Roof domes (acrylic or polycarbonate) and potential failure
Acrylic and polycarbonate domes can fail without warning under people and maintenance loads, becoming brittle with age and possibly causing serious injury and death.
This information was confirmed as current in February 2016. It originally appeared in Codewords newsletters prior to January 2014.
Overview
Workers may stand on roof domes to carry out maintenance, for repairs or for painting. People may accidently fall on roof domes and there is a high likelihood that people will walk or sit on them in flat roof areas that people use for recreational purposes.
Where domes are to be installed in a new building, designers and building consent authorities need to consider the likelihood of loads that will be applied during the intended life of the dome. To meet the structural requirements of the Building Code, the domes must be designed for the worst effects from wind, snow or concentrated actions due to people or maintenance loads.
These loads must be determined, according to the design brief, from the loadings standard. Currently, NZS 4203 is cited as a means of compliance, but an updated B1 Compliance Document that cites AS/NZS 1170 instead was effective from 1 December 2008.
For actions that need to be undertaken due to people or maintenance loads, the details of the application will determine whether these loads need to be considered. Details such as the size of the dome, where it is being installed and the likely use of the roof area around the dome are all relevant. For example, a small dome with a raised profile on a steeply sloping roof is unlikely to be subject to people loads, but a large dome installed on a flat roof in an area that can be accessed by people will need to be designed to carry the appropriate loads or alternative safety measures taken.
Alternative measures could be to install a barrier to prevent access to the dome or to install a safety grillage over or beneath the dome designed to carry the appropriate loads should a person fall through it.
Durability requirement
The durability requirement for a roof dome and its fixings will depend on the ease of access and the ability to detect failure. If failure (for example, becoming brittle) would go undetected during both use and normal maintenance, then the durability of the dome must be for 'the life of the building, being not less than 50 years' (B2.3.1(a)(iii)). If such failure can be detected during normal maintenance, then the required durability is 15 years (B2.3.1(b)(ii)) and if detectable during normal use, five years (B2.3.1(c)(ii)).
Where there is evidence that the dome will sustain the design loads but the design life is unknown (that is, a dome which may become brittle at some point during its life and this 'failure' would go undetected), then the requirements of B1 can only be met if either the dome itself incorporates a means of arresting the fall or there is a grillage directly over or beneath the dome designed to carry the appropriate loads should a person fall through it.
This means manufacturers need to provide sufficiently detailed technical literature to show that the dome will meet the claimed performance requirements of the Building Code. Designers are then able to determine the appropriate dome for their application.
Technical literature should include:
- loads that can be sustained by the dome
- expected life
- maintenance requirements
- ancillary components (for example, grillage details)
- any special requirements (such as avoiding placing weight on domes when maintaining)
- what to look out for (for example, star cracking).
At least one manufacturer is proposing to install a label on domes that clearly states the year beyond which they may no longer meet the performance requirements of the Building Code. This would give a clear instruction to building owners to get the domes assessed for further life.
Once building consent applicants know the claims of the manufacturers, they can then assess that information. It will help them decide on the appropriate dome, and whether further protection is necessary, for the particular work they are planning. Building consent applications should include the dome maintenance requirements so this is available to future building owners. Building consent authorities also need to look at the proposals and satisfy themselves of Building Code compliance, including the requirement to account appropriately for the consequences of failure.
Information available suggests the life of a polycarbonate/acrylic dome before it becomes brittle is in the order of 15 to 20 years. Therefore, it is important that the life of the dome is specified and that there is evidence that it will not become brittle and will provide the appropriate structural resistance for its specified life. Some manufacturers claim their domes have high impact resistance, but there are no statements on life or other Building Code compliance.
With existing domes, the main concerns are buildings such as educational institutions, hospitals and commercial buildings where there are often several large roof domes installed in areas able to be accessed by people and maintenance staff.
We believe many of these domes may have been consented based on 15-year durability and may now be beyond this. The owners of buildings where these domes are installed should inspect their domes and seek advice from the manufacturer, a building surveyor or a consulting engineer about ongoing performance. In such cases these domes may need to be replaced or alternative safety measures taken if there are any doubts about brittleness or deterioration of fixings.