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Hi,
I'm building a deck on the back of my house. Unfortunately the previous owners put a useless crappy 1m wide concrete patio on. I can't remove it as it supports the brick veneer which was added later. I've attached a photo of the distance between the slab and the sliding door frame. I'll be using 90 X 45 for the joists, but am concerned about the raised height of the 90 X 45 joist with the 19mm decking on top. I could use 70 X 35 on the 1 meter section and then drop the headers to make sure the 90 X 45 joists meet the height of the 70 X 35 for the section of the deck that isn't on the concrete slab. My question is. Is there any other materials I can use to lower to height of the deck that covers the 1m concrete patio that meets the glass sliding door. Could I use 90 X 45 on its side , and then just build a box /ladder frame with 450 centres, that would have the deck height lower to the sliding door, then switch to the 90 X 45 for the portion of the deck supported by bearers . Unless there is another low profile alternative to 70 X 35 , like aluminium or something? Any ideas?
Welcome to the Bunnings Workshop community @Gongrat. It's wonderful to have you join us, and many thanks for your question about decking.
There are a few ways you could approach this situation, but none of them follow standard deck construction techniques. When you start building over an existing concrete patio, you’re creating what’s known as an ultra-low deck. Once your joist height drops below about 90mm, there’s very little air circulation beneath the boards, which can trap moisture and speed up the decay of both the joists and the decking. Turning 90 x 45mm joists on their side may physically reach the required height, but the deck will be more susceptible to moisture damage. The same applies if you were to drop down to 70 x 45mm. While you can technically build it that way, you should expect a shorter lifespan for the deck structure.
If you’re aiming to keep the deck flush with the sliding door, it’s important to maintain as much airflow as possible and to isolate the timber from direct contact with the concrete. Use plastic packers or battens to slightly raise the joists off the slab, allowing air to circulate underneath and preventing moisture from being trapped. Also, apply a bituminous paint on the bottom of the joists before fixing your decking boards.
I'm not aware of any other products which could be used as joists on your low level deck which wouldn't be affected by moisture.
Please let me know if you have any questions.
Mitchell
Support is not necessarily dictated by the calibre of timber you use @Gongrat. It comes down to how that timber itself is supported. So, if you were to add packers or supports every 1000mm for 90 x 45mm on edge, if you lay it on its face, you might support it every 500mm to ensure it is not spongy.
Mitchell
