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). As this section is quite involved, the webpage is just a guide, for full details on joists please refer to NZS3604.
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 decks with balustrades a minimum joist size of 190x45 is required.
Cantilevered decks with a balustade
With decks in this category, things get a bit more complicated. There are strict building code regulations in place for the attachment of handrail balustrades on a deck. The two diagrams (from NZS 3604 diagram 7.10b & 7.10c) shown below are for the handrail balustrades mounted on top of the deck and to the perimeter of the deck using the boundary and edge joists. There is also a third diagram in NZS 3604, but this is for a continuous channel support balustrade. The fixing of the balustrades is strengthened considerably due to the high torsional loads of the handrail. For the fasteners these are all stainless steel 304, for more information please see the webpage on durability. Please note in the original diagram 7.10b & 7.10c on NZS3604, the M12x250 coach screw is wrong (the size doesn't exist), but the text in the document is correct.
Blocking
Extra lateral support is required between the joists. In NZS304 clause 7.1.2.4, this can being either be:
- Blocking the same height as the joist but a minimum of 35mm thick
- Herringbone strutting diagonally placed between adjacent joists.
In reality sometimes its just easier using block the same size as the joists using offcuts.
Where blocking is required
Blocking is required in these following situations:
- Along the bracing lines @1.8m centres (NZS3604 7.1.2.2)
- Along the outmost end of the joists
- Along the foundation wall (by the ledger), this needs to be continuous over 1.8m metres (NZS 7.1.4.2)
- Midspan blocking may also be required if the height/width ratio is 4 or more. Joists of 200x50mm and 190x45mm fall into this category. These would be midspan and can be offset (NZS 3605 7.1.2.3).
NZS 3604 also outlines many other requirements on joists such as fastenings, please refer to this document for more information.
Joists used in Decks using ROBO Deck Designer
With ROBO Deck Designer, all the decks use 100x50mm joists for the low and pergola decks, and a joist size of 200x50mm for the medium height decks. Also with the medium height decks the handrail balustrades are mounted on the perimeter (the boundary and edge joists) only. All the blocking requirements are built into the design.
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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. | 
