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 4x4 posts on one 2-ply 2x8 beam.
Where these numbers come from. The deck's guard and footing depth follow the Ontario Building Code 2024, Part 9, with its article numbers. Joists, beams and posts are sized from the CWC Prescriptive Residential Exterior Wood Deck Span Guide (2016) (incised treated lumber; 1- to 3-ply beams; 4x4 posts up to 6.5 ft, where the plan asks your building department to confirm them, since OBC 9.17.4.1 calls for 6x6); footing diameters use the US AWC DCA 6 table, for reference, since neither NBC Part 9 nor the guide has one for decks. The US reference 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), the load the guide is for. A town with heavy snow can need more: pick yours below, or have it 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. OBC 2024's Part 9 is based on NBC 2020. Joist, beam and post tables: Canadian Wood Council, Prescriptive Residential Exterior Wood Deck Span Guide (2016), designed to CSA O86-14 and NBC 2015.
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 Canadian Wood Council's Deck Span Guide (joists Table 3a, beams Table 5b, posts note 5), and sizes each footing for the deck area it carries (from the US AWC DCA 6 footing table, for reference: neither NBC Part 9 nor the guide has one for decks). It then designs the deck to the Ontario Building Code 2024, Part 9: a guard from 600 mm up, 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 Canadian Wood Council's Deck Span Guide (Table 5b), which lists 1-, 2- and 3-ply beams with posts 4, 6 or 8 ft apart: a 2-ply 2x10 of D.Fir-L, Hem-Fir or S-P-F carrying joists that span 12 ft from a ledger can have posts up to 8 ft apart. Posts can be 4x4 up to 6.5 ft tall under a 1- or 2-ply beam, else 6x6; OBC 9.17.4.1 calls for 6x6 posts unless calculations show less, so the plan asks your building department to confirm a 4x4. 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 AWC DCA 6 table, for reference: neither NBC Part 9 nor the CWC guide has a footing table for decks) 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.