Decking joist spacing guide

Planning a deck? Use the decking calculator to work out joists and boards. This guide covers the spacing rules.

What are joist centres?

Spacing is measured from the centre of one joist to the centre of the next, not the clear gap between them. The calculator uses that centre-to-centre distance.

Maximum spacing by board size

The TDCA (Timber Decking and Cladding Association) publishes maximum joist centres based on board dimensions and timber grade. For example:

• 120mm × 27mm board (C16 timber): max 400mm centres
• 144mm × 27mm board (C16 timber): max 400mm centres
• 144mm × 34mm board (C16 timber): max 500mm centres

See the full table on the decking calculator page or the TDCA Code of Practice.

Can I use wider spacing?

You can go up to the maximum for your board size and timber grade, but not beyond. Wider spacing saves on joists but increases deflection (bounciness). Wickes notes it's better to have a narrower gap at the edge than exceed 400mm.

Joist size and orientation

Joists run perpendicular to the deck boards. The TDCA recommends joists 170mm × 44mm or larger, installed at 300–600mm centres depending on board size. Always install on edge (not flat) for strength.

What about noggins?

Noggins (short cross-pieces between joists) add stiffness and prevent twisting. Wickes recommends staggered noggins spaced no more than 1200mm apart along the joist run.

Diagonal boarding

If you're laying boards diagonally (at 45°), use closer joist spacing than the table shows for straight boarding. The TDCA notes diagonal installations need tighter centres.

Sources: Wickes how to lay decking guide and TDCA Code of Practice TDCA/RD0801 E6 (Table 5), both read 26 October 2026.