How Overflow Scuppers Save Flat Roofs from Collapse

Photo source: www.reddit.com/r/CrappyDesign

The primary drainage system of a flat roof can fail due to debris clogging it or because it can’t withstand an intense downpour. Whatever the reason, water will pool, which can collapse your roof. Because of this, secondary (emergency overflow) scuppers are required by the IPC (International Plumbing Code) and IBC (International Building Code) for roofs with parapets. 

The main function of overflow or secondary scuppers is to back up the primary one by draining rainfall water to prevent it from overloading your roof.

How Overflow Scupper Drains Prevent Roof Collapse

The main goal of overflow scuppers is to drain water to prevent a roof’s structural failure. Their discharge also alerts that the primary scuppers are clogged or poorly sized.

The above is why overflow scuppers must not drain into downspouts, as stated in the IPC (International Plumbing Code), section 1108.2.

The flowing water is a warning signal to clear debris blocking the primary scuppers or vertical drainage immediately.

Photo source: www.reddit.com/r/CatastrophicFailure

Overflow Scuppers Adequate Positioning and Size

It’s not the intent of this article to discuss the design of overflow scuppers in depth. However, you should at least know the following:

Overflow scuppers must be next to the primary ones because:

• Those are the lowest points of a flat roof.
• It lets you quickly locate and clear a clogged primary scupper.

Regarding their size, the UBC (Uniform Building Code), section 1506.3, states that overflow scuppers shall be three times the size of the primary ones. However, many old buildings have undersized primary drains, so following the UBC recommendations would also lead to undersized secondary scuppers.

According to codes such as IPC, IBC, and FM Loss Prevention, to prevent the depth of ponding water from exceeding that for which the roof was designed:

• The size of overflow scuppers must be able to handle the anticipated rainfall event (considering the primary scupper or vertical drains are 100% clogged).
• Their inlet elevation shall be not less than 2 in. nor more than 3 in. above the low point of the adjacent roof surface.
  

Photo source: www.reddit.com

Choosing the Overflow Scupper Material

Copper and aluminum are the most common materials for scuppers because they are corrosion-resistant.

Choosing between one and the other depends on:

• The ability to create an oxide layer when exposed to air, moisture, rain, or seawater in a particular environment.
• The roof material where they’ll be because of dissimilar metals. Dissimilar metals have significantly different properties that react when they come into contact, causing galvanic corrosion.
•  Our article “What is a Box Scupper? What You Need to Know” delves deeper into dissimilar metals.

Installing Overflow Scuppers

Install overflow scuppers the same way as box scuppers:

1. Cut a hole in the parapet wall at the right height and diameter.
2. Install membrane or base sheet over the roof and parapet per the manufacturer’s instructions.
3. Cut through the membrane to expose the hole for the overflow scupper.
4. Apply sealant around the hole entrance as an adhesive for the back of the scupper flange.
5. Insert the scupper drainpipe into the hole, ensuring it extends at least two inches past the outside of the parapet.
6. Fasten the scupper onto the parapet with nails.
7. Rough up and clean the scupper flange.
8. Install membrane or cap sheet over the flange per the manufacturer’s instructions.
9. Bolt the clamping ring using the provided wrench to secure the overflow scupper onto the membrane.
10. Seal the drainpipe outside the parapet wall.

Overflow Scupper Maintenance

Overflow scuppers are less likely to get clogged by debris because they don’t discharge into a downspout.
Nevertheless, cans, bottles, and plastic bags can block secondary scuppers. Periodic roof maintenance and debris removal are necessary for proper roof drain and scupper performance, as Kim Koontz states in his article on the effects of debris on the flow rates of roof drains and scuppers.

Conclusion

Well-designed and maintained overflow scuppers prevent the collapse of flat roofs with parapets and long-term damage by ensuring ponding water never reaches a live load surpassing the structural capacity. They are also a visual alert of a clogged or undersized primary drainage.

Whether building from the ground up or planning on re-roofing, Menzies has overflow scupper drains to help ensure your roof complies with the latest codes, regulations, and safety standards. Get your high-quality scuppers today to prepare your flat roof for the next wet season.