No room for joists under deck

[MikeS1]

Hi

I want to build an outdoor deck over the top of pressed concrete. Because of the internal floor level, I only have 60 mm between the bottom of the decking and the pressed concrete.  Can I put joists directly on the pressed concrete with a barrier of some sort?  The area is covered so minimal moisture from rain.  Happy to hear of options and alternatives.

[JacobZ]

Hi @MikeS1,

Welcome to the Bunnings Workshop community, it is fantastic to have you with us.

While it would technically be possible, you are going to be at extreme risk of premature rot, and there is no decking board manufacturer that would offer any kind of warranty if their products were used this low to the ground.

The only way I can see it being possible is if you use 70 x 45mm MGP10 H4 Treated Outdoor Structural Radiata Pine and plane it down so that it is 60mm in height and is directly supported by the concrete along the full length of the timber. This is H4-treated timber, so it is capable of being in direct contact with the concrete.

It can be done, but you can't expect your deck to last long-term. If you wanted it to last long term, you would need to look at removing the concrete so there is more of a gap beneath the deck and thus much better airflow to dry out any moisture that gets underneath it.

Otherwise, you might look at an alternative such as pavers or decking tiles.

Let me know what you think, and if you have further questions, please don't hesitate to ask.

Jacob

[MikeS1]

Hi Jacob,

Thank you for your reply.  Yes, planing the joists down is an option I was considering, and as you pointed out, even under cover, moisture will eventually eat way at the wood.  I would probably get around 10 years out of it - maybe more, however to replace it would be a pain.

I am currently also looking into aluminium joists.  The joist height is less but probably need more (closer together).  Would still be able to use low level feet adjusters. Definitely an added cost with that type of system.

[AlanM52]

G'day @MikeS1,

What about foundation blocks.

Cheers

[JacobZ]

Hi @MikeS1,

I'd be weighing up the cost of having to replace the deck in, let's say, 5-10 years, versus the cost to have the concrete cut out and removed, versus the cost of using another more expensive decking system. Ultimately, the choice is yours, and you will bear the cost, so it's best to know all of your options and what they would mean.

I just thought I'd add that in the scenario where you use the H4 framing, the decking boards are more likely to fail than the framing, so you might like to check out DecoDeck, which are aluminium decking boards. Cost-wise, they are pretty up there, but aluminium decking would not be subject to rot like timber or composite decking would.

Jacob