Convert the roof area and the design rainfall rate (3 inches per hour in Phoenix) into gallons per minute, divide it among the drains, then pick a drain whose published flow rate covers that load. The leader and storm pipe come from the code's capacity tables, which also depend on pipe slope.
What rainfall rate do you design for in Phoenix?
Three inches per hour. The 2024 IPC as adopted in Phoenix says the sizing of "vertical conductors and leaders, building storm drains, building storm sewers, and any horizontal branches" is based on "an hourly rainfall rate of three (3) inches per hour." On the Uniform Plumbing Code side, Phoenix's amendment to Section 1101.12.1 sets the same rate, as a storm "of 60 minutes duration and 100 year return period." The city's own justification says the model codes list Phoenix at 2.2 inches per hour (UPC) and 2.5 (IPC), and 3 is used "for ease of using the sizing tables." Our page on what code requires for roof drains on a flat roof covers overflow drains and ponding.
How do you turn roof area into gallons per minute?
Section 1106.2.1 converts rainfall intensity and roof area to gallons per minute using Equation 11-1. The math behind it is simple: one inch of rain on one square foot is about 0.62 gallons, so one inch per hour over one square foot is about 0.0104 gallons per minute. Multiply that by the rain rate and the area each drain serves.
At 3 inches per hour, each square foot of roof sends about 0.031 gallons per minute to a drain. A 2,500 square foot drainage area is about 78 gallons per minute. Section 1106.4 adds one more input: "one-half of the area of any vertical wall that diverts rainwater to the roof" counts as extra roof area, which matters for a roof with a tall parapet or a taller neighboring wall.
How do you pick the roof drain body itself?
From the manufacturer's flow data. Section 1105.2 says the "published roof drain flow rate, based on the head of water above the roof drain, shall be used to size the storm drainage system," and that the flow used "shall be based on the maximum anticipated ponding at the roof drain." Section 1106.2 adds that the calculated flow "shall be checked against the roof drain manufacturer's published flow rate for the specific roof drain model and size." In short, a drain is not simply "4 inch" or "6 inch." Its rated flow depends on how deep the water ponds around it, and that depth is limited by what the roof structure was designed to carry. The IIBEC explains the structural side in its article on ponding and secondary drainage.
How are the leader and storm pipe sized?
From capacity tables. Vertical leaders use Table 1106.3, and the flow through a leader "shall not exceed that specified in Table 1106.3." That table lists 3 inch leaders at 92 gpm and 4 inch at 192 gpm. Storm piping, vertical or horizontal, uses Table 1106.2, where horizontal capacity climbs with slope. A 4 inch horizontal storm drain carries 115 gpm at 1/8 inch per foot and 163 gpm at 1/4 inch per foot; a 3 inch pipe carries 55 and 79 gpm at those slopes.
What does a worked example look like?
Say a roof has four drains and each serves about 2,500 square feet of roof, with no tall walls shedding onto it.
- 1Flow per drain: 2,500 sq ft x 3 in/hr x 0.0104 = about 78 gpm.
- 2Drain body: check that the chosen model's published flow at the planned ponding depth is at least 78 gpm.
- 3Vertical leader: a 3 inch leader (92 gpm) covers it.
- 4Horizontal run at 1/8 inch per foot: a 3 inch pipe (55 gpm) is too small. A 4 inch pipe (115 gpm) works.
- 5Combine runs: when two drains share a pipe, add their flows (about 156 gpm) and size that pipe from the table, which now calls for a larger size.
The secondary (overflow) drains are sized the same way. Section 1108.3 says they are sized "based on the rainfall rate for which the primary system is sized," and the flow through the primary system is not counted toward them.
Why do tenants and owners care?
Because a drain that is too small, or partly clogged, turns into a pond. Re-roofing, adding rooftop equipment or capping a leader can all change the load, so ask for the revised drainage calculation before the work starts. Our guide to roof vents and roof drains that need cleaning covers upkeep, and floor drain versus trench drain versus area drain shows what must stay off the sanitary side. Phoenix's amendments also state that the city does not allow combined sanitary and storm drainage systems.
Exact sizes depend on the drawings and the code the designer used. The tables above are the 2024 IPC as Phoenix adopted it; a UPC-based design uses that code's own storm tables. HQ is licensed and insured and gives free estimates, including for commercial drain and storm piping repairs.