Piles, Bearers and Joists
After you have chosen what type of decking you want to use we can then move on to designing the structure. Again we are designing the deck in the opposite direction to the way we would build the deck.
Remember the joist spacings are max:
- (a) 600 mm joist centres for 32 mm decking; or
- (b) 450 mm joist centres for 19mm decking.
When choosing what size timber to use when framing up your deck it is important to note that they all relate to each other and have flow on effects. The distance you space your piles one way determine the size of the bearer and the distance you space your piles the other way determine the size and span of your joists.
Continuing with the example deck
Joists
Following on from our example, the distance between bearers is 1.8m. Our joists are going need to span this distance between the support of our bearers.
You can use the table below to determine what distances certain timber can span. If you are building a low level deck you may require smaller timber (90x45mm) spaced closer together. If ground clearance is not an issue then you may opt to use bigger lengths of timber (190x45mm) which will be stronger and can span bigger distances.
An exterior deck is prone to water so according to NZ 3604 section 7.1.1.1 we must use the 2kpa floor load table when determining our framing sizes. Using table 7.1 (b)
140 x 45mm joists can span more than 1.8m regardless of if they’re spaced 400mm, 450mm or 600mm apart.
Note: 140x35mm is not a standard timber size so has been disregarded
Design tip – If you are using 32mm thick decking you can save timber and labour by spacing joists 600mm apart. If you’re using hardwood or other decking thats 19mm thick the joist spacing will have to be reduced to 450mm centres or less.
Additional deck design tips:
- Floor joists shall be laid so that any bow in them will straighten under load.
- Floor joists shall have minimum bearing on their supports of 32 mm.
- Joints in floor joists shall be made only over supports, but not where the joist is cantilevered beyond the support.
- Joints in floor joists may be butted over supports
Bearers
The bearers are the main load bearing beams of a deck. As they are responsible for carrying the weight we will need to work out the loaded dimension for each one.
Design Note: If you require crawl space under the deck for storage or access it may be better to use smaller timbers that span less distance.
Using the example deck diagram above we work out the loaded dimension for the bearers:
- Bearer one – (1.8+1.8) ÷ 2 = 1.8
- Bearer two – 1.8 ÷ 2 + .6 = 1.5
As with the joists we must use the 2 kPa floor load figures.
Going off of table 6.4 (b) a max bearer span of 1.65m – at a max loaded dimension of 1.8 – shows:
The bearers must be at least 140x90mm (6’x4′)
This can be made up of two 140x45mm timbers nailed together.
Note if you wanted to space the piles 2m apart you would need 190x70mm minimum bearers.
Cantilevered joists
Span 3 of our deck has joists cantilevered over the last bearer. We need to make sure the joist size we have selected previously can cantilever the distance required. For this we check out table 7.2. When building a deck we use the last column of the table for 2kpa floor load.
According to table 7.2 a 140x45mm joists that are spaced at 600mm centres can extend up to 900mm past the last support. The cantilever on the example deck is 600mm so we are well under.
Design tip – cantilevering your deck can give you more deck area without having to install additional support. However it leaves nothing to fix base boards to if you plan to close off the underside of the deck.
Stringers
A stringer uses the houses existing foundation to support part of the deck. Stringers can be used instead of having a row of piles and a bearer up against a house.
For our example deck we are attaching a stringer to the house.
Effectively the stringer is a bearer attached to the house and saves having to dig and concrete additional piles.
For a timber pile house the bearer can be fixed directly to the existing house piles using nails and engineers bolts or coach screws.
Design tip – the stringer can be substituted for another bearer with the same properties as ‘bearer 2’.
For the purpose of this example we have assumed the stringer for the deck will be below the bottom of the cladding. Extra steps have to be taken to prevent water ingress where a stringer attaches to cladding. It would be best to consult a builder before comencing work.
Piles
Generally for all low level decks we use 125mm x 125mm H5 piles. These come in a variety of different lengths and can be cut down to suit.
Pile spacings
As determined by the size of the bearers and joists, our pile spacings are:
- Parallel with the house – max 1.65m apart (bearer span)
- Perpendicular to the house – 1.8m (joist span)
Pile Length
When determining the best length piles to buy you can work out what the height of the finished deck is minus the thickness of the decking and size of the framing timber plus the depth of the hole you want to dig.
Eg. finished deck height minus thickness of decking minus joist size minus bearer size + hole depth = min pile length. (900mm – 32mm – 140mm – 140mm + 300mm = 888mm)
Note there is no allowance for lifting the pile up 100mm from the bottom of the hole. This gives you a little extra to make sure the piles aren’t too short. You can always cut a pile down but you cant add length to it!
Pile to bearer connections
In order to maintain the structural integrity of the deck, skewed nails alone might not provide enough strength to hold bearers to their piles.
Ordinary piles
Ordinary piles need to be fixed to there bearers with 2 x wire dogs (or z nails)
Braced pile
Braced piles need to be fixed to bearers using a 12kn connection to help distribute the bracing from the piles to other framing components. Joists on each side of the braced pile needs twist straps.
Bracing requirements
As the example deck is approx 25.8m2 it requires 193.5BUs
For our hypothetical deck the corner piles and the two closest piles at right angles need to be bracing piles. Braces should extend from the bottom of one brace pile to the top of another.
The standard says the bracing units must be spread evenly across the deck where possible. One corner bracing provides 120BUs in each direction well exceeding the requirements for the deck. However to provide an even spread across the deck the other corner furtherest from the house must be done as well.
One corner of bracing piles have been marked on the initial diagram of the deck. In the drawing the diagonal braces have been removed for clarity.
Any bracing is better than no bracing so feel free to figure out a way that suits you best.
