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 Southern Pine joist that works is a 2x10, with 2 ft to spare.
US tables. Spans use US prescriptive tables until licensed Canadian tables are added; confirm with your building department. In Ontario, the deck falls under the Ontario Building Code 2024, Part 9.
| Size | Allowed span | Margin | Max cantilever | Result |
|---|---|---|---|---|
| 2x6 | 9 ft | 3 ft over | — | Fails |
| 2x8 | 11 ft 10 in | 2 in over | — | Fails |
| 2x10 | 14 ft | +2 ft | 2 ft 10 in | Pass |
| 2x12 | 16 ft 6 in | +4 ft 6 in | 3 ft | Pass |
Max cantilever: the lesser of a quarter of the span and the table limit for the size (R507.6). Shown for sizes that pass.
Where these numbers come from. Spans use US prescriptive tables (IRC 2021 Table R507.6, No. 2 lumber at 40 psf live and 10 psf dead load) until licensed Canadian tables are added; confirm with your building department. In Ontario, decks fall under the Ontario Building Code 2024, Part 9. 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.
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 IRC 2021 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 IRC 2021 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 IRC 2021 table, 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.
Use them as a starting point only. These numbers come from the IRC 2021, a US model code: 16Span uses US prescriptive tables until licensed Canadian tables are added. 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 table's 40 psf live load covers only that minimum, so in a town with heavy snow the joists may need to be deeper or closer. 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.