Enter the span and spacing to see which joist sizes work, how much room each has to spare, and how far it can cantilever.
For a 12 ft span at 16 in on center, the smallest D.Fir-L / Hem-Fir / S-P-F joist that works is a 2x10, with 10¼ in to spare.
CWC deck guide spans. Written for outdoor treated lumber; the decking braces the joists, so no cross bridging. In Ontario, the deck falls under the Ontario Building Code 2024, Part 9.
| Size | Allowed span | Margin | Max cantilever | Result |
|---|---|---|---|---|
| 2x6 | 8 ft 8 in | 3 ft 4 in over | — | Fails |
| 2x8 | 10 ft 6½ in | 1 ft 5½ in over | — | Fails |
| 2x10 | 12 ft 10¼ in | +10¼ in | 1 ft 11½ in | Pass |
| 2x12 | 14 ft 11¼ in | +2 ft 11¼ in | 1 ft 11½ in | Pass |
Max cantilever: CWC Table 3a's limit for the size (400 mm for 2x6 and 2x8, 600 mm for 2x10 and 2x12). Shown for sizes that pass.
Where these numbers come from. Spans come from the CWC Prescriptive Residential Exterior Wood Deck Span Guide (2016), Table 3a (incised treated lumber, No. 2 or better), the lowest of D.Fir-L, Hem-Fir and S-P-F, with its cantilever limits. The guide is engineered to CSA O86 for outdoor decks, but it is not part of the building code: in Ontario, decks fall under the Ontario Building Code 2024, Part 9. 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.
Joist span is the clear distance a joist covers between supports: from the ledger board on the house, or a beam, to the face of the beam at the other end. Each row shows the longest span the DCA 6 table allows for that size at your spacing, and the margin: the room left over, or how far over the limit you are. The smallest size that passes is usually the cheapest.
Spacing is measured on center: from the middle of one joist to the middle of the next. 16 in is the usual choice under wood decking. Closer spacing lets a smaller joist span further. Composite boards and diagonal decking often call for 12 in, so check the decking maker's instructions before you settle on a size.
A cantilever is the part of the joist that runs past the beam. It lets you set the beam and footings in from the edge of the deck. The limit is a quarter of the span behind the beam or the table value for the size, whichever is less.
Go one size deeper when the margin is only an inch or two, or when you want a stiffer floor that bounces less. The table assumes ordinary deck use and No. 2 lumber. A hot tub, heavy planters or an outdoor kitchen need an engineered design, not a span table.
At 16 in on center, the smallest joist the AWC DCA 6 span table allows for a 12 ft span is a 2x10 in all three species groups. A 2x8 falls just short: Southern Pine 2x8s at 16 in stop at 11 ft 10 in. At 12 in on center, a Southern Pine 2x8 reaches 13 ft 1 in.
In the AWC DCA 6 span table (Table 2), for No. 2 lumber at 40 psf live load: 11 ft 10 in for Southern Pine, 11 ft 1 in for Douglas Fir-Larch, Hem-Fir or SPF, and 10 ft 7 in for Redwood, Cedar, Ponderosa or Red Pine.
Without. The span is the clear distance between the supports: from the ledger on the house, or a beam, to the face of the next beam. The part of the joist past the beam is the cantilever, and it is checked against its own limit.
For a deck in Ontario, the calculator reads the Canadian Wood Council's Prescriptive Residential Exterior Wood Deck Span Guide instead (Table 3a, incised treated lumber, No. 2 or better, the lowest of D.Fir-L, Hem-Fir and S-P-F). It is engineered to CSA O86 for outdoor decks, with no cross bridging. The answers above quote AWC DCA 6, the American Wood Council’s US deck guide. In Ontario, decks fall under the Ontario Building Code 2024, Part 9, which designs a deck for its snow load or 1.9 kPa (40 psf), whichever is greater (OBC 9.4.2.3). The guide is for 1.9 kPa, so in a town with heavy snow the deck needs an engineered design or the building department's review. Confirm the joist size with your local building department.
Describe the whole deck and 16Span sizes the beams, posts and footings too, with a blueprint, a 3D model, a priced materials list and the build steps.