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Not all engineered timbers are the same

With recent supply chain issues, builders have needed to look at alternate products – but do all engineered beams meet equivalent spans and if you use a different type of beam do you need a new approval?

With demand for new housing hitting an all-time high in 2021-22, delays are being experienced due to a shortage of timber. This has forced contractors to think outside the square, either looking at alternate products or modifying the design to suit the availability of materials to ensure work is finished.

Any change, as minor as it may seem, needs be done so without breaching building code compliance. Further to this, it’s important not to just assume one materials performance may be equivalent to another, even if it looks very similar, as for example different engineered wood products (EWP’s) such as LVL’s will have different maximum spans or loadings specific to that LVL type.

Here are some simple tips to ensure you don’t fall out of line when alterations occur on-site or when the materials you are seeking for the project is not available.

Ask first rather than ask afterwards

Like the traditional saying goes, measure twice cut once, the simplest thing to do is to ask the building certifier/surveyor for the project if the proposed changes trigger a different approval and if it will have any impact on approval or require additional validation. The best time to do this is prior to purchasing or installing the alternate product as it can be costly and difficult to remediate after the fact.

If it does trigger an amendment to approval or require further detailed information to support the use of the alternate product, find out what evidence is needed, this can be different depending on the method of verification and may require an engineers report or re-submitting the compliance information for a particular products technical specifications.


Where approval is needed for timber modifications, compliance can be achieved by using the housing provisions within the NCC. Plans given for approval should confirm the frame meets AS 1684 or AS 1720. Modifications to timber sizes, grading or treatment will trigger the need for a building assessment. The building certifier/surveyor is likely to request documentation including amended timber framing layout and/or roof truss computations.

Altering a beam type is most instances is unlikely to be “like-for-like” and will therefore require comment from the relevant authority.

As an example, changing an engineered timber product from LVL 13 to LVL 15 will have an impact on the member sizing, so it’s important your building certifier/surveyor is alerted to the change before it is done on site.

As an example, in looking up a span table for a 3.5 metre long LVL 15 floor joists spaced at 450mm centres, it specifies that it must be a minimum size of 170 x 42 mm, this is increased to 170 x 45 mm for LVL 13 floor joists in the same situation.

As a general rule of thumb, depending on the member length, span tables for different types of beams can vary between 100 mm and 300 mm.

Engineers asessment or performance solutions

Where modifications fall outside timber framing code requirements or the builder chooses a different material, the NCC allows the use of alternative methods to justify the result.

This is done through either engineers assessment using the likes of AS 1720 (timber structures) or via use of an alternate product such as use of a steel beam in place a timber member that meets relevant loading requirements of AS 4100 or AS 4600. The other option is to develop a Performance Solution with appropriate evidence and validation to support compliance.

For each of these options it is important to discuss this with the building certifier/surveyor for the project. Whilst they are not able to provide with design advice they will be able to provide information on what level or type of documentation and validation they will require for you to be able to demonstrate compliance for the alternate material/modification and whether it requires additional approval.

Preparing documentation

The NCC evidence of suitability provisions set out the required product evidentiary and documentation requirements. These are listed in Part A5 of NCC Volumes One and Two.

These evidentiary requirements apply to materials and designs meeting either the Deemed-to-Satisfy Provisions of the NCC or where a Performance Solution in used.

These provisions list 6 different options for how to demonstrate that the material, product or design meets the NCC. These include:

  • A CodeMark certificate of conformity
  • A certificate of accreditation under a state or territory accreditation system where they may be in place (such as the Victorian building product accreditation system)
  • A certificate by a JAS-ANZ accredited product certification scheme
  • A certificate or report by a NATA accredited testing laboratory
  • A report or certificate by a registered engineer or other appropriately accredited person
  • Another form of documentary evidence that demonstrates the product or design meets the NCC, this could include a Product Technical Statement or span tables and material specifications produced and validated for that product.

Where a Performance Solution is to be used to support the use of particular product or design that is outside of the Deemed-to-Satisfy Provisions, before a building certifier/surveyor can stamp off its approval, they must be presented with a performance-based design brief.

There is an obligation the applicant (builder) consults with all project stakeholders including (but not limited to) the owner, engineer and designer. Each party should agree to the changes and nominate how the solution proposed is to be assessed by the regulatory body.

The brief shall show

  1. all performance requirements and/or Deemed-to-Satisfy Provisions.
  2. the assessment method(s) used.
  3. an outline of the steps taken to reach a resolution.
  4. confirmation that the Performance Requirement(s) has been met; and
  5. include any conditions or limitations.
  6. There are no restrictions on content size; it is possible a brief may be covered within one page.

Expert verification

Experts used to justify a performance solution should be able to substantiate their experience and/or qualifications. It is common for qualified structural engineers to supply advice on structural changes.

On site inspections

Once changes are finalised and the framework is up, the applicant may need to notify the relevant authority and/or may need a specific inspection as part of compliance. The relevant authority must be satisfied mandatory notifications have either been appropriately administered or adequately addressed before confirming the frame is ok.

Additional information on approval process for minor structures can be found on our website Building permit - Do I need one?

To find out more, contact HIA’s Building Services team.

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