Structural Timbers

While most people often only consider the boards that will be laid on their deck, just as much consideration should be given to the structure underneath their deck. While most people will be using treated pine timber, it is important to know that not all treated pine timber is the same. As well, there is hardwood structural timbers, engineered timbers, and composite timbers. 


Treated Pine

Treated pine is the most common structural timber used in the construction industry in Australia. It is also used extensively in the construction of decks. It has the advantage of being readily available, at a reasonable price. 

When it comes to using treated pine outdoors, it is important to understand the different levels of treatment and when to use which one. 

  • H2 is for inside use only and protects against termites
  • H3 is for outside, above ground only. It provides protection against termites and wood rot
  • H4 is for outside, non-structural, in contact with ground, e.g. garden edges, and posts
  • H5 is for outside, structural, in contact with ground e.g. decks and pergolas with the posts imbedded into concrete footings.
  • H6 is for outside, structural, in contact with salt water. e.g. boardwalks in coastal areas, piers, jetties, etc.

There are also different stress grades. Stress grading refers to the strength of the timber, and a higher stress grade means a particular size timber will span a greater distance than a lower graded timber. Most outdoor treated pine timber is sold in the F5/MGP10 rating, and design work should reflect this unless you are willing to source a higher grade.

Treated pine is also available in a number of different finishes. The following is organised from most affordable to most expensive.

  • Rough header - recognisable from its distinctive lines. Quality varies among suppliers so beware of a cheap price.
  • Treated pine DAR - DAR stands for dressed all round, and this has a smooth finish to all four sides
  • Treated pine DAR Primed F/J - This is a manufactured pine product, made from smaller pieces joined and glued together and then finished with a coating giving it a very smooth, straight appearance.
  • Treated Pine DAR Primed - Same as previous, but made from solid timber rather than finger jointed smaller pieces

When building in treated pine, care and attention should be given to any exposed end grain, and it is recommended to protect the top of bearers and joists with a protective rubber membrane.

Hardwood Structural Members

Whilst Treated pine is great for most residential decks, some situations require a higher level of strength and durability. Hardwood timber is available in much higher stress grades than pine, and therefore can span much greater distances. As well as greater strength, careful choice of timber species combined with good design and careful maintenance can mean a deck that will last for 40 years and more.

Choosing a timber to use for hardwood structural members should be influenced by availability, durability, and strength requirements. Also expect to pay significantly more for hardwood bearers and joists.

Hardwood Bearers and joists are commonly available in unseasoned F17 stress grade, and up to F27 in Kiln dried DAR. 

Engineered Timbers

There are a number of engineered timbers on the market, however most of them are unsuitable for a deck that will be exposed to the weather. Engineered timbers are very strong in comparison to ordinary structural timber, however they get this strength from many thin sections glue laminated together. Since the laminations are prone to separating when exposed to the weather for a prolonged period of time, they are not usually recommended for external use. There are a few products that have been treated for external use on the market, and combined with careful construction techniques may be used satisfactorily, there is usually no compelling reason to use these outside.

If your site or design calls for much larger free spans, consider incorporating steel beams with plates welded to accept timber joists, or a proprietary steel framing system with the decking boards screwed to the steel joists.

Composite Timber

You may be familiar with the recent arrival of composite decking boards. These are made of a combination of timber fibres and recycled plastics, commonly old milk cartons. These offer the promise of being warp free, defect free, rot free, and maintenance free. The same base material is now being used to manufacture all sorts of building materials, including bearers, joists, handrails, and balustrades. These have an attractive woodgrain appearance, and are particularly suited to marine environments that are quite harsh on the timber. 

If you are after an environmentally friendly option for your deck, these may be the right option for you. Combined with composite timber decking boards, you can both save landfill, and have a maintenance free deck for many years to come. It isn't cheap however, and expect to pay a significant premium over a standard timber deck. There is also the issue of embedded energy in the building material. Normal structural timber has a very low energy input in the manufacturing process compared with composite timber boards. Also there are some good arguments for using timber from renewable resources like plantation forests that also act as net carbon sinks.

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