Why external timber cladding is OK for Class 2 and Class 3 Low-rise
Forest & Wood Products Australia
Tuesday, September 4, 2018 - 15:15
Why external timber cladding remains acceptable for Class 2 and Class 3 Low-rise Buildings
If your building surveyor questions the use of external timber cladding a low-rise project, this article provides you with the reasons why you’re within the requirements of the Code.
A recent amendment to the National Construction Code (NCC) regarding the use of external wall claddings has led some people to assume that external timber cladding is no longer acceptable in Class 2 and 3 low-rise buildings. Boris Iskra, National Codes and Standards Manager for Forest and Wood Products Australia (FWPA) and a structural engineer with additional qualifications in building fire safety, argues that a careful reading of the NCC shows that this is not the case.
Following the recent devastating and tragic building façade fires, (e.g. the Lacrosse building in Melbourne, and Grenfell Tower in London), the National Construction Code (NCC) was amended to address risks associated with the use of external cladding products on high-rise buildings. The amendment, NCC 2016 Volume One Amendment 1, contained changes relating to fire safety in high-rise buildings and the evidence of suitability provisions. However, the existing mandatory provisions of the NCC remain unchanged.
There is a major difference in fire safety risk between a 20-storey high-rise tower and a 3-storey low-rise building. Although the ban on the use of combustible cladding products on mid- and high-rise buildings is clear, the use of timber cladding products on low-rise buildings is still permitted. This being said, interpretations of the Amendment have had unfortunate flow on-effects for low-rise building developments which have caused a great deal of unnecessary angst, concern and uncertainty.
Since the introduction of a “Concession” for timber-framed construction systems in 1994 (BCA 1990 - Amdt 7) for Class 2 apartment buildings and recently (NCC 2014) for Class 3 buildings (e.g. hotels, motels, backpacker accommodation etc.), the use of timber products has been permitted as external timber cladding for 3-storey Class 2 and 3 buildings, and 4-storey if the lowest storey is constructed of concrete or masonry and used for car parking, under the Deemed-to-Satisfy provisions in non-sprinkler protected buildings.
Specifically, for Type A construction, the NCC 2016 Amendment 1 Specification C1.1, Clause 3.10(a) states (my underlining):
“3.10 Class 2 and 3 buildings: Concession
(a) Class 2 or 3 building having a rise in storeys of not more than 3 need not comply with Clause 3.1(d) of Specification C1.1 and the requirements of C1.9(a), (b) and C2.6 for non-combustible material, if it is constructed using—
(i) timber framing throughout; or
(ii) non-combustible material throughout; or
(iii) a combination of (i) and (ii),
(v) any insulation installed in the cavity of a wall required to have an FRL is non-combustible; and
(vi) the building is fitted with an automatic smoke alarm system complying with Specification E2.2a.”
Therefore, 3-storey Class 2 and Class 3 timber-framed buildings fitted with an automatic smoke alarm system need not comply with the following requirements for Type A construction:
- Specification C1.1, Clause 3.1(d) – concrete, masonry or fire-protected timber requirements for loadbearing internal walls and loadbearing fire walls,
- Clause C1.9(a) – non-combustible external walls and common walls, including all components incorporated in them including the façade covering, framing and insulation; the flooring and floor framing of lift pits; and non-loadbearing internal walls where they are required to be fire-resisting.
[My note: Any installed insulation must be non-combustible as per the “Class 2 and Class 3 buildings: Concession” – Clause 3.10(a)(v)]
- Clause C1.9(b) – non-loadbearing, shafts (e.g. lift, ventilating, pipe, garbage or similar) to be of non-combustible construction, and
- Clause C2.6 – the non-combustible material requirements for vertical separation of openings in external walls.
The Concession also applies to 3-storey timber construction over a ground floor concrete/masonry carpark (4-storey total construction) – refer Specification C1.1, Clause 3.10(b) as well as for Type B 2-storey timber construction or 2-storey timber construction over a ground floor concrete/masonry carpark (3-storey total construction) – refer Specification C1.1, Clause 4.3.
The WoodSolutions Technical Design Guide 2 Timber-framed Construction for Multi-residential Buildings – Class 2 & 3 provides the technical information and construction detailing in relation to sound and fire-resisting timber construction systems that comply with the NCC DTS provisions. The supporting timber system fire test and assessment reports from an Accredited Testing Laboratory can be found here.
Whilst it is understandable that there has been some nervousness shown by building professionals in specifying external cladding materials and products, the NCC 2016 Amendment 1 does not change the Concession that permits the use of external timber cladding products on low-rise Class 2 and Class 3 buildings (3 or 4-storey apartment, hotel, motel developments etc.) which has been in the Code since the mid-90s.
The WoodSolutions Technical Design Guide 2 Timber-framed Construction for Multi-residential Buildings – Class 2 & 3 is available for free download here.