available on the UpCodes website.
3. Technical: "Appendix B: Rates of Rainfall for Various Cities" on the ICC website provides the rainfall data for major US cities required for the sizing calculations mandated in the building codes. If your town is not listed, visit NOAA's Precipitation Frequency Data Server (PFDS) to look up the 100-year one hour rainfall data for your area.
4. Technical: For guidelines on proper sizing and placement of roof scuppers, see FM Global's DS 1-54 Roof Loads for New Construction (Data Sheet). You'll have to sign up in order to view data sheets on the FM Global website, but it's free and definitely worth your time.
5. Technical: An excellent technical article on scuppers is available here at the website of the Copper Development Association. Provides construction details for various scupper types. They talk about scuppers being made out of copper in that article, but that doesn't mean that scuppers have to be made out of copper or that the information provided is only applicable to copper scuppers. (Copper is a great scupper material, though).