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How can I use timber joists over a concrete slab?

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How can I use timber joists over a concrete slab?

I have a 6.3 x 2.8 newly poured concrete slab. The slab has been poured with a significant fall as a result of incorrect information being given to the concreters.

From one side ( 2.8 metre) to the other the fall goes from zero to 115 mm bottom of joist.

So I need to use either brackets or perhaps timber to raise the height at the required intervals for a 70 x 45 joist.

I was looking at the Klevaklip system to raise the joist but due to the sloping concrete they are unsuitable.I have spoken with the manufacturers.

Can anyone suggest some options please? I was thinking that I could use timber almost like  mini stumps which would have Bitumen paint as waterproofing and perhaps raised a few mills with plastic spacers to allow water to pass under. I could also put waterproofing around the base of the timber to stop water getting under completely. 


Any assistance appreciated 

I've seen on previous posts where a deck was put down over concrete and galvanised angle brackets were used with a small clearance off the concrete.


I have 15 x  2.8 m joists and timber size is 70x45




70x45 joist level70x45 joist level





Bunnings Team Member
Bunnings Team Member

Re: How can I use timber joists over a concrete slab?

Welcome to the Bunnings Workshop community @Pedro462. It's wonderful to have you join us, and many thanks for your question about constructing a low-level deck.


Typically, concrete slabs are not flat and have a fall/slope. The slope doesn't inhibit the use of adjustable supports. As with any adjustable support system, using the appropriate sized adjustable fixing is essential. It's not unusual to need to use two different support styles, one for the lower section and another for the higher. This is due to their limited adjustment heights. If the slope of your slab is an issue, a simple fix would be to grind a small section flat where the support is to be installed.


If you'd like to go with mini stumps cut from 90 x 90mm H3 posts, then using the TuffBlock 300 x 300 x 90mm Instant Foundation System Deck Support would be a fantastic option.


Using angled brackets would be suitable for the lower areas, but once you get up close to the 115mm mark, then there aren't brackets large enough to suit.


Please let me know if you have any questions.


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Re: How can I use timber joists over a concrete slab?

Thanks Mitchell appreciated 

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