Building-code answers, cited
Quick, plain-English answers to the questions contractors and homeowners ask most — each pointed at the exact IRC section. Need yours scoped to your jurisdiction? Ask Donnie.
What is the minimum ceiling height for a habitable room?
Habitable rooms must have a ceiling height of not less than 7 feet (IRC R305.1).
What is the minimum basement ceiling height, and can beams or ducts hang lower?
Non-habitable basement areas need 6 ft 8 in, and beams, girders, or ducts may project down to 6 ft 4 in above the floor (IRC R305.1.1).
What is the minimum floor area for a habitable room?
Habitable rooms must have at least 70 square feet of floor area; kitchens are exempt (IRC R304.1).
What is the minimum width or dimension for a habitable room?
Habitable rooms must be at least 7 feet in any horizontal direction; kitchens are exempt (IRC R304.2).
How much window area do I need for natural light and ventilation in a room?
Glazing must be at least 8% of floor area for light, and openable area at least 4% for ventilation (IRC R303.1).
Does a bathroom need a window, or is an exhaust fan enough?
A bathroom needs 3 sq ft of glazing with half openable, or a mechanical exhaust fan vented outdoors at 50 cfm intermittent / 20 cfm continuous (IRC R303.3).
Where are smoke alarms required in a house?
In each sleeping room, outside each sleeping area, and on every story including basements (IRC R314.3).
Do smoke alarms have to be interconnected?
Yes - when more than one is required, all smoke alarms must be interconnected so that triggering any one sounds them all (IRC R314.4, 2015 edition).
When does a house need a carbon monoxide alarm?
CO alarms are required in dwellings that contain a fuel-fired appliance or have an attached garage (IRC R315.2, 2015 edition).
Where should carbon monoxide alarms be installed?
Place a CO alarm outside each separate sleeping area near the bedrooms, plus inside any bedroom that contains a fuel-burning appliance (IRC R315.3, 2015 edition).
What is the fire separation between units in a duplex?
The two units of a two-family dwelling must be separated by walls and floor/ceiling assemblies with at least a 1-hour fire-resistance rating (IRC R302.3).
What kind of door is required between an attached garage and the house?
A solid wood or solid/honeycomb-core steel door at least 1-3/8 inches thick, or a 20-minute fire-rated door - and it cannot open into a sleeping room (IRC R302.5.1).
What drywall is required between an attached garage and the house?
At least 1/2-inch gypsum board on the garage side, and 5/8-inch Type X gypsum board on the ceiling when there are habitable rooms above the garage (IRC R302.6).
What size egress window does a bedroom need?
Min 5.7 sq ft net opening (5.0 sq ft at grade), 24 in min height, 20 in min width, sill ≤ 44 in (IRC R310).
How high can an egress window sill be above the floor?
The egress window sill can be no more than 44 in above the finished floor (IRC R310.2.2).
Does a basement or basement bedroom need an egress window?
Yes — basements with habitable space and every sleeping room must have at least one operable emergency escape and rescue opening (IRC R310.1).
What size does an egress window well need to be?
An egress window well needs at least 9 sq ft of horizontal area, with a minimum projection and width of 36 in (IRC R310.2.3).
When does an egress window well need a ladder or steps?
Window wells deeper than 44 in must have a permanently fixed ladder or steps (IRC R310.2.3.1).
What is the minimum size for a required exterior (egress) door?
The required egress door must be side-hinged and give at least 32 in clear width and 78 in clear height (IRC R311.2).
What size landing is required outside an exterior door, and how far can the floor drop at the threshold?
Exterior doors need a landing at least as wide as the door and 36 inches deep in the direction of travel; at the required egress door the floor can be no more than 1.5 inches below the threshold (IRC R311.3).
What is the minimum width for a hallway in a home?
Hallways must be at least 3 feet (36 inches) wide (IRC R311.6).
How big a step down is allowed at an exterior door threshold?
At the required egress door, the floor or landing can be no more than 1½ inches below the top of the threshold (IRC R311.3.1).
What are the maximum stair riser height and minimum tread depth?
Max riser 7¾ inches, min tread 10 inches (IRC R311.7.5).
What height should a stair handrail be?
Handrail height: 34 to 38 inches above the stair nosing (IRC R311.7.8.1).
How wide does a staircase have to be?
Stairways must be at least 36 in wide above the handrail (IRC R311.7.1).
What is the minimum headroom above a stairway?
Stairs need at least 6 ft 8 in of headroom (IRC R311.7.2).
How high should a stair handrail be and what size is actually graspable?
Handrails sit 34–38 in high, with round grips 1¼–2 in in diameter (IRC R311.7.8).
How tall do guardrails and deck railings need to be?
Guards must be at least 36 in high, or 34 in on the open side of stairs (IRC R312.1.2).
How far apart can balusters be on a railing?
Openings can't allow a 4-inch sphere to pass through (IRC R312.1.3).
Do I need a landing at the top and bottom of the stairs?
Yes — a landing at least 36 in deep is required top and bottom (IRC R311.7.6).
What are the tread depth rules for winder (pie-shaped) stairs?
Winder treads must be at least 10 in deep at the walkline and never less than 6 in at the narrow end (IRC R311.7.5.2.1).
What's the steepest slope allowed for a residential ramp?
Ramps are limited to a 1:12 slope, about 8.3 percent (IRC R311.8.1).
How high does a window sill have to be above the floor to meet fall-protection rules?
Where an operable window opening is more than 72 inches above the exterior grade, the bottom of the clear opening must be at least 24 inches above the interior floor, or approved fall-protection devices must be used (IRC R312.2.1).
When does glass next to a door have to be tempered safety glazing?
Glazing within a 24-inch arc of a door's vertical edge, with its bottom edge less than 60 inches above the floor, must be safety glazing (IRC R308.4.2).
Does glass around a bathtub or shower have to be tempered?
Yes — glazing in a tub or shower enclosure with its bottom edge less than 60 inches above the standing surface must be safety glazing (IRC R308.4.5).
How wide and thick does a footing need to be?
Footings must be at least 12 inches wide and about 6 inches thick, with the exact width taken from Table R403.1 (IRC R403.1).
How deep do exterior footings have to be below the ground?
Exterior footings must sit at least 12 inches below undisturbed ground, and below the frost line if that is deeper (IRC R403.1.4).
How deep below the frost line do footings need to go?
Footings must extend below the frost line set by your local jurisdiction in Table R301.2(1), or use an approved frost-protection method (IRC R403.1.4.1).
How thick does a foundation wall need to be?
Concrete and masonry foundation wall thickness is set by the tables in R404.1 based on wall height and unbalanced backfill, and can never be thinner than the wall it supports (IRC R404.1).
What is the anchor bolt spacing for a sill plate on a foundation?
Minimum ½-inch anchor bolts no more than 6 feet apart, embedded 7 inches, with two per plate section and one within 12 inches of each end (IRC R403.1.6).
How much bearing do floor joists and girders need at the ends?
At least 1½ inches of bearing on wood or metal, and at least 3 inches on masonry or concrete (IRC R502.6).
How do I attach a deck ledger to the house?
Use ½-inch lag screws or through-bolts spaced per Table R507.2 into a solid band joist — never nails — with proper flashing (IRC R507.2).
What is the deck lateral load connection requirement?
Hold-down tension devices at no fewer than two locations, each rated for at least 1,500 pounds, installed in line with the deck joists (IRC R507.2.3).
What size does an attic access opening need to be?
Minimum rough-framed opening of 22 by 30 inches, required when the attic is over 30 sq ft with at least 30 inches of clear height (IRC R807.1).
How much attic ventilation do I need?
Net free ventilating area must be at least 1/150 of the vented attic area (IRC R806.2).
Where is an ice barrier (ice and water shield) required on a roof?
In areas with a history of ice damming, an ice barrier must run from the eave edge to at least 24 inches inside the exterior wall line (IRC R905.1.2).
What is the minimum roof slope for asphalt shingles?
Asphalt shingles require a slope of at least 2:12, and double underlayment is required for slopes from 2:12 up to 4:12 (IRC R905.2.2).
When are rafter ties required, and how far apart can they be?
Where ceiling joists do not connect rafters to resist outward thrust, rafter ties are required, spaced no more than 4 feet on center; otherwise the ridge must be a structural ridge supported by a wall or beam (IRC R802.3.1).
What is the minimum size for a roof ridge board?
A ridge board must be at least 1 inch nominal thickness and no less in depth than the cut end of the rafter (IRC R802.3).
How much attic/ceiling insulation does the energy code require by climate zone?
Ceilings need R-30 in zone 1, R-38 in zones 2-3, and R-49 in zones 4-8 (IRC Table N1102.1.1).
What blower-door air leakage rate does code require for a new house?
Tested envelope leakage must not exceed 5 ACH50 in zones 1-2 and 3 ACH50 in zones 3-8 (IRC N1102.4.1.2).
How much insulation do HVAC ducts in the attic need?
Supply and return ducts in unconditioned attics must be insulated to R-8, and other ducts outside conditioned space to R-6 (IRC N1103.2.1).
How far does a toilet have to be from a side wall or vanity?
A water closet must sit at least 15 inches from its centerline to any side wall, partition, or vanity, and 30 inches center-to-center from an adjacent fixture (IRC P2705.1).
What is the minimum shower size allowed by code?
A shower must have at least 900 square inches of interior area and be no less than 30 inches in any dimension, maintained up to 70 inches above the drain (IRC P2708.1).
Does every plumbing fixture need a P-trap?
Yes — each plumbing fixture must be separately trapped by a water-seal trap, with no more than 24 inches vertically from the fixture outlet to the trap (IRC P3201.6).
Does a water heater need a temperature and pressure relief valve?
Yes — every water heater must have a temperature-and-pressure relief valve (or separate temperature and pressure valves) set to open by 210°F and 150 psi (IRC P2804).
When does a water heater need a drain pan, and what size is the drain?
A pan is required where a leak would cause damage; it must drain through a pipe at least 3/4 inch in diameter to an approved location (IRC P2801.6).
Do outdoor hose bibbs need a backflow preventer?
Yes — sillcocks, hose bibbs, and wall hydrants must have a vacuum breaker to prevent backflow into the potable supply (IRC P2902.4.3).
How much combustion air does a gas furnace or water heater need indoors?
Under the standard method, the appliance space must contain at least 50 cubic feet of volume per 1,000 Btu/h of total appliance input rating (IRC G2407.5).
Does a water heater or furnace in a garage have to be raised off the floor?
Yes — any appliance with an ignition source installed in a garage must have that ignition source at least 18 inches above the floor (IRC M1307.3).
What is the maximum length of a clothes dryer exhaust duct?
35-foot maximum developed length (2012 IRC M1502.4.4, with the 35-ft figure in M1502.4.4.1), measured from the transition-duct connection to the outlet terminal and excluding the transition duct. In the 2012 IRC, bend/fitting deductions are taken from Table M1502.4.4.1: a standard 4-inch-radius mitered 45-degree elbow deducts 2.5 feet and a 90-degree elbow deducts 5 feet, with smaller deductions for larger-radius smooth elbows. Greater lengths are permitted where the dryer manufacturer's installation instructions allow (M1502.4.4.2).
Where can a clothes dryer vent terminate and does it need a damper?
It must terminate outdoors at least 3 feet from any building opening, with a backdraft damper and no screen (IRC M1502.3).
How many CFM does a bathroom exhaust fan need?
A bathroom local exhaust fan must move at least 50 cfm if intermittent, or 20 cfm if run continuously (IRC Table M1507.4).
Can HVAC return air be pulled from a bathroom, kitchen, or garage?
No — return air must not be taken from a closet, bathroom, toilet room, kitchen, garage, mechanical room, or a space containing a fuel-burning appliance (IRC M1602.2).
How many CFM does a kitchen exhaust fan need?
A local kitchen exhaust must move at least 100 cfm intermittently or 25 cfm continuously, vented outdoors (IRC Table M1507.3).
How far apart do wall outlets have to be (the 6/12 foot rule)?
No point along a wall may be more than 6 feet from a receptacle, which works out to outlets no more than 12 feet apart (IRC E3901.2).
How many outlets do I need on a kitchen counter and how far apart?
A receptacle is required at every counter space 12 inches or wider, placed so no point is more than 24 inches from an outlet (IRC E3901.4).
Where does the outlet have to be in a bathroom?
At least one receptacle is required within 36 inches of the outside edge of each bathroom sink basin (IRC E3901.6).
Which outlets in a house have to be GFCI protected?
GFCI protection is required for receptacles in bathrooms, garages, outdoors, kitchens (counter-serving), unfinished basements, and crawl spaces (IRC E3902).
Do I have to use tamper-resistant outlets in a house?
Yes — 15- and 20-amp, 125-volt receptacles in dwelling units must be listed tamper-resistant type (IRC E4002.14).
Where is AFCI (arc-fault) protection required in a house?
AFCI protection is required on 120-volt branch circuits serving outlets in most living areas — kitchens, bedrooms, living/family/dining rooms, hallways, closets, and laundry areas (IRC E3902.16).
How much clearance is required in front of an electrical panel?
Panels need clear working space at least 36 in deep, 30 in wide (or the equipment width), and 6½ ft high (IRC E3405.2).