Residential Building Inspector B1Roof-Ceiling ConstructionHard
A homeowner is installing new gutters on a residence. The roof area draining into a single downspout is 1,200 square feet (111.5 m²). The local design rainfall rate is 6 inches (152 mm) per hour. What is the minimum required cross-sectional area for the downspout, assuming a standard rectangular downspout?
- A20 square inches (129.0 cm²)
- B8 square inches (51.6 cm²)
- C12 square inches (77.4 cm²)
- D16 square inches (103.2 cm²)
Show answer & explanationAnswer & explanation
Correct answer: C. 12 square inches (77.4 cm²)
The IRC provides a table for downspout sizing. For a roof area of 1,200 sq ft and a rainfall rate of 6 inches per hour, the table indicates a minimum downspout cross-sectional area of 12 square inches.
Why the other options are wrong
- A. 20 square inches is excessive for the given parameters.
- B. 8 square inches is too small for the given roof area and rainfall rate.
- D. 16 square inches would be sufficient but is not the minimum required.
Downspout Sizing
Downspouts must be sized to adequately drain rainwater from the roof, based on the roof area served and the local design rainfall rate.
- Sizing is determined by roof area and rainfall intensity.
- IRC provides tables for determining minimum downspout size.
- Insufficiently sized downspouts can lead to gutter overflow and water damage.
Memory trick: Rainfall rate, roof size, check the table, don't compromise.