lining for homemade trough ?

Hi , I'm currently building a "trough" to put on my patio in order to plant sweet peas and other wild flowers. The trough will be about 2m long by 40cm wide and +/- 30cm deep.(The patio itself is wood decking) My question is about lining : what would be the best to use to line the inside of the trough for water retention/ evacuation and what would be the best mix for the earth ? In addition is there anything I need to do to protect the underlying patio decking ?
I thought of some kind of "plastic" lining but am afraid it wouldn't let the water out... any ideas ?

ps : I am hopeless at math so could anyone tell me how much earth I will need to fill this size trough ? :dh:

Any suggestions are welcome, as you can see I am a novice at all this but really willing to have a go ! Thanks alot...

 

The answer to how much 'earth' is easy. 2m x 0.4m x 0.3m = 0.24 cubic metres or about a quarter of a cubic metre.
As far as lining goes I'd pop down to a garden centre and buy some black plastic which should be available on rolls. You need a bit 2.6m x 1m. (2m long plus 0.3m for each end, and 0.4m across the base plus 0.3m for each side) An alternative might be to split a wheelie bin liner, but without going out and measuring one of ours I'm not sure if it would be wide enough.
Whatever you line the trough with all you need to do is poke a few drainage holes in the base of the plastic. Another thing I'd do is to raise it up from the decking by putting it on slates/tiles/chunks of polystyrene/bricks or similar - that will give an air space and help keep the the decking dry.
As far as the 'earth' is concerned In our situation I'd fill the base with broken up polystyrene to aid drainage, then add some compost from the compost heap and top up with bought potting compost.
One other thing you might like to consider is adding water-retaining gel as this will help prevent the trough drying out when (and if!) things get dry and hot.

 

I would agree with Dave about adding some polystyrene, on the grounds of weight. A cubic metre of water weighs a metric tonne. But soil weighs perhaps 1.5 times as much. So a quarter of a cubic meter would weigh well over a quarter of a tonne.

 

Hello Dave W & PeterS
Thanks for your replies... I would have answered before but the site was down for a while... Good idea for the polystyrene but can I use just any old stuff , ie: packaging polystyrene and just chop it up ?

 

Yes just break it into chunks - any kind will do. Most of the pots and planters in our garden have a layer of polystyrene in their bases.