Raised bed drainage holes on elevated patio
I am new to this forum, so apologies if there is already a thread dealing with this topic.
I am getting a patio built and since there is a drop of 70 cm to the grass below I need some sort of barrier. I settled on a long 5 metre raised bed area at the edge of the patio. I am planning to put herbs in it and also some small evergreen and winter-flowering shrubs.
The raised bed will be in rendered concrete block with a small capping stone. It will be 45 cm high and 60 cm in total thickness, with 40 cm of soil width for the plants to grow in.
The blockwork will be only 10 cm thick at the front and back of the raised bed (10 + 10 + 40 gives the 60 cm) but I will put four crosssections along the 5 metres to provide support. I will end up with five areas of approximately 90 cm in length to grow the plants in.
My question is as follows: I imagine I will need some sort of drainage holes. How far should these be apart and at what level? (I have seen someone suggesting to drill 1 cm holes in the jointing every two blocks. Is this a good idea? If so, how high should these holes be from the bottom of the raised bed?
I do want drainage but don't want "soil runoff" to stain the wall behind (or "underneath") the raised bed, i.e. between the patio and the garden below. Can I avoid this somehow?
Also, how do I stop the drainage holes getting clogged up with soil? Would Geotex fabric or something like that be useful there, and if so where should I put it?
Thanks for any advice you can give!