What to use as a substrate for a Douglas fir wood patio?

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In the next few weeks I would like to try my hand at building a patio. I would like to concrete the formwork stones on compacted ground (still need to be 30 cm higher), and pour concrete over it. On top of that is the substructure, and on top of that are the decking boards. I chose Douglas fir to make the boards. Do they need to be painted or oiled right away?

I wanted to use pressure-impregnated pine as the substrate. Is that possible or should I also use Douglas fir? But I can only get it untreated and it is too fragile for me.



Then please also think about weed fleece, padding (to keep the substructure from standing in water on the rocks), and structural protection of the boards from moisture.

For substructure, I would only use a suitable hardwood that does not weatherize as quickly as Douglas fir or even pine (such as Bangkirai or Bongossi).

But why such an elaborate structure with formwork stones? As an alternative, fill KG pipes with concrete. There are plenty of inexpensive and easy options for the non-professional. Last weekend I stacked our substructure on mortar bags (= freeze bags).

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