I have an area to deck. It's undercover with internal height of 2.0m. What is the minimum I can put my joists to ground level? I have H3 treated and coated with a tar membrane. Im a bit lost with what I can do.
Welcome to the Bunnings Workshop community @jodia. It's fabulous to have you join us and many thanks for your question on building a deck.
You might like to read through this helpful guide: How to build a low-level deck.
When it comes to low-level decking, there isn’t a strict minimum clearance set in stone, but more of a sliding scale based on risk and longevity. As a general guide, you’d want at least around 10mm clearance to allow for a bit of airflow and for any water to drain away. Even in undercover areas, you’ll still get some wind-driven rain and moisture buildup over time.
The key thing to understand is that the closer your joists sit to the ground or substrate, the more you restrict airflow. Reduced airflow means moisture hangs around longer, and that significantly increases the risk of early timber decay, even with H3-treated timber and a protective membrane.
So while you can go quite low if space is tight, it really comes down to your risk tolerance. The less clearance you allow, the more you’re trading off long-term durability for height. If you can afford to give it a bit more space, even 20–40mm, you’ll get much better airflow and a longer-lasting deck.
Please let me know if you have any questions.
Mitchell
Thanks for that advice. Im looking at 20mm deck jacks to place under joists. When reading about clearance underneath it becomes very cloudy regarding heights. Your advice has helped lots as I thought the minimum was around 150. So around 20 should be OK with cross wind airflow.
Does the decking have to be floating or can it be fixed to ground with bolts.
As long as you're not in a location that experiences high winds or cyclonic conditions, having the deck entirely floating is an option @jodia.
