How to build a low-level deck over a concrete slab?

[petebest52]

New to this group. Looking for some wisdom!

I have an old queenslander room at the back of my property and want to knock the walls out and turn it into a deck, but there are couple issues I need help with:

1. The concrete slab is flat/has no fall. Not ideal, but can I get away with it?

2. The height from the slab to the height of the back door is 110mm. How could I build a deck under this height? I had been planning on using build tuff tuff blocks (50mm height) to stop the joists sitting on any water that collects on the flat slab, but with 70mmX45mm joists and 19mm merbau decking I now realise this will mean the deck level is higher (139mm) than the back door/ground level of the house.

I would rather not tile it if possible, any help or suggestions would be greatly appreciated🙏

[MitchellMc]

Welcome to the Bunnings Workshop community @petebest52. It's terrific to have you join us, and many thanks for your question about create a deck.

When building a low-level deck over a concrete slab, it’s important to consider proper drainage, especially when the slab is flat or lacks a gradient. Water accumulation on a flat surface can lead to significant moisture issues, including mould growth, wood rot, and structural damage. It’s essential to ensure that any water that gets under the deck can drain away effectively. If modifying the slab is not feasible, consider incorporating drainage solutions like weep holes or creating a slight slope towards a designated drainage point.

With only 21mm of clearance under your joists, attaching them directly to the concrete using angle brackets is a practical option. This approach will help keep your joists elevated above any water that may collect on the surface, ensuring they remain dry and protected. However, it’s important to ensure that the brackets are securely fixed to provide adequate support and stability for your deck. 

You might like to read through this guide: How to build a low-level deck.

Please let me know if you have any questions.

Mitchell