I am building a raised bed for vegetables out of pressure treated sleepers and I need to put them over concrete.
I live in UK and there is more than enough humidity so I do want to prolong their life and I wonder if I should put some kind of spacers between the sleepers and the concrete so they could dry easier.
I plan to put a lining of landscape polypropylene to keep the soil from washing out into wood or concrete below but I doubt this would help too much with humidity. I already red that putting non-permeable sheets could do more damage than benefits, so I suspect that could be a decent trade off.
I am going to put ~5cm (2in) of gravel on the bottom for draining
My big question is about the sleepers themselves which are 20 cm x 10 cm and I will mount them so the walls will be 10 cm wide. If they stay directly on the concrete there will be almost permanent wet there as my garden does not have very good drainage. After heavy rains it takes two hours for the pool to drain into the concrete.
Should I put some spacers, how high and of which nature? The raised bed is going to be quite big (260 cm x 70 cm).
Please note that this is for growing organic vegetables, so picking the right materials can be tricky.