I think it's important to stand it on solid ground - so slabs are fine, or if not slabs inside then they need to go all round the edges to prevent foxes digging under the wood, iyswim...
We built our run onto an old shed we use for the house, and I dug around the perimeter about a foot down and sank the wire into this before filling it in and slabbing over the top, all round the perimeter.
Never had a fox dig in so far (two years-ish) and they have a nice, dusty, dry earthy floor which sadly has zero foliage but they can use it all year round.
I covered the roof with some scrap corrugated sheets I got from the builders merchant so even when it is snowing or raining, only the wire covered area (round a huge tree) gets muddy.
Hens like dry and dusty - I would free range ours still but plenty got taken by the fox when we used to, so that's why I built the run - it's about 2.5 metres wide (same as shed) and 5 metres long so three chooks have loads of room even if it's a bit boring for them. We had six in there, lost some to illness etc. but you could keep probably 15-20 in such a space.