How do I calculate the FLW for bearers and joists on low level freestanding deck

[laingstoked]

Hi community,

I am building a low level outside freestanding deck using treated pine bearers and joists and hardiflex decking boards. I am wondering if I can receive advice on calculating the spacing of posts, bearers and joists, and also the depth of footings as well. Bearers are 140x45mm H3 treated pine and joists are 90x45mm treated pine. The deck is 6000x7000mm. 

Currently I am working on post spacing of 1.5m along each bearer along the 6m side, making a total of four posts. Then bearer spacing of 1.5m along the 7m side making a total of 5 bearers. 

The joists I am thinking to space at 45mm making a total of 14 joists along the 7m side. 

Current calculations:

Total Deck height: 500mm

Depth of footings: 500mm

Post spacing along bearer: 1500mm

Bearer spacing: 1500mm

Joist spacing: 450mm

Decking boards: Hardiflex fibre cement.

Thanks for your time, I appreciate any reflections on this.

[EricL]

Hello @laingstoked 

Welcome to the Bunnings Workshop community. It's pleasure to have you join us, and thank you for sharing your question about how to calculate the floor load width (FLW) of your deck.

The floor load width (FLW) is generally defined as the area of the floor supported by an individual bearer. Based on the measurements you've given. If the spacing between each post is 1.5 meters you'll need five posts instead of four to support the 6-meter bearer. If the spacing is the same on the 7-meter side you would have an overhang of 865mm. To cover the entire length you would need six bearers unless you wish to have an overhang. 

The bearer is supported by the posts. It is the span between the posts that determines the FLW. Using your measurements as an example if the span between posts is 1500mm we need to divide that into two giving us a value of 750mm. But the next post is inside the floor area now so it now supports the weight of two sides. to get the value of the next post you need to add the span of the first post to the second and the third post and divide it by two. This will give you a value of 1500mm. This goes on until you reach the end of the posts. The values you get will now be placed into the span table and you'll be able to tell the right size bearer for your build.

Your local council has rules and regulations when it comes to building decks. I suggest having a look at the rules in your area to get a better idea of what you need to do to plan your deck. Because of the size of your deck, the council might require you to submit a validated engineering plan. I'm sure you're keen to do as much as you can with this project, but sometimes we need to engage the services of a professional in order to make sure everything is safe and in accordance with local council rules. 

There are online calculators that you can refer to and since you have a good idea of the size of timber you want to use, it will be easy to input the values.

I've placed a video at the bottom showing how the FLW is calculated. Please note that this was done in 2015 the regulations and standards for your area could have changed. Make sure to refer to the latest regulations and standards before you begin your build.

If you need further assistance, please let us know.

Eric

[Brad]

Install 

Seal 

450mm on a standard pattern install will give you a low commercial grade rating. 500mm is max spacing for a standard patern install for domestic use.

Diaganal installation has a 300mm spacing and is domestic use only.

Soil type will play a big part in what depth of footings to use.