Decks that project more than 2 metres from a building must have subfloor bracing (see NZS3604:2011 18.104.22.168). In reality you have to work this out at the beginning of your project in order to design your piles correctly. It can look a bit intimidating but work through it.
HOW TO WORK IT OUT
The rule for decks is half the bracing demand required by table 5.8 (in NZS3604:2011) for light/light/light cladding at 0 degrees roof slope and subfloor structure.
So this is 0.5 x 15 (value from table for light/light/light 0 degrees) = 7.5 (this is what bracing units you require per square metre).
In addition to this you need the multiplication factor for the soil type and earthquake zone your deck is built on. In this example we will use the worst soil class and worst earthquake zone in order to cover all situations. At the bottom of table 5.8 (in NZS3604:2011) soil class deep and very soft and earthquake zone 4 give us a multiplication factor of 1.5.
Therefore the equation now looks like this: 7.5 x 1.5.
The last part of the calculation for brace requirements is the area of your deck. In our example the deck is nearly 4 metres long and 4.8 metres deep. In order to work out the area just multiply the length by the depth i.e. 4 x 4.8 = 19.2 metres square.
Now the equation looks like this 7.5 x 1.5 x 19.2 = 216, meaning that this deck requires a minimum of 216 bracing units.
In order to work out your requirements, the rules are stated in section 5.5 of NZS3604:2011. Try to think about it like this: you will need to brace the deck in each direction to ensure that it is stable and will not wobble. The rules very simplistically are half of the bracing units required divided by how many braces you have in that one direction or 15 times the length of the external wall.
In this example, for the lines running parallel to the house we plan to have two braces in that direction (one in each corner of the deck) so that is 0.5 x 216 (half requirement) divided by 2 (how many braces we have in that line) 0.5 x 216 / 2 = 54. So we need 54 BU’s but the calculation of the length of the wall is 15 times 4 (length of deck) which equals 60. At first glance this might appear to be all we need, but in accordance with the NZS3604:2011 5.5(e), we actually must use “bracing greater than 100 BU’s” so we need at least 100 BU’s along string A and string C.
For string B and string D, which are running perpendicular to the house, we do the same 0.5 x 216 / 2 = 54. 15 x 4.8 = 72. Again we must have at least 100 BU’s so between B and D combined they need 100 BU’s.
Be careful with how many braces you intend to use and what the requirement is. For example, in our second calculation if we had planned to only have 1 brace then the equation would have been 0.5 x 216 / 1 (number of braces) then our bracing requirement would have been 108 and that would have been greater than the minimum required.