Determinations issued
Determination 2008/51
Building consent for a house with a membrane roof
The matter for determination
The matter for determination was the decision by the building consent authority to refuse to issue a building consent for a house unless the design of the membrane roof was changed. The building owners applied for the determination.
The building
The building is a two-storey house with a structure consisting of concrete foundations, concrete block external walls, suspended concrete floors and a roof of a steel frame with timber rafters.The main roof, which was the subject of the determination, has a fall of 1:40 and consists of 17 mm ply-wood substrate clad with a 2-layer torch-on bituminous membrane having a total thickness of 6.8 mm. The building consent authority required the roof to have a 1:30 fall, in order for it comply with the design guidelines for membrane roofs it had published as a practice note.
The submissions
The owners submitted that changing the roof pitch would require a significant redesign of a building that already complied with the Building Code and the membrane manufacturer's installation information. The height restrictions on the site prevented the roof and parapet from being raised and the owners did not wish to lower the ceiling height. The owners said careful attention had been given to the design, detailing and choice of materials, in order to provide a high quality, durable and weathertight roof. The building consent authority noted that, while it accepted the torch-on membrane itself was Code-compliant, it had concerns regarding the supporting structure, which it considered to be prone to deflection. The building consent authority said other industry literature and guidance information specified minimum falls that were greater than those described in E2/AS1.The building consent authority considered that the 1:40 minimum pitch did not adequately allow for building tolerances and likely future deflection in timber structures that would inevitably lead to ponding of water on membrane roofs. The building consent authority provided photographic evidence of membrane roofs to timber structures where ponding had occurred, thus reducing their ability to shed water. The building consent authority noted that the torch-on membrane fell outside the scope of Acceptable Solution E2/AS1 because the membrane was not of a type listed in E2/AS1 and because the roof also exceeded the 10 m2 limit for such roofs.
Discussion
The determination did not accept that the building consent authority's practice note could take precedence over the requirements of the Building Act and the Building Code. The determination noted that the building was considerably more rigid than a conventional timber structure, with the steel roof beams parallel to the fall of the roof and short-span timber rafters across the line of fall. There was therefore little likelihood of the roof deflecting in such a way as to cause 'ponding'. While the torch-on membrane was considered to be an alter-native solution, it could be evaluated and compared with the membranes included in E2/AS1. Following such a comparison, it was concluded that the torch-on membrane would perform at least as well as the membrane systems included in E2/AS1 and that the torch-on membrane in this case would therefore comply with Clause E2 of the Building Code.The determination acknowledged and accepted the building consent authority's concerns about the performance of low-pitched membrane roofs. However, it was noted that the building consent authority could have sought information about the expected deflection of the roof structure and made its decision as to compliance based on the information received.
The decision
It was determined that the building consent authority's decision to refuse to issue a building consent for the house be reversed.
Determination 2008/62
Dispute over a building consent for alterations to a house due to the slope of the roof
The matter for determination
The matter for determination was the decision by the building consent authority to refuse to issue a building consent for alterations to a house because the replacement roofing lacked sufficient falls. The building owner applied for the determination.
The building
The proposed alterations to the house included the replacement of the existing roof cladding to an end section of the existing roof, which also involved extending the roof edges by 600 mm to match adjacent eaves, and adding a new skylight. The existing roof has a 1º pitch and is clad with trough section galvanised steel roofing; the new roof cladding is to match the existing in material and profile. According to the owner, the building consent authority required the altered section of the roof to have a 3º pitch in order to comply with the requirements of Acceptable Solution E2/AS1.
The submissions
The owner submitted that increasing the roof pitch would be costly, would spoil the appearance of the house, and would pose additional weatherproofing problems. The owner also said that there had been no leaks or problems with the existing roof over the house's 35-year history.
Discussion
While the Acceptable Solution E2/AS1 describes a 3º minimum roof pitch for this roofing profile, E2/AS1 provides only one way of complying with the Building Code. Therefore, the new roofing can be considered an alternative solution. The determination considered the bulk of the new roof cladding was a replacement of the original for which a building consent was not required because it was considered exempt work under Schedule 1 of the Act, as the replacement roof cladding uses:
- comparable materials, or replacement with a comparable component or assembly in the same position, of any component or assembly incorporated or associated with a building…
The area of new roof that was not considered a replacement of the existing was approximately 5 percent of the total area of new roof cladding. The new roof cladding contained no ridge flashing and the only junctions in the cladding occurred over an open deck. The existing roofing had performed adequately for 35 years. Increasing the roof pitch would create weather-tightness risks to a roof that had performed adequately in the past.It was concluded that the new roof cladding would perform at least as well as the existing cladding and that it would comply with Clause E2 of the Building Code.
The decision
It was determined that the building consent authority's decision to refuse to issue a building consent for the proposed alteration be reversed, in respect of the proposed roof pitch only.
These are summaries only. The full determinations (along with all other determinations issued) can be viewed at www.dbh.govt.nz/determinations