Simple wood floor on top of concrete, for garden room

9 Jan 2020
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United Kingdom
I've been converting a detached Victorian greenhouse into a garden room. The floor is fairly rough concrete slab (no damp-proof) with one end suspended and one resting on the ground.
I had planned to paint the floor to start with at least, but I wonder about doing it better.

There isn't really the space to build a suspended floor using joist hangers - and the walls are already panelled (all very DIY ;) ) so my plan was to build a platform (maybe 2x2) fixed into the concrete slab, and then lay flooring on top. Probably put insulation between the 'joists'.

I have used a paintable DPC slurry on the end which suffers damp but am still concerned about laying wood directly on the concrete, even treated external timber. I have seen in old properties the put slates between joists and walls and this made me wonder, could I just put plastic DPM strips under the wood to act as a barrier?

This is not a 'room' as such more a fancy shed - everything I've done has been the first time I did it so there are lots of things wrong I'm sure. All done by me and on a budget.
Given how it currently is, what is least horrible?

Here are a couple of photos to show the core structure, and where I got to with it:



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