Joists
As with bearers the NZS3604 takes quite a bit of interpreting. The size of the joists are dependant on the span between the bearers, the spacing between the joists and the type of decking used. Furthermore there are further restrictions whether the joists are cantilevered as well (some of the decks designed using ROBO Deck designer used cantilevered joists)
SG8 grade joists for wet in service use using a 2KPa floor load from table 7.1b from NZS 3604.
| Floor Joist Size | Maximum span of joists at a maximum spacing (mm) of |
| 400 | 450 | 600 |
| mm x mm | (m) | (m) | (m) |
| 90 x 45 | 1.60 | 1.50 | 1.30 |
| 140 x 35 | 2.20 | 2.05 | 1.80 |
| 140 x 45 | 2.50 | 2.35 | 2.05 |
| 190 x 45 | 3.40 | 3.20 | 2.75 |
| 240 x 45 | 4.30 | 4.05 | 3.50 |
| 290 x 45 | 5.20 | 4.90 | 4.25 |
| * Spans may be increased by 10% for joists continuous over 2 or more spans |
Maximum cantilever length of joists using Grade SG8 (Wet) with a 2KPa floor load (for decks only) from table 7.2 from NZS 3604
| Floor Joist Size | Joist Spacing | 2 KPa Floor Load |
| mm x mm | mm x mm | mm x mm |
| 190 x 45 | 600 | 1300 |
| 450 | 1500 |
| 400 | 1600 |
| 240 x 45 | 600 | 1650 |
| 450 | 1900 |
| 400 | 2050 |
| 290 x 45 | 600 | 2000 |
| 450 | 2350 |
| 400 | 2500 |
Please note for cantilevered balustrades a minimum joist size of 190x45 is required.
NZS 3604 also outlines many other requirements on joists such as fastenings and blocking, please refer to this document for more information.
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Disclaimer: Whilst this website contains a lot of information on the design of decks which has been produced with the highest level
of professional care, ROBO Deck Designer, ROBO Design Solutions or Paul Roberts (the developer of this website) does not accept any responsibility for
the accuracy of the information on this website. This website is intended as a guide only, please refer to the New Zealand building code for the
appropriate regulations. | 
