Residential Code Explained
The International Residential Code in plain language, section by section, as it applies to building a home in the Arizona desert.
IRC R303: Light, Ventilation & Heating
IRC R303 sets the minimum natural light, fresh air, and heat for habitable rooms. Each room needs window glass equal to at least 8 percent of its floor area and openable area equal to 4 percent, unless you swap those windows for mechanical ventilation and electric light.
Read the referenceIRC R305: Ceiling Height
IRC R305 sets the minimum ceiling height for the rooms you live in at 7 feet (84 inches). Bathrooms, laundry rooms, and basements can drop to 6 feet 8 inches, and beams or ducts may hang down to 6 feet 4 inches without failing the room.
Read the referenceIRC R308: Safety Glazing
IRC R308 requires safety glazing, usually tempered glass, in spots where someone could fall through and get cut. That means glass in and beside doors, around tubs and showers, near stairs, and large low panes. Tempered glass breaks into dull pebbles instead of sharp shards.
Read the referenceIRC R311: Means of Egress
IRC R311 sets the safe path out of a house. Stairs allow a maximum 7.75-inch riser and require at least a 10-inch tread, hallways must be at least 36 inches wide, and every home needs one side-hinged egress door giving at least 32 inches of clear width.
Read the referenceIRC R311.7: Stairways
IRC R311.7 sets stair geometry so people climb safely. Risers cap at 7.75 inches, treads run at least 10 inches, headroom is at least 6 feet 8 inches, the stair is at least 36 inches wide, and a graspable handrail sits 34 to 38 inches above the nosings.
Read the referenceIRC R312: Guards & Window Fall Protection
IRC R312 requires a guard at any walking surface more than 30 inches above the ground, at least 36 inches tall, with openings too small to pass a 4-inch sphere. R312.2 adds window fall protection: an operable window sill lower than 24 inches needs a limiter or guard.
Read the referenceIRC R302: Fire-Resistant Construction
IRC R302 sets the fire separation between your garage and the living space. A wall shared with the house needs at least 1/2-inch gypsum board on the garage side, and a ceiling under habitable rooms above the garage needs 5/8-inch Type X gypsum board, plus a self-closing rated door.
Read the referenceIRC R310: Emergency Escape & Rescue Openings
IRC R310 requires every bedroom and basement to have at least one window or door big enough to climb out of in a fire. The opening must give 5.7 square feet of clear space, at least 24 inches tall and 20 inches wide, with the sill no higher than 44 inches off the floor.
Read the referenceIRC R313: Automatic Residential Fire Sprinklers
IRC R313 calls for an automatic fire-sprinkler system in new one- and two-family homes. But Arizona law (A.R.S. 9-807) bars cities from enforcing that mandate on most new homes, so it is deleted by amendment almost everywhere. Fountain Hills is the local exception.
Read the referenceIRC R314: Smoke Alarms
IRC R314 says where smoke alarms go in a house: inside every bedroom, outside each sleeping area near the bedrooms, and on every story including the basement. In a new home the alarms must be hardwired with battery backup and interconnected so when one sounds, they all sound.
Read the referenceIRC R315: Carbon Monoxide Alarms
IRC R315 requires carbon monoxide alarms in a home that has a fuel-fired appliance or fireplace, or an attached garage that opens into the house. The alarms go outside each sleeping area near the bedrooms, and inside any bedroom that contains a fuel-burning appliance.
Read the referenceIRC R401-R405: Foundations, Soils & Drainage
IRC R401 through R405 cover the foundation chapter: how the soil is classified, how the ground must drain, and when a foundation has to be engineered. The headline rule is that the lot must slope away from the house at least 6 inches over the first 10 feet.
Read the referenceIRC R403: Footings
IRC R403 sets the minimum size and depth for the footings under your house. A standard footing must be at least 12 inches wide and 6 inches thick, placed at least 12 inches below undisturbed grade, and footings on expansive soil must be engineered to handle the soil movement.
Read the referenceIRC R506: Concrete Slab-on-Ground Floors
IRC R506 sets the rules for a concrete slab poured directly on the ground. The slab must be at least 3.5 inches thick, and in the 2021 IRC a 10-mil vapor retarder goes under any heated slab to keep ground moisture from wicking up into the finished floor.
Read the referenceIRC R602: Wood Wall Framing & Bracing
IRC R602 sets the rules for wood-framed walls: what size studs to use, how far apart to space them, and how much braced wall panel a house needs to resist wind and earthquakes. A standard braced wall panel must be at least 48 inches long.
Read the referenceIRC R802: Roof-Ceiling Framing
IRC R802 sizes the wood that holds up your roof: rafters and trusses. It sets the maximum span a given rafter can cover based on its size, the lumber, and the roof load, and it requires rafter or ceiling ties so the roof does not push the walls outward.
Read the referenceIRC M1401 & Manual J: HVAC Sizing
IRC M1401.3 requires heating and cooling equipment to be sized from a real load calculation, not a square-foot guess. The load is figured with ACCA Manual J, and the equipment is then picked with Manual S, so the system matches the actual house.
Read the referenceIRC M1601: Duct Construction & Insulation
IRC M1601 sets how ducts are built, sealed, and insulated. Joints and seams must be sealed against air leaks, and supply ducts run through an unconditioned attic must be insulated to at least R-8, which in Arizona heat is one of the biggest factors in a comfortable, efficient home.
Read the referenceIRC R324: Rooftop Solar Photovoltaic Systems
IRC R324 governs how rooftop solar panels are installed. Panels must meet the same wind and fire ratings as the roof, and R324.6 keeps clear access pathways for firefighters: two 36-inch paths to the ridge and an 18-inch clear setback below the ridge on most homes.
Read the referenceIRC R806: Roof/Attic Ventilation
IRC R806 requires a vented attic to have ventilation equal to at least 1/150 of the attic floor area. That ratio can drop to 1/300 when the vents are split evenly high and low and a vapor retarder is used, which keeps hot air moving out of the attic.
Read the referenceIRC R807: Attic Access
IRC R807 requires an attic access opening of at least 22 by 30 inches whenever the attic is big enough to enter, meaning 30 inches of height over at least 30 square feet. The opening also needs 30 inches of headroom above it so the space can be reached and serviced.
Read the referenceIRC 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.
Read the reference
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