Roof Scuppers: Facts and Guidelines

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Roof Scuppers: Facts and Guidelines

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What is a Roof Scupper?

The International Building Code defines a scupper as “an opening in a wall or parapet that allows water to drain from a roof”.

Roof scuppers are occasionally referred to as “scupper drains”, for obvious reasons. Scuppers with spouts that project out beyond the wall are known as “roof canales” in the American Southwest, where they are often used for rainwater collection.

Scuppers provide a pathway for water to flow through parapet walls or any other raised edge around a roof, such as a gravel stop edge metal.

Primary scuppers normally channel water off the roof into a conductor box that connects to a downspout or a gutter. Overflow scuppers will let the water run freely down the side of the building.

A scupper is usually 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).

Where roof sections on the same building are separated by walls, there will often be scuppers in the walls that drain water from one roof section to another.

Scuppers through a wall separating two roof sections on the same building
Scuppers through a wall separating two roof sections on the same building

Types of Roof Scuppers

Scuppers mainly come in two forms: open scupper (a 3-sided scupper, open at the top, also called a “channel-type” scupper), or through-wall scupper (basically a hole in the wall, usually rectangular). Occasionally, you’ll see a round scupper or a decorative scupper* in some other shape.

Thru-Wall Scuppers

Through-wall (or “thru-wall”) scuppers are typically lined with sheet metal such as galvanized steel or copper. Aluminum is not normally used, because it’s more difficult to form and solder aluminum on site than other metals used in roof construction.

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.

A thru-wall overflow scupper on a (really dirty) TPO roof
A thru-wall overflow scupper on a (really dirty) TPO roof

Channel-Type Scuppers

Channel-type scuppers are also typically lined with metal, but the metal is integrated with the coping or other edge metal as well as being flashed into the roof membrane.

Less commonly, other materials, such as liquid-applied waterproofing resin, are used to line the scupper hole. Often with single-ply roof membranes such as PVC or EPDM, the same membrane material will be used to completely line the scupper box, bonded to the metal.    

A channel-type overflow scupper on a ballasted EPDM roof
A channel-type overflow scupper on a ballasted EPDM roof

Roof Drainage Systems and Scuppers

Scuppers will play one of two roles in a roof system: they will either be a part of the primary drainage system, or they will be a part of the secondary (overflow) drainage system.

primary drainage system will simply be designed to remove water from the roof as quickly as possible.

An overflow drainage system will be designed as a complete second drainage system which functions independently of the primary system.

Its purpose is to drain water from the roof if the primary drains or scuppers become blocked so the roof structure doesn’t become overloaded by the weight of the water. Overloading can lead to deformation of the roof structure or even roof collapse.

Location of Primary Scuppers

Primary scuppers take the place of internal roof drains (for various reasons).

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.

They are located in the parapet wall next to the lowest points of the roof.

Location of Secondary (Overflow) Scuppers

Scuppers that are part of the overflow drainage system are also installed at the lowest points of the roof, but slightly higher than the level of the roof surface.

They are typically installed at a point in the parapet wall that is as close as possible to the primary drain for that area. The overflow scupper is located as close to the primary drain as possible so that it will drain the same roof area that the drain does if the drain becomes clogged.

If there is a primary scupper instead of a drain, the overflow scupper will be right next to it, but about two inches higher.

Sometimes water can’t flow directly to the wall because of an obstruction (such as a curbed expansion joint, for example) between the primary drain and the wall. In this case the overflow scupper will be installed in the nearest, lowest part of the wall that water can directly flow to from the drain location.

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.

This is to prevent overflow scuppers from acting as primary drains.

Overflow scuppers should only drain the roof if the primary drains or primary scuppers are blocked and water is ponding on the roof as a result.

A primary scupper in a parapet wall with a conductor head and a downspout, and an overflow scupper.
A primary scupper in a parapet wall with a conductor head and a downspout, and an overflow scupper.

Roof Scupper Code Requirements

There are different code requirements for scuppers depending on which role they play.

It’s important to note that the language regarding both primary and overflow scuppers in the most recent version of the International Building Code states that “the quantity, size, location and inlet elevation of the scuppers shall be chosen to prevent the depth of ponding water on the roof from exceeding the maximum water depth that the roof was designed for as determined by Section 1611.1 of the International Building Code”.

This is not very helpful if you don’t have access to the structural design specifications for the building. The two-inch height was a rule of thumb in the industry for decades, and is still considered standard by most industry professionals.

Roof Scupper Size

The only unconditional guideline for scupper sizing provided by the code is that “scupper openings shall be not less than 4 inches (102 mm) in height and have a width that is equal to or greater than the circumference of a roof drain sized for the same roof area.”

So we can at least be certain that round scuppers can’t have a diameter of less than 4 inches, and rectangular scuppers can’t have any side be less than 4 inches.

If you do know the diameter of a properly-sized drain for a roof area, however, you can easily calculate the required size of the equivalent scupper using the guideline above.

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

Scupper Maintenance

Sealant Deterioration: The sealant around the flange or faceplate on the exterior wall surface around a thru-wall scupper is a maintenance item.

Sealant deteriorates over time. It’s important to check the condition of this sealant every year and replace it when it starts to go.

This sealant plays a critical role in preventing roof leaks. It keeps water from getting between the metal scupper box and the wall masonry, and then backing up into the roof system and into the building.  

Looking straight down at the exterior faceplate of a thru-wall roof scupper. You can see the deteriorated building sealant and why it might be a problem.

Overflow Scuppers Can Tell You if the Drains are Clogged

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).

If interior roof drains are your primary drainage system, and the scuppers are your secondary drainage system, you do want to see this water coming out of the scuppers, because it lets you know that your drains are clogged.

This is why overflow scuppers are basically just holes in the wall without a conductor head and downspout. Your view of 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 Can Block Scuppers

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.

Scuppers and Roof Replacement

Re-Roofing: The existing scuppers in a parapet wall will have been designed for the height of the original (or previous) roof system.

Recent changes to energy codes demand higher R-values, and therefore thicker roof insulation, 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 probably going to have to raise the height of the scupper holes when you install a new roof.

Related Pages

1. Building Codes: “1502.3 Scuppers” and “1502.2 Secondary (emergency overflow) drains or scuppers” from the 2018 International Building Code. Available on the UpCodes website.

2. Building Codes: “R903.4.1 Secondary (Emergency Overflow) Drains or Scuppers” from the 2018 International Residential Code. Also note exception 2 under subsection 1 under “Section R908 Reroofing“. Available on the ICC website.

3. Building Codes: “Secondary Drainage and Ponding Requirements in the IBC and IEBC” is a good place to start if you’re trying to understand code requirements regarding scuppers as dictated by the 2015 International Building Code (including major changes from previous versions of the IBC). Article is from 2017 and is on the IIBEC (formerly RCI) website.

4. 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.

5. 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. 

6. Technical: An excellent technical article on scuppers 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). 

7. Technical: Construction Details: See these scupper/roof tie-in details for various roof types in the technical database at the Firestone Building Products website.

8. Technical: Basic instructions for the installation of a thru-wall scupper box are available at the website of Metal-Era.

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