Posted: 13/02/2017 at 21:21
The thicker the wood the better and sleepers will last the longest. However, timber decking boards are treated, good value and will last a decade or more. Most of my raised beds are on soil but when I've had to build on concrete I used woven landscape fabric, stapled to the inside with galvanised staples to prevent soil running out of the bottom. You'll need to make them deeper than when on soil and it's advisable to put down a layer of gravel etc to help drainage (put that down and lay the landscape fabric on top of it then fill with growing medium.) I used bought-in topsoil mixed with well rotted farmyard manure. In my early attempts, I filled them with bagged multi-purpose compost but found it degraded and plants stopped growing well after a few years, hence the switch to the topsoil/manure mixture which only needs top dressing with more FYM every couple of years or so to stay fertile.