Deck Footing Size and Post Height Tables

Use the deck footing calculator to work out the area one post carries from your joist length and post spacing, then read both tables at once. For the full deck footprint and a scope every contractor can price, continue with the deck planning guide.

A deck’s posts stand on footings, and the residential code sizes both with tables: a footing by the deck area its post carries and the soil under it (Table R507.3.1), and a wood post’s greatest height by its size, its species and that same area (Table R507.4). The code calls that area the tributary area: the deck surface a post and its footing hold up. The tables below are the code’s rows for a live load of 40 psf (pounds per square foot), the model code’s minimum for a deck, which Austin keeps; Austin’s ground snow load is 5 psf, so those rows govern there. Los Angeles designs decks for California’s 60 psf, which the last section below covers. The posts are No. 2 grade or better, with the wet service factor built in: the strength reduction for wood that stays damp outdoors. The post heights also assume a 10 psf dead load, the weight of the deck itself, with no snow at the same time as the live load (Table R507.4).

How big should a deck footing be?

Read down to the deck area the post carries, in the table for the soil under the footing. Each row gives a square footing’s side or, for a round footing, its diameter, and in either case the thickness of plain (unreinforced) concrete. A post carrying 60 square feet of deck on clay, at 1,500 psf, needs a square footing 17 inches on a side or a round one 19 inches across, 6 inches thick; on sand, at 2,000 psf, it needs 15 inches square or 17 inches round, still 6 inches thick. The code allows reading between two rows, but not past the 5 to 160 square feet the table covers. The footing is wide enough for the post to bear on it completely, and where a brick or concrete block (CMU) pier stands on it, the footing projects at least 2 inches past the pier on every side.

Which soil column to read

Unless a complete geotechnical evaluation says otherwise, the code has you assume the load-bearing values below (Section R401.4.1). Clay and silt are taken at 1,500 psf, the lowest value the footing table has. Where soil data show that expansive, compressible, shifting or otherwise questionable soil is likely, the building official decides whether to require a soil test, and where the building official finds soil bearing less than 1,500 psf likely, a soils investigation sets the value. Ask the city which value it accepts for your lot before reading a firmer soil’s column.

How deep do deck footings need to be?

A deck footing goes at least 12 inches below undisturbed ground (R507.3.2). Where the deck is attached to a house whose own foundation is protected from frost, the deck’s footings are protected too: below the frost line in the city’s Table R301.2, on solid rock, or by another approved method (R507.3.3). Austin puts its frost line at 12 inches and Los Angeles at 12. A free-standing deck whose joists rest on the ground their whole length, or a small, low one whose joists sit on precast concrete pier blocks, can need no footings at all; the deck joist and beam span guide gives the conditions.

How tall can a deck post be?

Table R507.4 gives the greatest height of a wood post under a single-level deck, measured from the underside of the beam to the top of the footing or pier. Read across from the deck area the post carries to its size. A Southern pine 4×4 carrying 60 square feet may stand up to 11 feet, and a 4×6 up to 13 feet 11 inches; 14 feet is the most these rows allow any post. NP, not permitted, means the code allows no post of that size and species at that area. The code allows reading between two areas, but not past the 20 to 160 square feet the table covers. A notched post, one cut to seat the beam, is sized for the beam as Section R507.5.2 requires.

Where a post bears on a concrete footing, it is held against sideways movement by an approved connector or by being embedded at least 12 inches in the surrounding soil or a concrete pier. Where the soil is expansive, compressible, shifting or otherwise questionable, the soil does not count toward that restraint (R507.4.1).

How far apart should deck footings be?

Along a beam, the footings sit under the posts, so they can be no farther apart than the beam’s allowed span, which the span tables give by species, beam size and the joists’ span and cantilever. Between two beams, or a beam and the house, the distance is the joists’ span. Spacing posts farther apart gives each one more deck to carry, and in the tables a larger area can mean a larger footing and a lower maximum post height: for 4×4 and 4×6 posts, and for 6×6 posts in the redwood group above 120 square feet.

Los Angeles

California’s code sets a deck’s live load at 60 psf, and the tables above are the 40 psf rows. The code’s footing table also has 60 psf rows, labeled for a live or ground snow load, but its post table’s 60 psf rows are labeled for a ground snow load, and it has no 60 psf live load row. Los Angeles’s code also narrows a permission for isolated plain concrete footings under columns or pedestals. California’s code permits them in detached one- and two-family dwellings of three stories or less with stud bearing walls; Los Angeles’s permits them there only in seismic design categories A, B or C, the code’s earthquake design classes, and its design criteria table (R301.2(1)) puts the city in D2/E. Neither code says whether this reaches a deck’s footings. Before sizing a Los Angeles deck’s posts or footings from a table, ask the city what it accepts; its code also allows an engineered design, under the Los Angeles Building Code, for any building or structure the residential code covers.

Whether the deck needs a permit at all is a separate question: the deck permit brief gives Austin’s and Los Angeles’s lines, and the deck calculator counts the joists and decking.