Compliance pathways for external wall cladding systems containing timber.
Large or intermediate scale fire tests
Historically, international large scale test methods did not include timber framing. In New Zealand, however, timber framing has been used extensively. Therefore, where intermediate and large scale tests are used to comply with the NZBC, it is recommended to take into account any timber framing used for the construction of the external wall cladding system.
NFPA 285 was revised in 2019 to include any type of building structure. This version of the standard permits the performance of an external wall cladding system with timber framing to be tested and assessed.
BR 135 does not specifically address the use of timber framing in the BS 8414 fire test standards. However, the general principles in BR 135 may still apply for timber framing, although suitable additional risk assessments and detailed reviews may be required.
Non-combustible or limited combustible systems
Where fire testing of an external wall cladding system is required, as an alternative approach to demonstrate compliance:
- All cladding, framing*, battens, insulation products*, rigid air barriers and filler materials (not including gaskets, sealants etc.) used in the external wall construction may be of limited combustibility.
- If vapour barriers, drainage mats, building wraps or similar are not non-combustible or of limited combustibility then all external wall cavities need to be fire stopped using cavity fire barriers at each floor level.
*Timber framing (or combustible insulation products within a framed wall assembly) may be used if a robust protective lining material (being of limited combustibility) is fixed to the exterior side of the framing and can be demonstrated to remain in place and protect the framing during the period of external fire exposure. 'Protect framing' can be assumed to be achieved if the protective lining material as part of a light timber frame wall exposed to the test conditions of AS 1530.4 can be shown to prevent charring of the timber frame for a period of 30 minutes. One way to determine this is to limit the temperature on the cavity side of the fire-exposed protective lining material during the test period to be no greater than 300 degrees Celsius.
Use of combustible rigid air barrier
A combustible rigid air barrier, for example plywood, may be used for any building if it has been included as part of a representative external wall subjected to a full scale fire test.