Joist Size and Spacing
There are lots of combinations of joist sizes and spacing. The correct choice depends on the decking material requirements, joist span, cantilevers and how solid you want the deck. First, you will want to meet the requirements of your building codes. When applying for a deck permit, you need to indicate what joist spacing and size of timber you plan to use.
| Joist Size | 8" OC | 12" OC | 16" OC | 24" OC | Cantilever |
| 2" x 4" | ≤ 7' 4" | ≤ 6' 5" | ≤ 5' 10" | ≤ 5' 1" | ≤ 8" |
| 2" x 6" | ≤ 11' 6" | ≤ 10' 0" | ≤ 9' 1" | ≤ 8' 0" | ≤ 16" |
| 2" x 8" | ≤ 15' 1" | ≤ 13' 2" | ≤ 12' 0" | ≤ 10' 2" | ≤ 16" |
| 2" x 10" | ≤ 16' 2" | ≤ 14' 0" | ≤ 11' 5" | ≤ 24" | |
| 2" x 12" | ≤ 16' 3" | ≤ 13' 3" | ≤ 24" |
What is a joist span?
The distance from the ledger board to the beam or from beam to beam is the span. Joists are anchored or resting on these foundation elements. In the photo below, the green portion of the joists is the span. They sit on top of the beam (Blue) and rest inside the joist hangers butted against the ledger board (Orange). In this example, the joists have a span of fourteen feet.
O.C.
On-Center is the distance between each joist—midpoint to midpoint. There are four standard spacing options available. | 24" | 16" | 12" | 8" |
It is the O.C. spacing and the dimension of lumber (e.g. 2x10) that determines the allowable span. For instance, 2x10 lumber with 12" spacing can have a span ≤ 16' 2". Composite materials often have requirements like 16" or 12" on-center joist spacing. This tighter spacing is needed to prevent warping in the boards. It also reduces any trampoline effects from joists spread out to far. Often joist spacing is a tradeoff between span needs, decking, rigidity and cost.
Common uses
- 24" O.C. - 2x dimensional lumber
- 16" O.C. - 5/4" decking and composite
- 12" O.C. - 45-degree angled composite
- 8" O.C. - Expensive, Avoid.
Cantilever
How far past the beam a joist hangs is the cantilever. Each joist height has a different allowable cantilever distance, which can be added to your span to lengthen your deck. In the example photo below, the joists are 2x10's; these have the option for a 2' cantilever. Combined with the 14' span, the deck can extend 16' with one beam. Sometimes your span and cantilever are not enough. If this happens, consider adding another beam. With a second beam, you may be able to downsize your timber or increase the on-center spacing to recoup some extra beam costs.
Pricing
To reduce costs, use larger on-center spacing between joists.
A 2x8 at 8" O.C. is going to be your most expensive framing option.
Spans Types Cost Chart
Here is a graph that shows the different joist size and spacing from least expensive to most expensive.
A cost-effective option for a 12-foot deck is 2x10 24" O.C.
Next, best is 2x8 16" O.C.
Span & Cantilever charts
Four charts with spans on the left and totals with cantilevers on the right.
Examples:
12' joists with 5/4" decking
2x8 16" O.C. | ≤ 12' Optional 16" Cantilever |
18' joists with composite decking
2x12 16" O.C. | ≤ 16' 3" Optional 24" Cantilever |
14'6"
joists with 45-degree angle composite decking
2x10 12" O.C. | ≤ 16' Optional 24" Cantilever |
12" spacing needed because of the angled deck pattern.
Material Type
Your joists should be pressure treated Spruce-Pine-Fir #1 or #2Do not use untreated lumber, yes it's cheaper, but it will rot. Adding weather sealer or decking tape is not adequate.
There are also some steel framing options from brands like Trex and Fortress building products. They have the advantage of longer spans and much bigger cantilevers. And are an excellent option for second-floor decks to minimize the number of posts and beams.
Permit
The City of Edmonton's Deck Design Form in section two will ask you to identify the joist size and spacing you intend to use. When the city inspects your deck, your joists' are part of the list. The only way you would fail is if you were under the minimum requirements for your deck size. If your ever in doubt, contact us.
