BRANZ Guideline July 2011 cantilevered joists for balconies

There is some confusion about what is covered in NZS 3604:2011 on the use of cantilevered joists in balconies. Cantilevered balcony joists in 3604 only cover the example that is shown in Figure 7.6, where lapped joists that are bolted to the floor joists cantilever out beyond the face of the cladding. The maximum cantilever length is covered in Table 7.2 and the far right column of the Table is the column that applies to balconies (2 kPa floor load, balcony floor, and balustrade only). Note also that only joists 190 mm or more deep are suitable for supporting a cantilevered barrier. (Where a barrier is not required on a cantilevered balcony, a smaller joist can be used.)

The overall length of the joist is governed by the cantilevered projection. The cantilevered projection (L) is multiplied by 2.25 to give the minimum overall length. For example, the maximum projection for 190 x 45 mm joists at 600 mm centres, from the 2 kPa floor load column in Table 7.2, is 1300 mm. The cantilevered section (L) is therefore multiplied by 2.25 to find the minimum overall length, which in this case is 2.9 m. Figure 7.6 gives the position and the sizes of the bolts where it attaches to a floor joist inside the building line.

Clause 7.1.5.3 of 3604 indicates that joists can be continuous over the outermost support. This is relevant only for a deck, as balcony joists require a step down of 50 mm to the deck surface (E2/AS1 Figure 14). Ripping a joist, and therefore reducing the depth by at least 70 mm (50 mm plus 20 mm thick decking), is not covered by NZS 3604 as the ripping of the timber invalidates the timber grade.

Cantilevered deck joists should also obey the 2.25 backspan rule.

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