You don't, one either end and as many in the middle as necessary to support the raised bed. I am presuming a timber base (maybe 35mm treated boards) with gaps to allow drainage. A sheet of 5mm steel with holes drilled for drainage would be better, but the sleeper surround will eventually rot out anyway.
You don't, one either end and as many in the middle as necessary to support the raised bed. I am presuming a timber base (maybe 35mm treated boards) with gaps to allow drainage. A sheet of 5mm steel with holes drilled for drainage would be better, but the sleeper surround will eventually rot out anyway.