Enter the deck's size and height to see the beam, how many posts and footings it needs, how far apart, and how big.
A 16 × 12 ft attached deck in Ontario needs 4 footings at least 16 in across and 48 in deep, under 4 6x6 posts on one 2-ply 2x8 beam.
Where these numbers come from. The deck's guard, footing depth and post size follow the Ontario Building Code 2024, Part 9, with its article numbers. Member sizes and footing diameters use US prescriptive tables (IRC 2021 Section R507, No. 2 lumber at 40 psf live and 10 psf dead load) until licensed Canadian tables are added; confirm them with your building department. Those table values are still being checked line by line against the code book. Snow: the calculator doesn't know your town, so it uses Part 9's minimum deck load of 1.9 kPa (40 psf) (OBC 9.4.2.3). A town with heavy snow can need more, so have yours checked; the full planner looks it up. Treat the result as a starting point and confirm your design with your local building department before you build.
The calculator runs the same instant designer as the 16Span planner. It picks the lightest joists that span your deck's width, using up to 2 ft of cantilever, then the cheapest beam and post layout that passes the US IRC 2021 beam table (until licensed Canadian tables are added), and sizes each footing for the deck area it carries. It then designs the deck to the Ontario Building Code 2024, Part 9: a guard from 600 mm up, 6x6 posts, and footings 1.2 m down unless the deck is small enough to be exempt.
An attached deck hangs its house side on a ledger bolted to the house framing, so it needs one beam and one row of footings. A freestanding deck replaces the ledger with a second beam, so it needs twice the posts and footings, but nothing is fastened to the house.
Post spacing is measured center to center along the beam. The beam table sets how far a beam can reach between posts. A deeper beam or an extra ply means fewer posts and footings, in exchange for heavier lumber.
Diameters assume 1500 psf soil, the lowest bearing class in the US footing table. Soft clay or fill may need bigger footings. Depths follow Table 9.12.2.2: with the soil unknown, at least 1.2 m (dug to 48 in) and below the frost line. A small, low, freestanding deck with no roof is exempt (9.12.2.2.(7)), so its footings can stay shallow.
Give yourself margin when posts are tall, the soil is soft or the deck will carry heavy loads: add a post or step up the footing. A failing check means the deck is too wide for one joist span and needs another beam line.
Attached to the house with a ledger and 3 ft up in Ontario, 16Span's design uses 4 footings under one beam, each at least 16 in across and 48 in deep. Freestanding, it needs 8 footings, because a second beam takes the place of the ledger. A heavier beam can mean fewer footings; a lighter one, more.
As far as the beam can span between them, which depends on its size, its number of plies and how far the joists span onto it. 16Span sizes beams from the US IRC 2021 beam table until licensed Canadian tables are added: there, a 2-ply 2x12 Southern Pine beam carrying joists that span 12 ft can have posts up to 8 ft 7 in apart, and a 3-ply 2x12 up to 10 ft 9 in. In Ontario, the posts themselves must be at least 140 × 140 mm (OBC 9.17.4.1), so 6x6. Confirm the sizes with your building department.
Big enough to spread each post's load over the soil. The footing table 16Span uses (the US IRC 2021 table, until licensed Canadian tables are added) works from the deck area each footing carries: at 1500 psf soil, about 16 in across for 40 sq ft and 20 in for 60 sq ft. Softer soil, such as wet clay or fill, needs bigger footings.
In Ontario, the Ontario Building Code 2024, Part 9 (Table 9.12.2.2) asks for at least 1.2 m (about 4 ft) where the soil isn't clearly known, and below the frost line. A small deck is exempt (9.12.2.2.(7)): one storey, not more than 55 m², no roof, not attached to the house, and not more than 600 mm from the ground to the underside of the joists. Ask your local building department for the frost depth where you are.
16Span draws this deck with its blueprint, a 3D model, a priced materials list, the code check and the build steps.