Residential Building Code Cheat Sheet (2021 IRC)

What this is: the numbers a home project most often has to meet under the 2021 International Residential Code (IRC), on one page: stairs, handrails, guards (railings), deck joists and decking, and concrete slabs. Each line gives the code section, so you can check it in the official text.

Before you build: states and towns adopt the IRC with their own changes, and some still use an older edition. Your building department’s version is the one that counts. Print this page (Ctrl+P) and bring it to the store or the job.

Stairs (IRC R311.7)

Rule	Requirement	Section
Riser height	7¾ in maximum	R311.7.5.1
Tread depth	10 in minimum	R311.7.5.2
Variation in a flight	⅜ in maximum (risers and treads)	R311.7.5.1–.2
Nosing projection	¾ to 1¼ in; not required if treads are 11 in or deeper	R311.7.5.3
Width	36 in clear above handrail height; 31½ in at the handrail with one handrail, 27 in with two	R311.7.1
Headroom	6 ft 8 in minimum	R311.7.2
Landings	At the top and bottom of each stairway (limited exceptions), at least as wide as the stairs	R311.7.6

To work out your riser height and number of steps from the floor-to-floor height, use the stair calculator, or read stair rise and run.

Handrails (IRC R311.7.8)

Rule	Requirement	Section
When required	On at least one side of any flight with 4 or more risers	R311.7.8
Height	34 to 38 in, measured up from the line of the tread nosings	R311.7.8.1

Guards and railings (IRC R312)

Rule	Requirement	Section
When required	Open sides of floors, decks, porches, stairs and landings more than 30 in above the floor or grade below, measured within 36 in of the edge	R312.1.1
Height	36 in minimum; 34 in on the open sides of stairs, measured from the nosings	R312.1.2
Openings	A 4 in sphere must not pass through	R312.1.3
Openings on stair sides	4⅜ in sphere; 6 in for the triangle under the bottom rail	R312.1.3

Deck joists (IRC R507.6 and R507.7)

Maximum joist spacing depends on the decking (Table R507.7, boards crossing three or more joists):

Decking	Straight across	Diagonal (up to 45°)
1¼ in (5/4) wood	16 in	12 in
2 in wood	24 in	24 in

Maximum joist spans at 16 in on center, 40 psf live load, No. 2 lumber (Table R507.6):

Joist	Southern pine	Douglas fir-larch, hem-fir, SPF	Redwood, cedars
2×6	9′ 0″	8′ 4″	8′ 0″
2×8	11′ 10″	11′ 1″	10′ 7″
2×10	14′ 0″	13′ 7″	13′ 0″
2×12	16′ 6″	15′ 9″	15′ 1″

Snow country uses shorter spans, and the full table with 12 and 24 in spacing and cantilevers is in deck joist spacing and span table. Count boards, joists and screws with the deck calculator.

Concrete slabs on the ground (IRC R506)

Rule	Requirement	Section
Thickness	3½ in minimum	R506.1
Base	4 in of clean graded sand, gravel or crushed stone under slabs below grade (not needed on well-drained Group I soils)	R506.2.2
Vapor retarder	10-mil, joints lapped 6 in, under house slabs. Not required for garages and other unheated buildings, or for driveways, walks and patios	R506.2.3

Work out cubic yards and bags with the concrete calculator.

Common questions

Does my town use the 2021 IRC?

Many do, but not all, and most add local changes. Call or check your building department’s website, which will list the code edition and amendments it enforces.

Do I need a permit for a deck or stairs?

Usually, especially for anything attached to the house or more than 30 inches off the ground. Rules vary by town, so ask before you build.

What’s the difference between a handrail and a guard?

A handrail is the rail you hold while using the stairs. A guard is the barrier that keeps people from falling off a raised edge. A stair can need both.

Sources

• International Code Council: 2021 IRC chapter 3: R311.7 stairways and handrails, R312 guards.
• International Code Council: 2021 IRC chapter 5: R506 concrete floors, R507 decks (Tables R507.6 and R507.7).