How Deep Should Footings Be? Depth and Size for Decks, Garages and Houses

How deep should footings be? Under the International Residential Code, every footing goes at least 12 inches below undisturbed ground, and footings for houses, additions, attached garages and decks attached to the house also go below the local frost line. Small detached buildings and freestanding decks can skip the frost line under the model code, but your building department has the final say.

The two depth rules every footing follows

The first rule is a flat minimum: IRC Section R403.1.4 puts exterior footings at least 12 inches below the undisturbed ground surface, and Section R507.3.2 sets the same 12 inches for deck footings.

The second rule is frost. Section R403.1.4.1 says foundations, piers and other permanent supports must be protected from frost by one of four methods: going below the frost line, a frost-protected shallow foundation, an ASCE 32 design, or bearing on solid rock. Footings can’t bear on frozen soil unless it stays frozen.

The code doesn’t print one frost depth for the country. Table R301.2 leaves a blank that each jurisdiction fills in with the minimum depth of footing below finish grade. Our guide to frost line depth by state shows which states print a number and how to find yours.

Whichever rule gives the deeper hole wins, and the bottom has to land on undisturbed natural soil or engineered fill, per Section R403.1, not on topsoil or loose backfill.

Footing depth by structure

Structure	Depth	Frost protection?	Size rule
House, addition or attached garage	12 in. minimum	Yes	Table R403.1(1), never under 12 x 6 in.
Detached garage or shed over 600 sq ft (light frame)	12 in. minimum	Yes	Table R403.1(1)
Detached light-frame accessory building (garage, shed), 600 sq ft or less, eaves 10 ft or lower	12 in. minimum	Not required	Table R403.1(1)
Deck attached to the house	12 in. minimum	Yes	Table R507.3.1
Freestanding deck	12 in. minimum	Not required by R507.3.3	Table R507.3.1
Freestanding deck on pier blocks, 200 sq ft or less, 20 in. high or less	No footing required	No	Precast pier blocks at grade

Deck footings

Section R507.3.3 requires frost protection where a deck is attached to a frost-protected structure, usually the house. If frost lifts the posts while the house stays put, the ledger connection takes the strain. A deck that stands on its own isn’t covered by that rule, though its footings still go 12 inches deep. Your building department can still require frost depth for a freestanding deck, so ask before you dig.

Section R507.3 drops footings entirely for two kinds of freestanding deck: joists resting on grade along their full length, or joists sitting directly on precast pier blocks with no beams or posts, 200 square feet or smaller, with the walking surface no more than 20 inches above grade at any point within 36 inches of the edge. Our freestanding deck plans guide covers how those are framed.

Garages, sheds and houses

An attached garage follows the house rules. A detached light-frame garage or shed can skip frost protection if it’s 600 square feet or smaller with eaves 10 feet high or less (400 square feet if it isn’t light-frame); anything bigger goes below the frost line or uses another approved frost method. Our foundation digging guide covers excavating for slabs, garage pads and pole barns.

Pergolas and retaining walls are different jobs. Pergola posts need real footings, explained in our article on anchoring a pergola to pavers, and segmental walls sit on a compacted stone pad, covered in our retaining wall base depth guide.

How big a footing should be

Depth keeps frost from moving a footing. Width keeps the soil from being overloaded. When nobody has tested the soil, the IRC assumes the bearing values in Table R401.4.1: 1,500 pounds per square foot for clay and silt, 2,000 for sand and clayey or silty gravel, 3,000 for sandy gravel and gravel, and more for rock. If the building official expects soil weaker than 1,500, a soils investigation sets the number.

Wall footings for houses and garages

Section R403.1.1 sets the floor: at least 12 inches wide and 6 inches thick, with the footing projecting at least 2 inches past the wall but not more than its thickness. Table R403.1(1) adds width for heavier buildings on weaker soil.

Light-frame building	1,500 psf (clay)	2,000 psf (sand)	3,000 psf (gravel)
1 story, slab on grade	12 x 6 in.	12 x 6 in.	12 x 6 in.
1 story with basement	16 x 6 in.	12 x 6 in.	12 x 6 in.
2 story with crawl space	15 x 6 in.	12 x 6 in.	12 x 6 in.
2 story with basement	19 x 6 in.	14 x 6 in.	12 x 6 in.

Deck post footings

Deck footings are sized by tributary area: the square feet of deck surface each post holds up. Table R507.3.1 gives the minimum footing for each area and soil.

Deck area per post	Clay, 1,500 psf	Sand, 2,000 psf	Gravel, 3,000 psf or more
20 sq ft	12 in. round, 6 in. thick	9 in., 6 in.	8 in., 6 in.
40 sq ft	16 in., 6 in.	14 in., 6 in.	12 in., 6 in.
60 sq ft	19 in., 6 in.	17 in., 6 in.	14 in., 6 in.
80 sq ft	22 in., 7 in.	19 in., 6 in.	16 in., 6 in.
100 sq ft	25 in., 8 in.	21 in., 6 in.	17 in., 6 in.
160 sq ft	31 in., 11 in.	27 in., 9 in.	22 in., 7 in.

Here’s how to find the area. Say the joists run 10 feet from the house ledger to a beam, with posts every 8 feet and no overhang. The ledger carries half the joist load and the beam carries the other half, so a middle post supports 5 by 8 feet, or 40 square feet. On clay that’s a 16-inch round footing. The end posts carry half as much, 20 square feet, so 12 inches works.

What size Sonotube for a 6×6 post

Table R507.3.1 sizes the footing, the concrete that bears on the soil, not the tube. If you pour a straight tube with nothing wider at the bottom, the tube’s diameter is your footing size and it has to meet the table.

That gets tight fast. Sonoco’s Builder’s Tube comes in 6, 8, 10 and 12 inch diameters, and Sonoco notes those sizes are nominal, so measure if it matters. At a 40 psf load, a straight 12-inch tube covers about 20 square feet of deck on clay and 40 on gravel. A post holding more deck than that needs a wider footing.

The usual fix is a wider footing at the bottom of the hole with the tube on top as a pier. The footing gets the bearing area, and the tube only has to fit the post and its hardware.

The code also says the footing must give the post complete bearing. A square post is wider across its corners than across a face, so a 6-inch tube is too small for a 6×6 to sit on. Measure your post corner to corner and buy the tube bigger than that.

When to call a pro or the building department

• Any footing for a house, addition, garage or attached deck. These usually need a permit, and the inspector may want to see the holes before the pour.
• Soft clay, wet ground, fill or a high water table.
• Rock or ledge above the frost line.
• Heavy loads such as a hot tub or a roof on a deck.
• Any digging at all: call 811 first so utilities get marked. We call before every job.

If you’d like the deck drawn with post spacing, beam sizes and footing sizes on paper before anyone digs, our online deck design plans lay it out for you or your builder.

Frequently Asked Questions

How deep should deck footings be?

At least 12 inches below undisturbed ground. A deck attached to the house also needs its footings below the local frost line, while a freestanding deck isn’t held to the frost rule under the 2021 IRC, though your building department may still require it.

How deep should footings be for a garage?

An attached garage follows the house: 12 inches minimum and below the frost line. A detached light-frame garage or shed of 600 square feet or less, with eaves 10 feet or lower, is exempt from frost protection; a bigger one is not.

What size footing do I need for a 6×6 deck post?

It depends on the deck area the post carries and your soil, not the post size. At 40 psf, a post holding 40 square feet needs a 16-inch round footing on clay, 14 on sand and 12 on gravel.

Can a footing sit on fill dirt?

Only if it’s engineered fill. Otherwise the IRC requires footings on undisturbed natural soil, so dig through loose fill until you reach it.

Sources: ICC, 2021 IRC R403.1 and Table R403.1(1); ICC, 2021 IRC R403.1.1 Minimum size; ICC, 2021 IRC R403.1.4 Minimum depth; ICC, 2021 IRC R403.1.4.1 Frost protection; ICC, 2021 IRC Table R401.4.1 Soil bearing values; ICC, 2021 IRC Table R301.2; ICC, 2021 IRC R507.3 Deck footings; ICC, 2021 IRC Table R507.3.1 Deck footing size; ICC, 2021 IRC R507.3.2 Deck footing depth; ICC, 2021 IRC R507.3.3 Deck frost protection; Sonoco, Sonotube Builder's Tube brochure

Written and reviewed by Thayne Muthler, owner of Muthler Landscaping & Construction (PA HIC #PA212584). How we check facts: editorial policy.