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The spacing your decking actually requires, and how many joists that means.
With these inputs:
| Result | Value |
|---|---|
| Joist spacing required | 16" on centre |
| Joists needed | 13 |
| Max span for 2x8 | 11.9 ft |
| Your span | 12.0 ft — TOO FAR |
| Deck area | 192 sq ft |
| Joist length each | 12.0 ft |
| Total joist lumber | 156 linear ft |
| Rows of blocking / bridging | 1 |
| Blocking pieces | 12 |
| Spacing limit set by | composite and PVC boards sag between joists, so 16" is the normal requirement |
A 2x8 only spans 11.9 ft at 16" on centre and you need 12.0 ft. Either go up a joist size, close the spacing, or add a beam so the joists span in two shorter runs.
Decking dictates joist spacing, not the other way round. Tell it what boards you are laying and at what angle, and it returns the spacing the material requires, the number of joists, and whether your planned span works at that spacing.
Nearly every composite and PVC brand requires 16 inches on centre for straight boards and 12 inches for diagonal installations. The boards are heavier and less stiff than wood, so they telegraph any extra spacing as a soft, bouncy feel. It is also a warranty condition — installing at 24 inches typically voids the cover even if nothing visibly fails.
Only with 2x wood decking laid straight across. 5/4 wood, composite, PVC and hardwood all need 16 inches or tighter. If you are re-decking an old frame built at 24 inches and want to move to composite, you have to add joists between the existing ones — which is the hidden cost that catches most re-decking projects out.
A board crossing the joists at 45 degrees has to bridge a longer unsupported distance — about 41% further than the spacing itself. So 16 inch spacing effectively becomes 22.6 inches of unsupported board. Dropping to 12 inch spacing brings the effective distance back to about 17 inches, which the board can handle.
Divide the width the joists are spaced along by the spacing in feet, then add one for the final joist. A 16 foot wide deck at 16 inch spacing needs 13 joists. Add extra for double joists under any butt joint in the decking and under anything heavy like a hot tub.
Yes, substantially. Closing the spacing shares the load between more joists, so each one carries less and can reach further. A 2x8 in southern yellow pine spans about 11 ft 10 at 16 inch centres but 13 ft 1 at 12 inch centres — so tightening the spacing for composite decking can remove the need for an extra beam.
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