Façade fire testing for timber structures

This Issue This is a part of the Fire feature

By , and - , Build 180

When combustible materials like timber are included in a façade system, the evaluation of vertical fire spread risk requires testing the system as a whole. There are several key things to consider.

TIMBER STRUCTURAL MEMBERS – including light framing – are used extensively throughout New Zealand construction. With increasing scrutiny of the external façade fire spread for higher-risk buildings and the introduction of new façade fire testing regimes globally, there have been questions about using timber structural members as a combustible component in façade systems.

Focusing on the right things

Talking façade fire testing (Build 180) discusses some of the basic principles of façade fire testing and different types of fire testing that can be performed.

When combustible materials are included in the façade system, its configuration is as important as the materials present. Consequently, the evaluation of vertical fire spread risk requires testing the system as a whole.

Lack of standardisation globally

In a fire test of a façade system, a representative sample of the construction is exposed to a standardised heat source and must pass certain criteria intended to result in a low risk of vertical fire spread.

Unlike fire resistance testing, to date, there has been limited international standardisation of the test methods and intended test outcomes for façade fire testing. Different countries test façade systems in different configurations with different heat sources and use different terminology. Table 1 lists some of the terms used throughout standards and documents in the UK, Australia and the US for testing a façade system. The number of terms and the overlapping meanings has added to the confusion.

It is not the terminology used that is important but a consideration of the associated fire risks of an individual façade system. Does the specific test assess the risk of the specific system?

Table 1 Terms used internationally in façade system fire testing

Tests for structural members

Fire tests for façade systems have not been specifically designed to test the structural members. However, structural members are included in the fire test along with insulation, membranes and any other components that make up the façade system test specimen.

Two façade fire tests, NFPA 285 and BS 8414, can be used to test a timber structure as part of a façade system.

NFPA 285 fire test standard

The NFPA 285 test standard was originally conceived as a method of evaluating the risk of combustible components, such as insulation or waterproof membranes, in buildings of otherwise non-combustible external wall construction (for example, a concrete structure).

The original stated scope of the standard was for ‘non-loadbearing wall assemblies’ intended for buildings ‘required to have exterior walls of non-combustible construction’. Over time, the desired application of the standard changed.

In 2019, the standard’s scope and purpose were amended to include any type of building structure. The 2019 version of the standard now permits the performance of timber framing to be assessed as part of the test standard.

The application of the standard is now set by the local building code where it is adopted. Other jurisdictions, such as the United States, have limited its use to specific construction types or provided other prescriptive measures around the application of NFPA 285 test results.

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BS 8414 fire test standard

Another fire test standard used for façade systems is BS 8414. This standard does not give a pass/fail outcome from the test, and separate classification is required through other means such as applying the criteria in BR 135 Fire performance of external thermal insulation for walls of multistorey buildings (Third edition).

The stated scope of BS 8414 and BR 135 does not specifically address timber structures, but BR 135 does state that, for timber construction, ‘the general principles given [in] this report may still apply although suitable additional risk assessments and detail design reviews would be required’.

In practice, there are timber-framed façade systems that have been tested and assessed in such a manner. A companion standard, BS 9414, also provides guidance on what can be assessed based on BS 8414 testing and what requires additional test evidence.

Protecting timber in a façade system

To supplement façade system fire testing, assessments can be made of the potential contribution of a timber structure to the fire spread risk.

To address the fire risk of timber structure in a building, encapsulation techniques are being widely used for timber building designs including those with mass timber construction. Some examples of buildings that have used encapsulation techniques to varying degrees include:

  • Brock Commons residential building at the University of British Columbia, completed in 2017
  • Mjøstårnet building in Norway, completed in 2019
  • Auckland City Mission, currently under construction in Auckland.

For façade systems, the goal with encapsulation is to limit the contribution of timber structural members to the fire spread and growth for a specified time. One approach is to protect the timber members from charring for a specified time or to protect timber to achieve a fire-resistance rating for a specified time.

Standards have been developed to assess the ability of sheet materials (such as gypsum plasterboard) to protect timber members and prevent charring for a specific period. An example is EN 14135:2004 Coverings – Determination of fire protection ability using criteria in EN 13501-2:2016 Classification using data from fire resistance tests, excluding ventilation services.

However, these standards are not specific to external wall construction. Further research is required to determine what performance parameters would be appropriate for façade systems incorporating timber structure.

To sum up

Timber structural members can be included in façade systems provided their performance is assessed and they are deemed suitable to mitigate the risk of fire spread. The assessment can be based on façade system fire testing or alternative assessments of performance.

Where gaps in the fire testing standards and Building Codes exist, further research is required. For specific systems in specific buildings, a detailed review is required for the façade system as a whole.

A good starting point for those wanting more information on the fire risks is BR 135 and the NFPA Exterior Façade Fire Evaluation and Comparison Tool – EFFECT™ User’s Guide.

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