New Zealand (NZ) Deck Design - ROBO Deck Designer
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Deck design plans NZ

Bracing Design for Decks

The design of a deck involves bracing design calculations for decks greater than 2m away from a dwelling. This is very detailed quite thoroughly in NZS3604 section 5. Though this can be a bit confusing to follow, the following form should be of help.

This form is for calculating the bracing units required and offers solutions how the bracing requirements can be achieved through braced or anchor piles. Please note this calculator is a guide only to aid in understand the bracing requirements for a deck. For more detailed information, please refer to NZS 3604 section 5.

Bracing Calculator for Decks

Length of Deck (m):
Width of Deck (m):
Earthquake Zone for details please click here:
Soil Class:
Bracing Design for NZ Decks

More Information about Bracing

The form above simplifies the requirement and design of bracing. This part of the webpage describes more in detail about what goes into the bracing design. The two main points here are:
  • The bracing design requires working out the required bracing and how this can be achieved through using bracing elements
  • The bracing design involves calculations in both the longitudonal and lateral directions
Bracing design involves bracing for both earthquakes and wind, though for the purpose of decks only the earthquake bracing design is considered as this is always greater than the wind bracing requirements.

Bracing design is worked out using Bracing Units (BU's)

For the purpose of bracing a 2KPa wet in service

Bracing is not required for decks up to 2.0m wide and attached to the wall of a property along one or more walls.

Working out the required bracing

The first step is working out the earthquake zone. In the NZS3604 document under figure 5.4 this outlines the the 4 different earthquake zones for New Zealand. The following table outlines the zones for some of New Zealand's major cities:
ZoneLocations
1Kaitaia, Dargaville, Auckland, Hamilton, Tauranga, New Plymouth, Asburton, Oamaru, Timaru, Dunedin, Invercargil
2Taupo, Rotorua, Wanganui, Whakatane, Christchurch
3Gisborne, Napier, Hastings, Palmeston North, Masterton, Wellington, Kairkoura, Queenstown
4Ward, Moana, Authurs Pass, Milford Sound
For full details please see NZS3604 figure 5.4.

The second step is working out the soil class, this is either rock, shallow or deep to very soft.

The third step involves working out the bracing requirement from the table 5.8. For the purpose of a deck this classed as a light roofed dwelling with light cladding, light and medium sub floor cladding and a roof pitch less than 25 degrees for the purpose of the table. This gives a rating of 15 BU's per square metre. For decks this rating is halved giving a value of 7.5 BU's per square metre.

The fourth step involves working out the multiplication factor from the bottom of table 5.8 from NZS 3604

Multiplication FactorsEQ Zone
Soil Class1234
A & B Rock0.30.50.60.9
C Shallow0.40.60.71.1
D & E Deep to Very Soft0.50.81.01.5

For an 5.0x4.0m deck with a deep to very soft soil class in Wellington, this would be: 5.0 x 4.0 x 7.5 (from the first part of table 5.8, halved) x 0.8 (the multiplication factor) = 120 BU's The 120BU's needs to be achieved in both a lateral and longitudonal direction.

Working out the achieved bracing

The bracing can be achieved using an external wall, braced piles & anchor piles. For more information on the braced and anchor piles please see the webpage on posts

For braced and anchor piles these are rated at 120BU's

For an external wall, this is rated at 15BU's per metre.

Bracing requirements from NZS3604

From NZS3604 section 5.5 states:
  • Bracing lines must not be at more than 5.0m spacings
  • Bracing units must be evenly distributed accross the lines of bracing as much as possible.
  • Bracing lines in perimeter foundation and subfloor framing, or internal lines parallel to these (paragraph 5.5.2.1), require not less than the greater of:
    • 100BU's
    • 50% of the total bracing demand divided by the number of bracing lines in the direction being considered (along or across). External subfloor bracing lines (paragraph 5.5.2.2) require not less than the greater of the two above from paragraph 5.5.2.1 or:
    • 15 BUs multiplied by the length in metres of the external wall.

Bracing Example

In our example of a 5.0x4.0m deck this requires a minimum of 120BU's. With the deck attached to the house along a 5.0m wall this allows for 50% of the bracing demands to be achieved through the decking posts.

For bracing line A or B, 50% of the bracing demand is required divided by the number of bracing lines. With a bracing requirement of 120BU's and two bracing lines in that direction, this calculates to a bracing requirement of 60BU's each for bracing line A & B. But NZS3604 section 5.5 states each bracing line must have a minimum of 100 BU's, so the bracing requirement for lines A & B is 100 BU's.

This can be achieved using either a braced or anchor pile which has a rating of 120BUs, which is in excess of 100BU's.

For bracing line M, 50% of the bracing demand is required divided by the number of bracing lines. With a bracing requirement of 120BU's and one bracing line in that direction, this calculates to a bracing requirement of 60BU's for bracing line M. But NZS3604 section 5.5 states each bracing line must have a minimum of 100 BU's, so the bracing requirement for lines A & B is 100 BU's.

This can be achieved using either a braced or anchor pile at each end which has a combined rating of 240BUs which is way in excess of 100BU's.

Bracing Design for NZ Decks

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.



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