In the SMACNA calculation example, the required plain round scupper is how many inches?

In the SMACNA calculation example, the required plain round scupper is how many inches?

• A. 3 inches
• B. 4 inches
• C. 5 inches
• D. 6 inches

Answer: (B)

Sizing a plain round scupper in the SMACNA approach comes from matching the roof’s expected water flows to what the opening can pass without causing overflow. You first determine the design discharge from the roof: multiply the tributary area by the design rainfall intensity to get Q (the water volume that must be drained per second). Then you convert that flow into what a circular opening can carry, using the opening’s cross‑sectional area and an allowable water velocity through the scupper. The cross‑sectional area of a round opening is A = πD^2/4, so the discharge is Q = V × A, where V is the chosen velocity limit for safe, unrestricted flow. Solving for diameter D gives the size needed to handle the design runoff at the specified velocity. In the example, performing those steps with the given roof area, rainfall rate, and the standard velocity assumed for scuppers yields a diameter of four inches. So the required plain round scupper is four inches. If the assumed rainfall, roof area, or allowable velocity change, the resulting diameter would change accordingly.

Sizing a plain round scupper in the SMACNA approach comes from matching the roof’s expected water flows to what the opening can pass without causing overflow. You first determine the design discharge from the roof: multiply the tributary area by the design rainfall intensity to get Q (the water volume that must be drained per second). Then you convert that flow into what a circular opening can carry, using the opening’s cross‑sectional area and an allowable water velocity through the scupper. The cross‑sectional area of a round opening is A = πD^2/4, so the discharge is Q = V × A, where V is the chosen velocity limit for safe, unrestricted flow. Solving for diameter D gives the size needed to handle the design runoff at the specified velocity.

In the example, performing those steps with the given roof area, rainfall rate, and the standard velocity assumed for scuppers yields a diameter of four inches. So the required plain round scupper is four inches. If the assumed rainfall, roof area, or allowable velocity change, the resulting diameter would change accordingly.