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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.

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    I think you would want to do more research before using pressure treated lumber for growing vegetables. It might be safe but certainly requires some care when cutting...finegardening.com/article/…
    – kevinskio ♦
    Jul 15, 2019 at 10:35
  • If there is any gap at all, even a sliver, you can skip and should skip adding a rock on the bottom. The water when it hits the concrete will naturally run off the sides. Putting down gravel might create a perched water table, creating a dam of water build up. Meaning your medium has to become fully saturated before it will pass through the gravel and drain away. deepgreenpermaculture.com/2019/09/06/…
    – GardenGems
    Dec 14, 2019 at 9:02
  • Masonry doesn't rot and isn't preserved with toxic chemicals...
    – Ecnerwal
    Aug 1, 2022 at 15:50

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Is that 10 millimetres a typo? Might want to correct that or clarify. Regarding spacers, you could consider something that does not corrode or rot itself and has a shape to contribute to stability. Two immediate suggestions: square tube aluminum and exotic hardwoods. Square tube aluminum will not corrode and if large enough dimension on the side will not contribute to rolling. However if you are located close to a garden furniture or shipping yard you might just have the opportunity of offcuts of teak or purpleheart. Very dense lumber is hard to work with but extremely durable in salty wet environments. In the ship construction industry there is a fair amount of waste due to odd shapes which must be cut from slabs to produce bow, keel and transom components. Take a trip to the seaside and talk to some old salty dogs puffing through their Popeye corn cob pipes, but try to leave the salty language where it belongs.

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