Posted: 05/01/2013 at 12:08
Sam, my two compost bins are on the ground and did have wire mesh across the bottom that was over twenty years ago, when I bottom out it has gone so I start with twiggy stuff that gives air space.
The boxes are square made of wooden slats on a frame with very slight gaps between the slats to let air in at all levels although the best way is to turn the compost mixing it well as you go. The boxes have lids.
People say leaving air gaps lets rats in, my answer to that is in all my years I have never once seen a rat, that could be because I always had terriers who had the run of the garden.
To sum up, on the ground or on slabs does not matter. If you have some small gap in the sides, drilled holes, a cut hole with fine wire over it or leave air gaps in the slats air will be drawn in by the heat of the compost and as I keep saying on here AIR HEAT MOISTURE, that is the secret of good compost, the hotter it is the sooner you can use it.