A flat commercial roof needs primary drains sized for a 3 inch per hour rain rate in Phoenix, plus secondary overflow drains set at least 2 inches above the roof surface. Phoenix requires the overflow to run in its own separate piping and discharge above grade where someone can see it.
What does Phoenix require for the primary roof drains?
Phoenix amends the Uniform Plumbing Code (UPC) in Section 1101.12.1. Roof areas are drained by roof drains, scuppers or gutters, and the drain locations and sizes have to be coordinated with the structural design and the pitch of the roof. Unless the building official requires something else, the primary drains, leaders and horizontal storm drains are sized for a rainfall rate of 3 inches per hour.
The city's own justification explains the number. The model code lists an annual rainfall rate of 2.2 inches per hour for Phoenix, and the amendment rounds it up to 3 for easier use of the sizing tables. It also makes scuppers an accepted primary drain, which the city says has been long-standing practice in Phoenix. A scupper has to be sized so ponding water never gets deeper than the roof was designed for. The opening must be at least 4 inches high and as wide as the circumference of the roof drain it replaces.
### How much water is 3 inches per hour?
More than most people expect. One inch of rain on one square foot of roof is about 0.62 gallons, because a square foot at one inch deep holds 144 cubic inches and a gallon is 231 cubic inches. At 3 inches per hour, a 10,000 square foot roof has to shed roughly 18,700 gallons in an hour, or about 310 gallons a minute. That number is why the drains, leaders and horizontal storm piping are all sized to the design rainfall rate and not to a typical shower. It also shows why one clogged drain matters. The roof around it does not stop collecting water, and the remaining drains and the overflow take the extra.
The exact drain and pipe sizes come from the sizing tables in the plumbing code and the roof's engineer of record, and they depend on the drained area. Do not swap in a smaller drain, or cap a leader, during a re-roof without checking with the designer. If a contractor proposes to change drain locations, ask for the revised drawing and confirm that it still shows a separate overflow, since that change can also need a permit and inspection.
What is a secondary (overflow) drain, and when is it required?
A secondary drain is a backup path for water. The model building code calls for one wherever the roof edge or a parapet would trap water if the primary drains stop working "for any reason." A flat roof with parapets on every side is the classic case, since a clogged drain has nowhere else to send the water.
Phoenix's amendment to UPC Section 1101.12.2.2 says secondary roof drains shall be provided, and it sets these conditions:
- The overflow drains are located at least 2 inches above the roof surface.
- The height is limited so the depth of ponding water never exceeds what the roof was designed to carry.
- They connect to a separate piping system, independent of the primary drainage.
- That discharge is above grade, in a location that building occupants or maintenance staff can observe.
- They are sized for the same rainfall rate as the primary system.
Why does Phoenix require separate overflow piping?
Phoenix removed the option of combining the primary and secondary drains into one system. The city's reason is short: a combined system has no way to show that the primary drain is blocked. With separate piping, water pouring out of the overflow is the visible sign that the main drain has failed, and someone can act before the roof carries the load.
That matters because water is heavy and a roof is only designed for a certain depth. The IIBEC's summary of the IBC says the structure has to carry the rainwater that collects if the primary drainage is blocked, plus the load from water rising above the secondary inlet, and warns that standing water can deflect roof members and, past the design limit, cause collapse.
### What does ponding do to a roof?
Flat roofs carry water as a load. The IBC's ponding rule, as the IIBEC summarizes it, requires the roof structure to carry the rainwater that collects if the primary drainage is blocked, plus the load from water rising above the inlet of the secondary drainage. If water does collect, the members deflect, and a deflected roof holds still more water. The IIBEC calls this ponding instability. Increasingly large deflections can overload the structure, and the result can be a roof collapse. This is the reason the overflow drain sits where it does. Its height sets the deepest pond the roof was designed to hold, so if you raise a roof surface, add insulation or install pavers without adjusting the drain and overflow heights, you can change how deep the water gets.
### Why does Phoenix's monsoon matter for a roof?
Monsoon storms bring strong winds and dust storms, and both put material on a flat roof: dust, gravel, plastic bags, palm fronds and leaves. When the rain arrives, that debris washes to the low spots, and the strainer over a drain is where it stops. Storms in the Valley can also drop a large amount of rain in a short time, so the drains have very little time to recover from a partial blockage. A roof can be code-compliant on the day it was built and still be unsafe in September if nobody has cleared the drains since May.
What does this mean for a building owner or tenant?
Most flat-roof trouble in the Valley starts with debris. Monsoon winds put leaves, gravel, plastic and dust on a roof, and the strainers over the drains clog quickly. A few habits help:
- 1Walk the roof before monsoon season and again after a big storm, and clear the drain strainers and scuppers.
- 2Find the overflow outlets and make sure they are open and not painted, screened over or buried.
- 3Treat overflow discharge as a warning. If water is coming out of the overflow, the primary drain is blocked.
- 4Ask before adding roof equipment. New curbs, pads or parapet changes can trap water where it did not collect before, so the drainage should be rechecked.
Repairs to a roof drain or its piping are plumbing work, and larger changes can need a permit. Our page on when a commercial plumbing repair needs a permit in Phoenix explains how that works. For the seasonal side, see how monsoon season affects plumbing and preventing a sewer backup before monsoon.
Codes get amended, and the building official's approved plans control for your roof. Phoenix adopted the 2024 code effective August 1, 2025, so a permit for a new roof follows the amended text above. HQ is licensed and insured and gives free estimates and upfront pricing on commercial plumbing work.