Joist Spacing and Bearer Sizing

Ensuring the correct spacing and sizing for joists and bearers is essential for building a sturdy, compliant deck. 
These elements form the backbone of your deck and must meet the requirements set out in AS 1684 – Residential Timber-Framed Construction.

Joist Spacing

The spacing of joists depends on the type of decking material and its thickness. Proper spacing prevents boards from sagging or flexing excessively.

Standard Joist Spacing for Decking Boards

Decking Material	Board Thickness (mm)	Maximum Joist Spacing (mm)
Softwood Timber Boards	19 mm	450 mm
Hardwood Timber Boards	19 mm	450 mm
Composite Decking Boards	19–22 mm	400 mm
Thicker Timber Boards	32 mm	600 mm

Joist Considerations

• Joist Orientation: Joists should run perpendicular to the decking boards for maximum support.
• Spacing for Wet Areas or High Loads: In areas subject to heavy use, like spas or BBQs, reduce joist spacing to increase load-bearing capacity.

Bearer Sizing

Bearers distribute the weight of the deck and support the joists. Their size and span depend on:

1. Timber Type: Hardwood or softwood.
2. Deck Load: Standard residential or higher load applications.
3. Span: The distance between supports (e.g., posts or stumps).

Typical Bearer Sizes for Single Span (Softwood)

Bearer Size (mm)	Joist Span Supported (m)	Maximum Bearer Span (m)
90 x 45	1.2	1.8
140 x 45	1.8	2.7
190 x 45	2.4	3.5

Typical Bearer Sizes for Single Span (Hardwood)

Bearer Size (mm)	Joist Span Supported (m)	Maximum Bearer Span (m)
100 x 50	1.2	2.0
150 x 50	1.8	3.0
200 x 50	2.4	4.0

Joist Sizing Based on Span

Joists must also be sized correctly for their span to prevent deflection.

Standard Joist Sizes and Spans (Softwood)

Joist Size (mm)	Maximum Span (m)
90 x 45	1.8
140 x 45	2.7
190 x 45	3.5

Standard Joist Sizes and Spans (Hardwood)

Joist Size (mm)	Maximum Span (m)
100 x 50	2.0
150 x 50	3.0
200 x 50	4.0

Tips for Correct Sizing and Installation

1. Use Span Tables: Consult span tables in AS 1684 for precise calculations based on your timber type, load requirements, and location.
2. Double Bearers or Joists: In areas with concentrated loads, such as posts supporting roofs or stairs, double the bearers or joists for added strength.
3. Durability Rating: Choose timber with a suitable durability rating for outdoor applications, especially if in contact with the ground.
4. Moisture Protection: Apply an anti-rot sealant to all cut edges, especially for joists and bearers in ground-contact areas.