Bowens - The Builders Choice

Bowens Formula to Building a Long Lasting Deck

Building Advice
Jeff Harvey

6 Key Factors

Continuing on the theme of last month’s article ‘Prevention is Better Than Cure,’ here we talk Decking. Decking is a product that can cause many post-installation problems, so using the formula to achieve a long-lasting deck is important.

With the above in mind and to avoid expensive remedial rectification, I list the following:


To achieve a good deck planning is vital, for the following reasons;

  • The site must have good drainage. If not, in winter the ground beneath the decking remains wet allowing the decking boards to absorb moisture causing expansion.
  • Choice of decking species. If good ventilation is not possible, consider using Merbau or a modified wood product because of their inherent higher stability.
  • Be aware that high tannin timbers can cause staining to surrounding concrete and pavers.


  • Ventilation is essential for board stability. Good ventilation aims to establish an environment below deck similar to above deck.
  • Gaps in surrounding base boards are essential, allowing air to ventilate the sub-deck space. These gaps should not be obstructed or blocked by structural timbers.
  • Effective sub-deck ventilation requires air to have a side entry point and an opposite side exit point.

Spacing of Decking Boards

  • The gap spacing between decking boards is very important, keeping in mind the boards will expand in winter and contract in summer.
  • Our recommendation is a gap of 4mm for boards up to 90mm wide and a minimum 6mm gap for boards up 140mm wide.
  • Decking boards with insufficient gap spacings often expand and come together in winter, there will be inevitable cupping and board distortion as they dry.


  • Coatings will enhance board stability and durability by acting as a sealing agent to minimize moisture ingress and assist in keeping the natural colour of the wood.
  • Factory-applied Precoating ensures the decking boards are coated on all sides prior to installation. It is important to re-coat all trimmed end grain including butt joins.
  • A coloured UV tint added to a clear finish will offer longer protection between re-coating periods.
  • For the long-term good condition of timber decking regular re-coatings are advisable. Depending on the decking’s exposure, wear and oil type used our suggestion is at least every 1 to 2 years.

Sub-deck Materials and Construction

  • Obviously, all the construction basics have to meet appropriate building regulations.
  • Stumps: If they are timber it should be a durable species, at least class 2 in-ground according to Australian Standard AS 5604—2005. We suggest a coating of bitumen paint for the in-ground component. If treated pine it must be at least a Hazard Level of H4 (suitable for in-ground contact), for critical use H5 may be required as per AS 1604.1 2012
  • All structural bearers and joists must be durable timbers. Perseverative treated timbers must be a Hazard Level H3 that is suitable for external use above ground. If a hardwood it should advisably be rated to class 1 in above-ground situations.
  • An important product often overlooked is a membrane installed over the joists. This is particularly important during the winter months so that the moisture content of the joists remains relatively low. If not used, treated pine, being a softwood, can become very soft affecting the deck fixings holding strength allowing the decking boards to cup and distort.
  • Good decking fixings will minimize board movement. Whether a screw or nail, the length is important. For 19mm thick decking, 65mm for a softwood joist or 50mm for a hardwood joist.


To ensure the long-term structural integrity and appearance of a deck, regular maintenance is vital. Annual inspections to identify any areas of concern early. Regular coatings will ensure ongoing good performance.

Perhaps in the original construction, an allowance is made (where possible) to inspect the sub-deck materials closely, particularly if that space has experienced periods of prolonged dampness.

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