IRC R905: Roof Covering
IRC R905 sets the rules for roof coverings by type and slope. Asphalt shingles need a slope of at least 2:12, with doubled underlayment below 4:12, while clay and concrete tile need at least 2.5:12, which shapes how a low-slope Arizona roof gets covered.
A roof covering does one job: keep water out while taking the full force of the sun. Section R905 of the International Residential Code (IRC) sets the rules for each covering type, and the key gate is slope. Use a covering on a roof flatter than the code allows and water can work back under it, so R905 ties every material to a minimum pitch and to the underlayment that backs it up.
What R905 requires
R905 has a subsection for each common covering: asphalt shingles, clay and concrete tile, metal panels, and more. Each sets a minimum slope and the underlayment beneath it. For asphalt shingles, R905.2.2 draws the line:
Asphalt shingles shall be used only on roof slopes of two units vertical in 12 units horizontal (2:12) or greater.
Slope is written as rise over run, so 2:12 means the roof climbs 2 inches for every 12 inches it runs out. Below 4:12, R905.1.1 requires a doubled layer of underlayment so the shallower pitch has more backup against water. Clay and concrete tile need a steeper minimum, 2.5:12. The underlayment itself, the felt or synthetic sheet under the covering, has to meet standards like ASTM D226, D1970, or D4869 and carry a label proving it.
What it means for an Arizona custom home
For example, a custom home in Cave Creek with a low 3:12 roof and concrete tile clears the 2.5:12 tile minimum, but because it sits below 4:12 it needs doubled underlayment under the tile. That underlayment is doing more in Arizona than almost anywhere, because tile roofs here are a rain screen, not a seal. Wind-driven monsoon rain blows under the tile, and the underlayment is the real waterproof layer. A common upgrade in the Valley is a high-temperature self-adhered membrane that survives the heat under dark tile.
The sun is the other half of the story. Roof covering color and the air space under tile affect how much heat reaches the attic, which feeds back into the ceiling R-value and the cooling load. Concrete and clay tile hold up well to decades of Arizona ultraviolet, which is one reason they are so common here, while asphalt shingles in this climate wear faster and are usually chosen for budget or a specific look. For how the roof and shell affect comfort, see are stucco homes good for Arizona .
Edition note
The slope minimums and underlayment standards in R905 are consistent across the 2018, 2021, and 2024 IRC. The 2021 edition reorganized the underlayment provisions into the shared R905.1.1 and updated the referenced ASTM standards, but the 2:12 asphalt and 2.5:12 tile thresholds and the doubled-underlayment-below-4:12 rule have held. Which edition your city enforces is on the Arizona building code editions by city page.
How it connects
The covering set here sits on the roof framing built under R802 and works with the attic airflow from R806 roof ventilation , which carries off the heat the covering soaks up. For how a hot roof affects panel output, see is solar worth it on a new construction home in Arizona .
Source
Read the section on the official ICC site: 2021 IRC R905.2.2 on codes.iccsafe.org . The ICC I-Codes are copyrighted, so only the short clause above is quoted; the explanation is ours. The underlayment requirements point to the ASTM D-series standards.
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