What Is the Difference Between Drains and Scuppers On a Flat Roof?

Historical Structures

Many commercial structures still in use today were originally designed with both internal drains and scuppers. Most historical structures were built before K-style aluminum guttering was available, so many used the trough, aka half-round, style of guttering.

In busy metropolitan areas where the buildings literally touch each other, internal drains and scuppers were often used to divert rainwater away from the front entrance. Even a hundred years ago these were still made from copper, and many are today.

Depending on the time period, internal drain systems were sometimes cast from concrete or cast from molten iron and sealed with molten lead.

Internal Roof Drain:

• Collects rainwater to a central location and drains the water to a downspout or drainpipe for safe transport to the ground.
• Points upward through the roof.
• Usually hidden from view.
• Best for roofs that slope towards the center of the roof, or a central location, like below a valley where the water naturally collects.

Scupper:

• Collects runoff and directs it to a central exit location, and discharges parallel to the ground.
• Points outward, away from the building.
• Usually expels the runoff directly to the ground, or leader-head, which is connected to a downspout.
• Always installed externally, and usually as part of, or attached to, an exterior wall.
• Not hidden from view, but located away from areas of pedestrian traffic to avoid flooding.
• Best for roofs that slope away from the center of the roof.

 

How Do Internal Roof Drains Work?

Both internal drains and scuppers require that the roof have a slight pitch, known in the roofing industry as a “low slope” roof. A “flat” roof is flat, but also has a pitch (also known as slope) of less than 3:12. This means the roof is never “level”, but rather slightly sloped.

The slope (or pitch) can be so slight that a person walking on the roof may not even realize they are walking uphill or downhill. This is why these roofs are known as “flat” by most people, but when shopping for materials look for “low slope” in the description.

This slope can be created in a number of ways, including standard carpentry, but the most common way is with tapered insulation panels. These panels are designed and installed as an interlocking system that both creates the required slope, and insulates the roof at the same time.

The slope represents a ratio of rise over run, which is the ratio of how high the roof is in relation to its length. Here’s an example:

• If the roof measures 30’ from the ridge (the uppermost part of the roof section), and 15’ from the top of the wall the roof sits on, we can calculate that the roof pitch is fairly steep at 6:12 (15 divided by 30 = ½).
• This means the ratio is ½ to 1, so for every 6 inches the roof rises, it extends 12 inches in length.
• We would say this roof has a sloped roof, with a pitch of 6:12. As mentioned earlier, if the roof slope is shallower than 3:12, the roof is considered “low slope”.

Low-slope (Flat Roof) Materials Are Different Than Materials For Sloped Roofs

Low slope roofing materials are designed to prevent curling and uplift, two main reasons for roof leaks. Low slope materials are designed to seal watertight, because water can curl under the material when it is moving slowly, or puddling.

Sloped roof materials also seal, they are not watertight. Sloped roof materials, like shingles, can shed water by using gravity to pull the water towards a gutter. However, the slower the water flows from the roof, the more likely the water is to curl back up under the shingle.

This is why sloped roofing materials, like shingles, shakes, and tiles, cannot be used on a low slope roof. Some materials are approved for a slope as low as 3:12, but most require at least a 4:12 pitch. A roof with a slope of less than 3:12 will always be considered low slope, so always use low slope materials, like ModBit, EPDM, TPO, PVC, or hot mopping.

 

What Building Codes Apply to Scuppers and Internal Drains?

Building codes usually dictate how many drains (or scuppers) are needed to effectively channel runoff from the roof safely to the ground based on total area. However, these codes mostly apply to commercial structures.

Many residential structures are not required to have drains, scuppers, or even gutters. However, erosion, groundwater contamination, and other maladies are regulated by stormwater management agencies.  Building code officials can step in if a homeowner’s irresponsible rainwater management causes damage to a neighboring property.

In such cases, a homeowner can be held liable, so managing rainwater is a great idea even when it isn’t legally required. Plus, damage to landscaping and foundations can occur when runoff is allowed to flow uncontrolled.

Flat roofs aren’t just for commercial properties. This residential roof has inadequate drainage, leading to water ponding.

 

 

How Do Scuppers Work?

Scuppers function more like a gutter than an internal roof drain, as scuppers are usually connected to some type of channel that pushes the runoff towards the scupper. This is why scuppers are located on the outside of the roof, often to the closest adjacent wall.

Scuppers are often painted, along with the downspout, to blend into the color of the wall. Scuppers are typically made from lightweight metal or plastic, as these are easiest to install and the least expensive.

However, the scuppers will need to be sized according to the roof area, as too much runoff can cause a backup where the leader-head meets the downspout. If the scuppers are unable to handle the large water volume water will pool up on the roof. Scuppers should be sized to meet the downspout without any reducers, as a reducer coupling could create a bottleneck.

Scuppers can take almost any shape as long as the inner part of the scupper is large enough to handle the runoff. As the roof size increases, the scuppers must either be enlarged, or the number of scuppers must be increased. When this is not feasible, many builders install a combination of internal drains and scuppers to accommodate the design or cost limitations.

In popular culture, scuppers are sometimes shaped into the head of a gargoyle, so that when it rains the water shoots from the gargoyle’s mouth. Scuppers can appear as a simple spout, or be concealed within a leader-head, which turns the water from horizontal to vertical, in the direction of the downspout.

 

When Is an Internal Drain Best For a Flat Roof?

As mentioned earlier internal drains are designed to accept runoff from the entire roof, which slopes slightly toward the drain, or drains. Often, this is done to prevent runoff from falling down over a doorway where customers may be walking.

Other situations, like having an obstacle in the path of the water, are perfect for internal drains because it’s usually easier to move a drain than to change the roof structure. Internal drains are usually fitted with a cleanout, which is a removable plug used to access the pipe for cleaning.

Internal roof drains are also ideal when discharging the runoff through a scupper is neither attractive, nor large enough, to control the amount of water. Internal roof drains can be essentially any size and installed in any number, making them a safe option for most flat roof styles.

The anatomy of the roof structure will often determine where the drains are located and where they empty. Ideally, the pipe attached to the drain will be as straight as possible. However, fittings can be used to avoid an obstacle, such as a framing brace.

 

When Is a Scupper the Best Option For a Flat Roof?

Scuppers are often a backup option for builders when the design or traffic flow permits the runoff to flow freely from the roof. Scuppers are generally less expensive to install and maintain, so builders use them whenever possible.

Scuppers are convenient for buildings with difficult attic access. If an internal drain fails, or becomes clogged, sometimes the easiest access to the inside of the pipe is from the attic or a chase.

Scuppers can be accessed much easier in most instances, even if the repair requires a mechanical lift. Many scuppers can be accessed via a ladder or bucket truck, making cleaning much faster and simpler.

Scuppers are not appropriate for areas where foot traffic may be present below, nor when the runoff will cause erosion of the ground surface. For example, a scupper in the wrong location might dump water across a sidewalk, which can then freeze and become a fall hazard.

They are ideal, however, when the structure is adjacent to an area where the discharge will not cause erosion, nor risk falling on pedestrians. Generally, parking lots are designed to prevent flooding by incorporating storm drains into the asphalt.

Strip malls, for instance, usually aren’t as concerned about the appearance of the rear side of the building as the front. Often, these buildings use scuppers on the rear of the building where shoppers won’t be walking.

 

What Type Of Flat Roof Material Can Be Used With Internal Drains and Scuppers?

There are many types of flat roof materials used in a commercial setting. EPDM, Modified Bitumen, TPO, and PVC roof membranes are commonly used in conjunction with internal drain systems. Scuppers aren’t connected to the roof membrane, so they won’t affect which type of material you choose.

EPDM (ethylene propylene diene monomer)

EPDM is a type of synthetic rubber membrane sold on a roll. EPDM is typically adhered to the roof with adhesive, instead of mechanical fasteners, like staples.

TPO (thermoplastic polyolefin)

TPO is a membrane like EPDM, but TPO has features EPDM does not, like resistance to airborne droplets of oil. TPO is usually white, while most other roofing membranes are darker in color. This helps TPO keep the roof cooler, as it reflects more of the UV radiation from the sun.

PVC (polyvinyl chloride)

PVC is a roofing membrane designed for heavy duty use. PVC roofing membrane is made from the same base material as water pipes, so PVC is excellent at repelling water. PVC is also more durable than most roofing membranes, and can last 30-40 years with minimal maintenance.

ModBit

ModBit is the shortened phrase for “modified bitumen”, which is similar to roll roofing. ModBit comes on a much smaller roll than TPO or PVC roofing membranes, and is installed like roll roofing in courses. The  main difference between roll roofing and ModBit is that ModBit is peel and stick, while roll roofing is installed with roofing tacks.

Roll Roofing

Roll roofing is made from the same material as fiberglass/asphalt shingles, but on a roll instead of in the shape of a shingle. Roll roofing can be installed much faster than shingles because the roll is simply rolled out on the roof deck and stapled or tacked into place.

Hot Mopping

Hot mopping, a traditional method of applying molten tar to a mixture of tar, fiberglass, and gravel to the roof deck, also accepts either internal drains or scuppers. Hot mopping is very effective, but the job is very hot, dangerous, and requires more upkeep than any type of roofing membrane. In fact, some of the oldest roofs still in use today were originally hot mopped, and integrated with internal drains and/or scuppers.

However, for newer builds or replacement roofs, hot mopping has largely been replaced with the membranes mentioned before, such as EPDM and TPO, as these more modern materials require less maintenance and last longer.

Ballast

A ballasted roof relies on placing heavy weight on the roofing membrane to prevent wind uplift, instead of adhesive or mechanical fasteners. Ballast is typically crushed stone, which is evenly distributed across the surface of the membrane. Ballasted roofs are very durable because there are no adhesives, nor fasteners, that could eventually fail and cause a leak.

Whether the roof is ballasted or not, all of these low slope roofing membranes are designed to work with internal drain systems. Some are attached only around the perimeter, and objects on the roof, while others are fully adhered to the roof decking.

What Is Roof Ballast?

Some flat roof designs use adhesive or mechanical connectors (like staples) to attach the membrane to the roof deck (also known as roof sheathing). Ballasted roofs don’t use either, and instead rely on weight, or ballast, to hold the membrane to the roof deck

Ballasted roofs have the advantage of easy repair, but the additional weight of the ballast requires special reinforcing of the roof structure. Ballasted roofs are ideal when the main concern for the roof is to avoid any perforations in the roof deck that could later become a leak.

Ballasted roofs are ideal for an internal drain system, as both technologies have been time tested to work together for many decades. Some ballasted roofs built over a century ago are still perfectly maintained today, and many still use the original cast iron internal drain system as well.

It’s important to note that the framing for ballasted roofs must be designed for ballast. Ballast is literally dead weight, so if the roof structure was designed for shingles, the roof will sag under the weight. Ballasted roofs are reinforced from the inside to distribute the weight more evenly without collapsing.

 

Who Installs Scuppers and Internal Drains On a Flat Roof?

Because of the pipes involved it may seem logical that a plumber would erect an internal drain system, but the work is actually performed by specialized roofing contractors. Scuppers are also often installed by these highly trained technicians after the roof has been constructed.

To be clear, an internal drain system can be roughed in before the roof is constructed when necessary, but accurate design and installation is critical. The contractors building the exterior walls will ensure no framing members are in the way of the scupper, and create an opening in the exterior facade.

 

What Are The Next Steps Or Resources If I Need Help?

If you would like to investigate internal roof drains and scuppers further, the best way is to schedule a visit with a quality local roofing contractor. In the Kansas/Lower Midwest region, if you choose to talk to Rhoden Roofing, we can help ensure you understand your options and make recommendations based on years of experience with flat roofs.

If you prefer, you can request an in-person appointment. We can answer any questions you have, and maybe a few you haven’t!