Reference data
Deck spans and spacing
How far each joist size spans by spacing and species, what spacing each decking material allows, and the gaps a guard has to close.
Written and edited by Pablo Ruiz Quintero
Last reviewed 2026-09-16
How far can a deck joist span, and how far apart do the joists go?
Two different questions that get answered as one. How far a joist can span is a structural limit set by its size, the spacing and the species of the timber. How far apart the joists go is set by the decking on top of them, which has to bridge the gap without flexing.
Get the first wrong and the deck bounces or sags. Get the second wrong and the boards feel soft underfoot between joists — and with composite it voids the warranty.
Joist spans, southern pine No. 2
| Joist | Depth | 12 in oc | 16 in oc | 24 in oc |
|---|---|---|---|---|
| 2 × 6 | 5.5 in | 9.9 ft | 9.0 ft | 7.6 ft |
| 2 × 8 | 7.25 in | 13.1 ft | 11.8 ft | 9.7 ft |
| 2 × 10 | 9.25 in | 16.2 ft | 14.0 ft | 11.4 ft |
| 2 × 12 | 11.25 in | 18.0 ft | 16.5 ft | 13.5 ft |
What to take off for your species
| Species | Factor | 2 × 8 at 16 in | Where you find it |
|---|---|---|---|
| Southern pine | ×1.00 | 11.8 ft | The species DCA 6 tabulates, and what most treated lumber east of the Rockies is. |
| Douglas fir-larch | ×0.96 | 11.3 ft | Common in the west. Spans about four per cent short of southern pine. |
| Hem-fir or SPF | ×0.90 | 10.6 ft | Softer and weaker. Ten per cent less span, which often costs you a joist size. |
| Cedar or redwood | ×0.83 | 9.8 ft | Rot resistant and structurally weak. Rarely used for framing, and never for a beam. |
That last column is the reason the species question is worth asking at the yard. The same 2 × 8 at the same spacing spans 11.8 ft in southern pine and 10.6 ft in hem-fir. On a twelve foot deck one of those works and the other does not.
Joist spacing by decking material
| Decking | Thickness | Perpendicular | Diagonal | Side gap |
|---|---|---|---|---|
| Pressure treated 5/4 × 6 | 1 in | 16 in | 12 in | 1/8 in |
| Pressure treated 2 × 6 | 1.5 in | 24 in | 16 in | 1/8 in |
| Western red cedar 5/4 × 6 | 1 in | 16 in | 12 in | 3/16 in |
| Composite, wood and plastic | 0.94 in | 16 in | 12 in | 3/16 in |
| Cellular PVC | 0.94 in | 16 in | 12 in | 3/16 in |
Gaps, guards and stairs
| What | Limit | Why it is that number |
|---|---|---|
| Drop requiring a guard | over 30 in | Measured to the lowest ground within a few feet of the deck, not to the nearest. |
| Guard height | 36 in | Residential decks. Commercial work is higher. |
| Baluster gap | under 4 in | A 4 in sphere must not pass anywhere, including under the bottom rail. |
| Baluster centres | 5.5 in max | With a 1.5 in baluster. Work tighter so the last gap at the post does not push you over. |
| Guard post spacing | 6 ft | The rail has to carry a person falling against it, and the load ends up in the post connection. |
| Stair riser | 7.75 in max | Divide the total rise by this and round up to get the number of risers. |
| Stair tread | 10 in min | Two 2 × 6 boards give 11.25 in, which is why that is the usual build. |
| Riser variation in a flight | 0.375 in | Tallest minus shortest. People climb on a rhythm set by the first two steps. |
| Stringer spacing | 16 in | 12 in under composite treads, which bend where a 2 × 6 does not. |
What these numbers are, and what they are not
The spans come from the wood council’s prescriptive deck guide, which is the document most residential deck construction in the United States is built to. The guard and stair limits come from the residential code. The decking spacings come from the manufacturers’ own installation instructions, and those are stricter than the code — a composite deck framed at the code maximum rather than the manufacturer’s can lose its warranty while passing inspection.
None of it replaces your building department. Snow load, seismic category and locally adopted amendments all move these figures, and the inspector arrives with a specific table in hand. Use this to size the deck and to know when you are near a limit; confirm before you buy lumber.
The one that surprises people
Diagonal decking. It looks like a styling choice and it is a structural one: a board crossing the joists at forty-five degrees spans about forty per cent further between supports than the same board laid square. That is why every material here drops to twelve inches on centre for a diagonal layout — a third more joists, before any decking is bought.
Using it
The deck calculator picks the smallest joist that spans your deck at the spacing your decking allows, corrects for the species you tell it, and warns you when a choice puts you over a limit rather than quietly sizing around it.
Send this to whoever is paying
The link carries your measurements, so whoever opens it sees the same numbers you are looking at — and you can come back to it yourself tomorrow with the delivery note in front of you.
Plain links, no tracking scripts. Nothing here reports back that you shared it.
About the author
Pablo Ruiz Quintero — Editor
Editor of MaterialsEstimator. Decides which documents count as a source, what every default value is, and where the line sits between what the reader parses and what the engines compute.
More about how this site is edited