IRC R903 Roof Weather Protection: Flashing, Coping, and Monsoon Drainage

A roof has two different jobs in the code. R905 sets which covering goes on which slope. Section R903 of the International Residential Code (IRC) handles the parts that actually stop water at the weak points: the flashing, the coping on a parapet, and the drainage. On a flat-roofed desert home taking a monsoon downpour, R903 is doing most of the work.

What R903 requires

Flashing is metal or membrane installed at every joint where water could sneak in: where the roof meets a wall, where the slope changes, and around every penetration like a vent, skylight, or chimney. Metal flashing has to resist corrosion and be at least 0.019 inch thick, with No. 26 galvanized sheet as the benchmark.

Chimneys get a specific rule. On the uphill side, water runs down the roof and piles against the chimney unless you divert it:

A cricket or saddle shall be installed on the ridge side of any chimney or penetration more than 30 inches wide.

A cricket is a small ridged structure that splits the water around the obstruction. A parapet wall needs a coping across the top, a weatherproof cap at least as wide as the wall, so water does not soak down into it. For drainage, a roof that cannot shed water over its edges needs roof drains at the low points. Where a blocked drain would pond water behind a parapet, R903 requires a backup. That secondary overflow drain or scupper sits with its inlet 2 inches above the roof low point, so if the primary drain clogs, water escapes before it loads the structure. Overflow scuppers are sized larger, with an opening at least 4 inches tall.

What it means for an Arizona custom home

For example, a contemporary home in north Scottsdale with a flat roof and parapet walls lives or dies on R903 details. There is no slope to carry water off the edge, so the roof drains to interior drains or through-wall scuppers, and the code wants a second path in case the first clogs during a hard monsoon cell. The parapet tops get coping, and every parapet-to-roof corner gets flashing tied into the water-resistive barrier on the wall. One Arizona break: the code's ice-barrier rule, which protects cold-climate eaves from ice dams, does not apply here, because the low desert does not freeze that way. Heat, not cold, is what ages these details, so the flashing metal and sealant choices matter.

Edition note and related rules

R903's flashing, cricket, coping, and drainage rules hold steady from the 2018 through the 2024 IRC. Which edition your city enforces is on the Arizona building code editions by city page. R903 works with the covering rules of R905, the framing under R802, and, when you reroof later, the prep rules of R908. For how the wall sheds water below the roofline, see exterior wall covering under R703. The section text is on the ICC site: 2021 IRC R903 on codes.iccsafe.org.