Correct timber treatment for enclosed skillion roofs

Note: The version of this article, including its figures, as printed in the June 2005 issue of Codewords contains some errors. The correct version is shown here. We apologise for any inconvenience caused.

 The Department is aware that there are still problems 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 those 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 either use the higher treatment set out in the Standard or some other method that complies with clause B2 of the Building Code. Table 1D in NZS 3602 (part table copied below with the kind permission of Standards New Zealand) 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.

Extract from Table 1D of NZS 3602