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Decking joists

New Contributor

Decking joists

Hi guys 

Im planning to build a deck on a existing concrete slab which  is 3.3m x 13m.

just wondering from the middle of the back sliding door the brick height to slab is 90mm but the end of the slab is 160mm drop this was for water drainage just wondering  how I can Make my joists  straight and level cause I believe it’s to high to use window packers always cause our decking boards are 86 x 19 what size joists should I use. Really butting my head up against the wall this is my first major home project. 
cant wait for some advice 

Junior Contributor

Re: Decking joists

Hi @clinda1 

You are correct that that is too high to use window packers. 

I would recommend a product called Klevaklip Adjustable joist supports. I have used them previously and found them to work great. They are fully adjustable and recommend for use on concrete slabs to support the joists for a deck. 

I have linked another post on this forum which I think would be a good read as I have posted pictures and some information as to how these work. In the original post there is also a span table which may help you select the best timber for your joists. 

Lots of information on the Klevaklip website as well.

I hope this helps and please let me know if you have anymore quesitons.


Bunnings Team Member
Bunnings Team Member

Re: Decking joists

Welcome to the Bunnings Workshop community @clinda1. It's fabulous to have you join us and many thanks for your question.


It's great to see you have already received some helpful advice from @bruce93.


The Klevaklip adjustable joist supports would be a fantastic option as you'll need to change each supports height to account for the drop. KlevaKlip products are available to be special ordered into stores.


In regards to what sized joist you should use you'll have limited options given your extremely low starting point. If you have 90mm total height at the door sill and remove the thickness of the board you are left with 71mm. This would allow you to stand 70 x 45mm H3 treated pine on its edge but would not leave any room for water drainage underneath. Without adequate water drainage you are going to limit the lifespan of the H3 timber. To provide adequate drainage you would need to use 90 x 45mm treated pine on its face. This might be fine for the immediate area in front of the door, but I'm not sure how that would adversely affect the deck's structural integrity if that construction method was used on the whole deck.


It would be worth viewing our Top 10 most popular deck projects for inspiration.


You should also find this step-by-step tutorial helpful: How to build a low-level deck by @Adam_W


Let me mention @ProjectPete and @Adam_W to see if they had some advice on what to use for joists when the total height of the deck is 90mm high.


Please let me know if you need further information or had questions.




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Junior Contributor

Re: Decking joists

Hi @MitchellMc and @clinda1 

In regards to the issue of the 90mm spacing at the door sill and using a 70x45mm joist. I was just thinking about this and I feel from the information we have been given that that clearance is only an issue at the door sill and then Clinda's slab seems to get more height in it due to the fall. It may not be an ideal situation but for the first row of joist supports maybe a 10mm notch could be taken out. It may mean for that hole row of joist supports they may neeed to be spaced a little bit closer together because the timber won't be as strong now but it would allow the use of the 70x45mm joist, adjustable joist support and make sure that the timber isn't in contact with the ground to prevent from water damage.


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