Posted: 18/04/2014 at 12:15
Hi daisymay, as you are building it on paving, you need to ensure some drainage and also ensure that the water draining out doesn't take soil with it, which would otherwise make a mess of the surrounding paving.
One way to do that is to build it in such a way that the vertical support posts project about 1cm from the bottom of the lowest board, so there is a slight gap between the bottom boards and the paving. Fix a layer of weed control membrane across the bottom and attach it all the way around each side (using short galvanised nails works well.) Then place a 3-4cm layer of gravel at the bottom and cover that with a layer or two of weed control membrane. The membrane will stop the soil mixing with the gravel and will also prevent muddy water running out of the bottom of the bed. Next, fill it with a mix of about 40% topsoil, 40% compost (any) and 20% well rotted farmyard manure.
In total, you'll need just over 400 litres of the mix to fill a bed of that size.