Decking Know How Tips & Advise

Species Durability Class Movement In Use Bleeding Splintering Length Specification Notes
Accoya 1 .0.8 - 1.5% Low Low 2.4 - 4.8M
Jarrah 2 5.0 - 7.5% Low Low 0.9 - 5.4M (2.4M avg) Watch swelling
Kwila 2 1.5 - 3.0% High Low 1.8 - 5.4M (3.0M avg) Watch bleeding
Pilularis/Blackbutt 2 4.0 - 7.0% Low-Medium Low 1.8 - 4.5M (2.7M avg) Watch swelling
Radiata (treated) 2 3.0 - 4.5% Low Medium 1.8 - 6.0M (3.0M avg) More likely to check and splinter
Rosewood 1 1.0 - 2.0% Low-Medium Low 1.5 - 4.5M (2.4M avg)
Saligna 2 - 3 5.0 - 9.0% Low-Medium Low 1.8 - 4.5M (2.7M avg) Watch swelling
Vitex 2 2.0 - 4.0% Low-Medium Low 1.5 - 3.6M (2.4M avg) Shorter lengths
 Spotted Gum 1   Low Low    


More about Slip Resistance

The Department of Building and Housing advises that decks, not on the route to the main entrance do not require a slip resistance of 0.4 to meet the Building Code clause  D1 (Access Routes).

However any exposed timber decking on the route to the main entrance will probably need to have some form of increased slip resistance.

More information click here

Expansion Gaps

Timbers commonly used in New Zealand vary enormously in terms of stability (how much they shrink and swell). Three factors need to be considered in determining the gap between boards:

  • Moisture Content: The moisture content of the framing must not exceed 20% at the time of fixing the decking as problems may occur later and cause excessive timber movement if framing is too wet. 
  • Species: Expansion gap for each species is different. Refer the following table for information. Decking species at ITI Timspec include Kwila, Vitex, Spotted Gum and Accoya®
  • Size (width of the board): Wider boards will shrink/swell more than narrower boards so require a larger expansion gap.

This table gives recommended expansion gaps for Timber Decking:

  90-120mm Wide 130-140mm Wide
Species Green / Shipping Dry Air Dried / Kiln Dried Green / Shipping Dry Air Dried / Kiln Dried
N/A 3-4mm N/A 4-6mm
Blackbutt 3mm 4mm Not recommended 6-8mm
Jarrah Not recommended 5mm Not recommended Not recommended
Kwila 3mm 4mm Not recommended 6-8mm
Spotted Gum Not recommended 4mm Not recommended


Vitex 3mm 4mm Not recommended 6-8mm

For more information download our hardwood decking brochure, visit our downloads page.