How far can a deck joist span?

Joist span is the one number that sets the shape of the whole frame. Set the depth of your deck below and the table works out every joist size that carries it, at every spacing, in the species your yard actually stocks.

The short answer

It depends on four things: the joist size, how far apart the joists are, the species and grade of the lumber, and whether the lumber was incised when it was treated.

For the common Alberta case, spruce pine fir at 16 in centres, the maximum spans in the span table are about 9 ft 1 in for a 2x6, 12 ft for a 2x8, 15 ft 2 in for a 2x10 and 17 ft 7 in for a 2x12 when the lumber is not incised. Incised pressure treated lumber loses roughly 5 percent: about 9 ft, 11 ft 5 in, 14 ft and 16 ft 3 in.

Where guards are required, the joists and rim boards have to be at least a nominal 2x8 so the guard posts have something to bolt to.

The four things that move the number

Depth of the joist. Stiffness goes up with the cube of depth, which is why a 2x10 does not span a quarter more than a 2x8, it spans a quarter more than a 2x8 and then some.
Spacing. Closer joists each carry less floor. Going from 16 in to 12 in buys roughly 1 ft on a 2x8.
Species and grade. Spruce pine fir, Douglas fir larch, hem fir and the northern species group all span differently, and the northern group is noticeably weaker. Buy what the table you used assumed.
Incising. Pressure treated lumber in 2x8 and larger is usually incised, the rows of little dimples that let preservative in. Those dimples cost roughly 5 percent of the span, and the span tables give incised and non incised values separately.

The span table, spruce pine fir

Joist	8 in	12 in	16 in	24 in	Cantilever
2x4	7 ft 4 in	6 ft 5 in	5 ft 10 in	5 ft 1 in	8 in
2x6	11 ft 6 in	10 ft	9 ft 1 in	8 ft	16 in
2x8	15 ft 1 in	13 ft 2 in	12 ft	10 ft 2 in	16 in
2x10	19 ft 3 in	16 ft 10 in	15 ft 2 in	12 ft 5 in	24 in
2x12	23 ft 5 in	20 ft 4 in	17 ft 7 in	14 ft 4 in	24 in

Maximum spans for spruce pine fir No 1 and No 2, not incised, from the Canadian Wood Council deck span guide. Joist stock over 16 ft is not normally available, so the longer entries are theoretical for most yards. Use the demonstration above for the incised values and the other species.

Cantilever is part of the span

A joist can run past the beam, which is how a drop beam hides the posts and the footings behind the fascia. The cantilever limits in the span guide are 8 in on a 2x4, 16 in on a 2x6 and 2x8, and 24 in on a 2x10 and 2x12, measured from the centre of the beam.

The trap is arithmetic. A 14 ft deck with a 2 ft cantilever is not a 12 ft span problem plus a cantilever: when you pick the beam, the number you look up is the joist span plus the cantilever, because that is the load reaching the beam. Getting that wrong undersizes the beam.

Guards push you to a 2x8

Both the deck span guide and the Edmonton guide say the same thing: where guards are required, the joists and rim boards should be at least a nominal 2x8. It has nothing to do with span. It is about having enough depth of wood behind a side mounted guard post to carry the load when someone falls against the guard.

So on a deck that needs a guard, a 2x6 joist is the wrong answer even where the table says it reaches.

Picking the spacing

The Edmonton guide sets joists no more than 600 mm (23 9/16 in) apart, the widest column in the span tables. What usually decides the spacing is not the joist at all, it is the decking:

Boards under 1 1/2 in thick, such as 5/4x6, are supported at up to 16 in.
Boards a full 1 1/2 in thick, such as 2x6, go to 24 in.
Composite, PVC and membrane systems follow their own evaluation report, which is often tighter than either.
Any diagonal or herringbone pattern shortens the effective span of the board, so the joists come closer together.

The decking answer works through that side of it.

What you may not do to a joist

Do not notch it deeper than about a quarter of its depth, and keep notches away from the middle third of the span where the bending is. Do not run a butt joint in mid span: joints land over a support. A joist with a plumbing stack cut through the bottom of it is a broken joist wearing a disguise.

Where this comes from

Canadian Wood Council, Prescriptive Residential Exterior Wood Deck Span Guide, 2016, Tables 2b and 3b, imperial, for joist spans not incised and incised, and the cantilever column.
City of Edmonton, Uncovered Deck Design Guide, Section 2, Joists, which states that the spans on its permit application come from that guide for spruce pine fir No 1 and No 2, and Section 3 for the decking spacing limits.

Nothing on this page is an approval or a design. Code editions change and every authority having jurisdiction has the final word. Check your numbers with yours before you build.