How far apart should decking joists be?

Two different spans are involved and people confuse them. The board spans between joists, which sets the joist centres. The joist spans between bearers or posts, which sets the joist depth. Getting one right and the other wrong produces a deck that either bounces or sags.

Step by step

1. Start from the board, not the joist.The board thickness and material set the maximum joist centres. 32mm softwood square to the joists takes 400mm; 25mm boards want 300mm; composite boards frequently specify 300mm or less.
2. Reduce the centres for diagonal boards.A board laid at 45 degrees spans roughly 1.4 times the joist spacing. Dropping from 400mm to 300mm centres restores the effective span.
3. Check the manufacturer's span table for composite.Composite boards vary widely and many require 300mm or even 250mm centres, closer still for diagonal or stair applications. This is not a place to generalise.
4. Size the joist for its own span.As a working guide for domestic decking, 150mm x 47mm treated softwood spans comfortably up to about 2.4m between supports, and 200mm x 47mm further. If in doubt, add a bearer and shorten the span.
5. Close the centres under point loads.Hot tubs, large planters, barbecues and seating all concentrate load. Double the joists or halve the centres locally, and check whether the load needs a designed foundation.
6. Keep the ventilation gap.At least 50mm of clear air below the joists, and airflow across the void from at least two sides. Joist spacing does not help if the void cannot breathe.
7. Add noggins on long joist runs.Solid blocking between joists at mid-span stops them twisting and stiffens the whole deck noticeably.
8. Check the deck height against the damp-proof course.The finished deck surface must stay at least 150mm below the damp-proof course of any adjoining wall. This often decides the joist depth available.

Tips

• When in doubt, close the centres. The extra joists cost far less than relaying a bouncy deck.
• Joists laid on edge with the crown upwards settle flatter than joists laid at random. Sight along each one before fixing.
• Use joist hangers where joists meet a bearer or a wall plate — skew-nailing into end grain is much weaker.

Common mistakes

• Using 400mm centres for composite boards — Most composite decking is less stiff than softwood of the same thickness and specifies closer centres. The result at 400mm is a deck that visibly sags between joists.
• Confusing joist centres with joist span — Joist centres are set by the boards; joist depth is set by the distance between the bearers underneath. Both have to be checked.

If it doesn't work

Questions people ask