raised flower bed on concrete floor: drainage issues

[dylanswall]

Hi, I have a small concreted garden and am thinking about putting a raised flower bed at the end (against a brick wall) using railway sleepers. The flower bed will be approx 80cm high and my question is what do I need to do about drainage.

I have a load of pebbles and rubble I was going to stick on the bottom of the bed and then put compost and soil over to help the drainage. Anyone forsee any problems with this? The brick wall has a few gaps at the base to let water drain out so I assume this will add to the drainage.

My only one thought is the rubble has a lot of dirt and a few weeds in it and was wondering will this just mean that my raised flower bed will be inundated with weeds in a short space of time?

All help is gratefully received. Thanks

 

[Thermo]

no problems with the drainage. depends what the weeds are. if its bindweed etc then it will need removing

 

[JMLanders]

A layer of permeable membrane over the rubble will stop weeds getting through whilst ensuring that soild doesn't get into this drainage layer and clog it up. Ironically, it will also prevent the soil above it from drying out too quickly.