Deck footings live or die by two questions: are they below the frost line, and are they big enough for the load? The IRC answers the first with your jurisdiction’s frost-depth map and the second with Table R507.3.1.
Footings must extend below the local frost line, bear on solid ground per R403.1.4, or be frost-protected — the local frost depth is set by your jurisdiction (Table R301.2)
IRC R403.1.4, R507.3.2
Bearing
Footings bear on undisturbed native soil or engineered fill; organic soil and uncompacted fill are not allowed
IRC R403.1, R507.3
Minimum size
Sized per Table R507.3.1 by tributary area, soil bearing capacity, and live/snow load — small decks on 1,500 psf soil commonly need larger footings than owners expect
IRC Table R507.3.1
Minimum thickness
Footing thickness minimum 6 in. per Table R507.3.1
IRC R507.3.1
Post-to-footing connection
Posts must be restrained against lateral displacement at the footing — a post base connector or embedment, not just resting on concrete
IRC R507.4.1
Free-standing exemption
A detached deck up to 200 sq ft, under 30 in. above grade, not serving the required exit door, may be exempt from a permit — the construction rules still apply
IRC R105.2
Worth knowing
Frost depth varies wildly by jurisdiction — from 0 in. in the South to 60+ in. in the northern plains. Find yours on your jurisdiction page or in the local Table R301.2 amendments.
Precast pier blocks sitting on grade rarely satisfy frost protection in cold climates.
Footings within 5 feet of a house wall generally must bear at the same elevation as the house footing or be evaluated for surcharge.
The IRC deck provisions (R507) were substantially expanded in the 2015-2021 editions — older span cards are outdated.
Common questions
What does the code say about frost protection for Deck Footings?
Footings must extend below the local frost line, bear on solid ground per R403.1.4, or be frost-protected — the local frost depth is set by your jurisdiction (Table R301.2). (IRC R403.1.4, R507.3.2)
What does the code say about bearing for Deck Footings?
Footings bear on undisturbed native soil or engineered fill; organic soil and uncompacted fill are not allowed. (IRC R403.1, R507.3)
What does the code say about minimum size for Deck Footings?
Sized per Table R507.3.1 by tributary area, soil bearing capacity, and live/snow load — small decks on 1,500 psf soil commonly need larger footings than owners expect. (IRC Table R507.3.1)
Adopted edition by state
Base-code year from BuildSpec’s adoption records. States often amend the base code, and your city or county may differ, so confirm with your building department.
This guide is BuildSpec’s own summary, not code text. It cites the 2021 International Residential Code and the 2023 National Electrical Code as model standards. The IRC 2021 text itself is in the Code Library for signed-in members; BuildSpec publishes no NEC text, which is at nfpa.org.Neither is an adoption record: your state may be on a different edition with amendments — check your jurisdiction page or get an Address Report.