Deck footing size, post spacing and bearer spans
Deck footing size depends on how much deck each post carries and what the ground is like. On stable ground in Australia, the NCC's pad footing table gives a 300 mm round pad for up to 3 m² of deck, 450 mm up to 8 m² and 600 mm up to 12 m². In the US, IRC Table R507.3.1 gives 8 to 31 in. round footings by area and soil, at least 12 in. deep and below the frost line for a deck attached to the house. Post spacing is simply how far your bearer (beam) can span: typically 1.5 to 2.4 m for a single pine bearer, or roughly 6 to 10 ft for a doubled beam.
These are general figures for planning. Footings depend heavily on soil, so have your builder, certifier or building department confirm them for your site.
How does the load reach the footings?
Decking sits on joists, joists on bearers (beams in the US), bearers on posts, and posts on footings. Each post carries its tributary area: half the joist span each side of its bearer, times half the bearer span each side of the post. Longer spans mean fewer posts but bigger footings.
How far apart should deck posts be?
As far as the bearer can span, and no further. The span allowed depends on the bearer size and its load width: how much deck it holds up, which is half the joist span each side.
Australia: single MGP10 pine bearers
| Bearer | 1.2 m load width | 1.8 m | 2.4 m | 3.0 m |
|---|---|---|---|---|
| 140 × 45 | 1.8 m | 1.5 m | 1.3 m | 1.1 m |
| 190 × 45 | 2.4 m | 2.0 m | 1.7 m | 1.5 m |
| 240 × 45 | 3.0 m | 2.4 m | 2.1 m | 1.9 m |
These are typical figures from Hyne's MGP10 bearer table, built on AS 1684. Doubled bearers go further: two 190 × 45s span 2.8 m at a 2.4 m load width (TABMA Technical Note 3). Deckulator lets a bearer run past its end post by up to a quarter of its allowable span, and never more than half the span behind it.
United States: No. 2 Southern pine beams, 40 psf
| Beam | 8 ft joists | 10 ft | 12 ft |
|---|---|---|---|
| 2-2×8 | 7 ft 7 in. | 6 ft 9 in. | 6 ft 2 in. |
| 2-2×10 | 9 ft 0 in. | 8 ft 0 in. | 7 ft 4 in. |
| 2-2×12 | 10 ft 7 in. | 9 ft 5 in. | 8 ft 7 in. |
| 3-2×10 | 11 ft 2 in. | 10 ft 0 in. | 9 ft 2 in. |
| 3-2×12 | 13 ft 3 in. | 11 ft 10 in. | 10 ft 9 in. |
From IRC Table R507.5(1). The 2021 code reads the column as an effective joist span: the actual span times a factor from 0.66 (no overhang) up to 1.00 (joists overhanging the beam by a quarter of their span). The 2024 edition labels the columns by span and overhang instead. A beam may overhang its post by up to a quarter of its span. Check the figures against your printed code before you build.
How big should deck footings be?
Australia
NCC 2022 Housing Provisions Table 4.2.13a, pads under stumps, Class A and S sites (100 kPa bearing):
- Up to 3 m² per post: 300 mm diameter
- Up to 8 m²: 450 mm
- Up to 12 m²: 600 mm
The table only covers Class A and S sites. On reactive clay or fill (Class M, H, E or P), have the footings designed to AS 2870. Your site classification may be on your house plans or in a soil report.
United States
IRC Table R507.3.1, round footing diameter at 40 psf:
| Area per post | Clay, 1,500 psf | Sand, 2,000 psf | Gravel, 3,000 psf |
|---|---|---|---|
| 20 sq ft | 12 in. | 9 in. | 8 in. |
| 40 sq ft | 16 in. | 14 in. | 12 in. |
| 60 sq ft | 19 in. | 17 in. | 14 in. |
| 80 sq ft | 22 in. | 19 in. | 16 in. |
| 100 sq ft | 25 in. | 21 in. | 17 in. |
Soil values are the code's presumptive ones (Table R401.4.1). Soil weaker than 1,500 psf needs a soils investigation.
How deep should deck footings be?
Australia
There is no single national depth for deck footings: it comes from your site classification and footing design. Deckulator's concrete count assumes 400 mm deep holes, so treat that as a placeholder until your ground is known.
Posts on galvanised stirrups need 75 mm clear underneath so termites can be seen. A post set in the ground needs H5 treatment, and framing within 150 mm of the ground needs H4 or better.
United States
At least 12 in. below undisturbed ground (IRC R507.3.2). A deck attached to the house must also go below the local frost line (R507.3.3); your building department sets that depth. The model code doesn't require frost depth for a free-standing deck, but check your local rules.
Posts need a connector at the footing, or at least 12 in. of embedment (R507.4.1).
What size deck posts do I need?
- Australia: Deckulator keeps posts to 15 times their width: 90 × 90 up to 1.35 m, 100 × 100 up to 1.5 m and 125 × 125 up to 1.875 m. Taller posts need an engineer. Every deck also needs bracing, whether diagonal braces, posts concreted in, or straps under the joists.
- US: IRC Table R507.4 limits a Southern pine 4×4 by the area it carries: 13 ft 8 in. at 40 sq ft, 11 ft 0 in. at 60 and 8 ft 4 in. at 100. A 6×6 reaches the table's 14 ft cap. DCA 6 is stricter and calls for 6×6 posts as a minimum.
Worked example
Australia. A deck 4.8 m wide and 2.8 m out from the house, 900 mm high, with 140 × 45 joists on a ledger and one bearer 300 mm in from the front. The joists span about 2.5 m, so the bearer carries a load width of about 1.5 m. That rounds up to the 1.8 m column, where a 190 × 45 spans 2.0 m. Three posts at 2.0 m centres, with 400 mm overhangs, cover the 4.8 m bearer. The middle post carries about 3.1 m², just over the 3 m² step, so it gets a 450 mm pad; the end posts carry about 2.1 m² and get 300 mm pads. That is about 0.12 m³ of concrete, or 13 bags of 20 kg.
United States. A deck 16 ft wide and 12 ft out, attached, with 2×8 joists at 16 in. spanning about 11 ft to a beam 1 ft in from the edge. A 2-2×8 beam spans 6 ft 9 in., so three posts carry it. On clay, the middle post (about 44 sq ft) needs a 19 in. footing and the end posts (about 30 sq ft) need 16 in. Because the deck is attached, those footings go below your frost line, not just 12 in.
Common questions
How far apart should deck posts be?
As far as the bearer or beam can span for the load it carries. Single MGP10 pine bearers typically span 1.5 to 2.4 m at a 1.8 m load width; doubled Southern pine beams under the IRC span roughly 6 to 10 ft.
How deep should deck footings be?
In the US, at least 12 in. below undisturbed ground, and below the local frost line for a deck attached to the house. In Australia the depth comes from your site classification and footing design, not a single national figure.
What size footing does a deck post need?
On stable Australian ground, the NCC gives a 300 mm pad for up to 3 m² of deck per post, 450 mm up to 8 m² and 600 mm up to 12 m². In the US, IRC Table R507.3.1 gives 8 to 31 in. depending on area and soil.
Do I need an engineer for deck footings?
In Australia, the standard pad table only covers stable Class A and S sites; reactive clay or fill needs footings designed to AS 2870. In the US, soil weaker than 1,500 psf needs a soils investigation.
This guide explains common rules in plain words. It isn’t engineering advice: span tables, footings and barriers depend on your timber grade, soil, wind and local rules. Have a builder, certifier or building department check your design before you build.