Low Profile Merbau Decking over Concrete Slab

[Kelpidor]

We are looking into installing a timber deck over an existing concrete slab of out outdoor entertaining area, but the space between the concrete and the termite barrier is only one standard house brick + mortar, so approx 80-85mm.

With the Merbau decking panels being 19mm deep, what would be the best product and method of laying these boards?

Area is approx 9000 x 5500

[MitchellMc]

Welcome to the Workshop community @Kelpidor. Many thanks for your question, Id's be happy to assist.

If you are working with 63mm of space to have joists and bearers it doesn't really leave the deck with much room to have adequate airflow. Consideration would need to be made as to whether that was indeed enough room to work with. The only readily available timber thickness which would fit in the space would be 45mm and H4 treatment is likely needed in such a confined space. Could you remove the concrete slab, lowering the level which we are starting, as that would open up more possibilities?

Let me mention @Brad and @ProjectPete to see if they would like to join the conversation.

Mitchell

[Brad]

Having had a read the other day to answer a similar question, the recommendation was for areas that may be damp to choose narrow boards with a large as possible gap between them, my decking jig will do 8mm gaps. Structural H4 seems to be almost non existent in Adelaide currently, not sure about the rest of the country.

I have used a floor of 135mm Ekodeck with 6mm gaps using steel purlins with a similar height that sits over the drain for the rain water tank and the only problem I have noticed so far is that in winter the gap is tighter in the boards on the drain end. It may be more luck than good judgement that I haven't had any problems yet from it.

[Kelpidor]

Thanks Mitchell,

Don't really want to remove the concrete as it is such a large area and apart from the time and costs of removal, the concrete is still in decent condition.

We are in Brisbane, so damp from cold isn't as big an issue, but agree that ventilation is still required.

Based on what Brad states, it would indicate that to 90mm boards are the better option than wide ones to allow improved ventilation.

Would if still require both joists and bearers if we are attaching the joists directly to the slab?

[MitchellMc]

Hi @Kelpidor,

In a standard deck, bearers connect to stumps or posts and joists connect to them for the decking to be fixed to. In your situation, as you are fixing straight to a slab having bearers seems somewhat obsolete. However, I would advise you to use plastic packers to raise the timbers slightly off the concrete slab.

Mitchell

[TrebleTrev]

Hi Kelpidor,
One of the carpenters maintenance guys from my work suggested I use galvanised roofing battens for installing deck with minimal clearance to concrete.

He said he has done this many times and secures the roof battens (see stock pic for example) by drilling through edge of batten to concrete and using galvanised screws with wall plugs at 300mm - 400mm spacings between screws.
He levels the battens by using galvanised washers underneath when screwing down if needed
https://www.bunnings.com.au/ramset-8-x-50mm-blue-wall-plugs-blue-pack-of-25_p2260643