Timber brings warmth, texture and a natural rhythm to commercial interiors. From feature panelling to acoustic ceilings and floors, it can lift a space that might otherwise feel clinical. The flip side is that timber is combustible, so design intent must be balanced with the National Construction Code (NCC) requirements for fire safety. That is where Group Number classifications and the right test evidence matter.

Group Numbers at a glance

In Australia, AS5637.1 sets out how to determine the Group Number of wall and ceiling linings. The Group Number reflects how a lining contributes to fire growth.

• Group 1 – minimal contribution to fire growth
• Group 2 – suitable in many commercial settings
• Group 3 – limited use where higher performance is required

The NCC then nominates which Group Numbers are required by building class, space type and location in the path of travel. Higher risk spaces such as exits, egress routes and some health or assembly areas typically require a higher performing group.

Note: floors are assessed separately using critical radiant flux methods. A Group Number certificate does not cover floor finishes.

How timber achieves compliance

As a rule, untreated solid timber, plywood and timber veneers should be assumed to be Group 3 (or worse) for wall and ceiling linings under AS5637.1, unless current, product-specific test evidence demonstrates otherwise for the exact build-up.

Practical pathways:

• Default assumption
  Locate Group 3 timber linings only where Group 3 is permitted by the NCC for the building class and space type.
• Proprietary systems with evidence
  Where a higher Group Number is required, rely only on a proprietary, factory-produced system that has AS5637.1 test evidence for the exact configuration specified. Evidence must match species, veneer thickness, substrate, adhesive, perforation pattern, acoustic backing, air gap and any other relevant details.
• Timber look without the risk
  In locations that require Group 1 or Group 2, consider non-combustible or mineral substrates with a timber-appearance finish that already carries the required classification.

Always ensure the tested assembly matches the specified assembly. Small changes to species, thickness, substrate or perforations can alter the Group outcome.

Acoustic panels and perforations

Acoustic linings often include slots, holes, backings and cavities. These features can change fire behaviour. Seek test evidence for the full system, not just the face veneer. Pay attention to perforation size and spacing, backing fabric, absorber type and cavity depth.

Documentation that smooths approval

• Current AS5637.1 certificates for the exact product and build-up
• A clear finishes schedule listing species, thickness, substrate and any perforation pattern
• Manufacturer installation instructions and any limitations of use
• A simple as-built confirmation that the installed system matches the tested system

Common pitfalls

• Assuming timber can be used wherever it is desired, then trying to justify it late in design
• Using generic letters or data sheets that do not reference AS5637.1 or the exact assembly
• Swapping substrates, backings or perforation patterns after certification has been secured
• Treating floors as if Group Numbers apply, rather than using the correct critical radiant flux evidence

WoodSolutions: a practical reference

For early screening and education, WoodSolutions hosts useful fire-hazard information on Australian timbers and common applications. Use it to orient options and terminology, then confirm the final specification with project-specific AS5637.1 evidence for the exact product system.

Balancing design and compliance

Timber and compliance are not at odds. With clear mapping of Group Number requirements, careful product selection and matching test evidence, you can deliver the warmth of timber where it makes the most impact and still meet the NCC.

How we help

Krneta works with clients and design teams to set the right compliance pathway, review product evidence, coordinate with fire engineers where required and package the certification for a smooth approval. If you have a preferred species or panel system in mind, we can confirm where it can be used as Group 3, and outline alternatives for higher-performance areas without losing the design intent.