IRC R403: Footings

A footing is the wide concrete base that spreads your house's weight onto the soil so the structure does not sink or tip. Section R403 of the International Residential Code (IRC) sets the floor for how big and how deep that base has to be. Get it wrong and you get cracked walls, sticking doors, and an uneven slab. This page explains the real numbers, then the part that matters most in the Valley: what R403 demands when the soil underneath swells.

What R403 requires

A conventional spread footing has three baseline numbers. It must be at least 12 inches wide, at least 6 inches thick, and set at least 12 inches below the undisturbed ground surface. "Undisturbed" means original, compacted soil, not loose fill someone dumped on the lot. Beyond those minimums, R403 sends you to Table R403.1 to size the footing to the load above it and the bearing strength of the soil below it. Heavier houses and weaker soils mean wider footings.

The depth rule normally exists to get the footing below the frost line, but most of Arizona has no meaningful frost depth, so the 12-inch minimum governs here instead. Many Maricopa and Pinal County jurisdictions still require deeper footings (18 inches is common) to reach stable soil below the zone where moisture moves up and down.

Here is the requirement the code points to for problem soils:

Foundation and floor slabs for buildings located on expansive soils shall be designed in accordance with Section 1808.6 of the International Building Code.

The expansive-soil rule (R403.1.8)

Section R403.1.8 is the provision that drives foundation design across the Phoenix metro. Soil is classified as expansive soil when it meets a set of lab thresholds, including a plasticity index of 15 or greater and an expansion index above 20. Expansive clay absorbs water and swells, then dries and shrinks. That movement lifts and drops parts of a footing at different rates, which is what cracks foundations.

When a site tests expansive, R403.1.8 stops you from using a plain footing and slab. The foundation has to be engineered, usually as a stiffened post-tensioned slab or a pier-and-beam system designed by a licensed engineer to ride out the soil movement. This is why a soils report comes before the foundation design on almost every custom home east of the I-17 corridor.

What it means for an Arizona custom home

For example, on a build-on-your-lot project in Cave Creek or Apache Junction, the sequence is: geotechnical engineer drills and tests the soil, the report classifies it, then the foundation is designed to that report. If the lot is clean granular soil, a conventional R403 footing at 12 to 18 inches deep may be all you need. If it tests expansive, expect an engineered slab and a thicker stem wall, which adds cost but prevents the cracking that haunts older Valley homes built before this was enforced. For the slab side of that decision, see why post-tension slabs are common in Arizona .

Edition note

The core footing dimensions in R403.1 (12-inch width, 6-inch thickness, 12-inch depth) are unchanged across the 2018, 2021, and 2024 IRC. The expansive-soil pointer to IBC 1808.6 is also stable. What changes is which edition your city enforces and what local amendments add on top. Scottsdale, Phoenix, and the counties each adopt and amend separately, so confirm the edition on the Arizona building code editions by city page before you design to a specific table.

Source

Read the section text on the official ICC site: 2021 IRC R403.1 on codes.iccsafe.org . The ICC I-Codes are copyrighted, so only the short clause above is excerpted here; the explanation is ours.