Correct timber treatment for enclosed skillion roofs
Note: 18/07/05: The version of this article, including its figures, as printed in the June 2005 issue of Codewords contains several errors. The correct version, minus the figures, as shown below will be reprinted in the July 2005 issue.
The Department is aware that there are still problems out there with incorrect treatments being installed in skillion framing. Treatments proposed to be used in building work need to be shown on the plans and specifications and this is something that needs to be carefully checked by building consent authorities before any consent is issued. However, we still field a number of calls from builders on site querying what is required or who have been told to replace incorrectly treated timbers in roofs already built.
NZS 3602: 2003 is the relevant Standard cited in the Department's B2 Compliance Document. The requirements for skillion roofs are clearly given in NZS 3602, as are the requirements for enclosed flat roofs. A 'flat roof' is defined in the Standard as one that has a pitch of less than 10 degrees. 'Skillion roofing' is also defined in the Standard with a skillion roof being one type of enclosed roof. If your roof is a skillion that also happens to be flat then you must look at the requirements for both and use the higher treatment required. Table 1D in NZS 3602 (part table copied below with the kind permission of SNZ) gives the requirements in each case. You will note that 'Ref No 1D.2' requires enclosed flat roofs to be treated to H3.1 whereas 'Ref No 1D.3' requires skillion roofs to be treated to H1.2. Therefore, if you have a flat skillion roof the required treatment is H3.1.
Table 1 - Requirements for wood-based building components to achieve a 50-year durability performance