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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!



  • pokhimpokhim Posts: 210

    I always had the understanding that the interior wall of bed was rendered to stop soil stains bleeding through and you lay bed on a normal soil base, perhaps with some rubble at the bottom to ease drainage...then you can plant ur lavender or rosemary etc in it..

    That's my plan anyway as I'm doing something similar to you... 

    But maybe I know nothing and would also love to hear the opinions of others

  • DervilaDervila Posts: 19

    Thanks, pokhim.

    I am planning to put drainage holes in the mortar near the base, or perhaps at the height of one block. Like you, I plan to put a base layer of something for drainage (pebbles, hardcore of about 40 cm - nothing too small that would block the drainage holes).

    I think I will also line the inside of each 1 metre planter area with black plastic. I might render the inside too as you have now suggested (but that will add to the cost..).

    I asked at a garden centre after posting my initial query and someone had the idea of stepping the planter out by a few centimetres so that the garden side of the planter projects out over the patio a little bit (as I said, I want to avoid streaks down the wall from the muddy water draining out of the planter area). 

    I would also love to hear a few people's ideas on these - am I thinking in the right direction? Is there something I have missed?

  • BobFlannigonBobFlannigon Posts: 619

    If I understand correctly you want to split the height (70cms) that the patio is raised by with a lower bed (45cm).  This is almost exactly the situation I have with my patio so I'll just tell you what I did and you can decided what to do with the information!

    I built two bricks walls (or a single L-shaped wall if you prefer) that contain the scalpings and rubble that I used to fill in under the patio (the other two are the house and the conservatory).  The paving slabs sit on top of this wall but overhang it by about 6cm.  Between this wall and the garden I used railway sleepers (proper ones, not the imitation stuff) to contain a raised flower bed.  

    I think you're suggesting that instead of sleepers you're going to use brick work but are worried that there will be water marks visible on the outside?  Personally I'd not be worried about that, and nor do I have any drainage; the rubble and flower bed are placed directly onto the ground so it just drains naturally.

    I couldn't quite tell if your bed was at the same level as your patio, obviously mine is lower so apologies if I've misunderstood.  Let me see if I have a photo...

  • BobFlannigonBobFlannigon Posts: 619


    Here you go, a little lower but same principle...

  • DervilaDervila Posts: 19

    Thank you so much for getting back to me Bob.  A photo would be really great. 

    My raised bed (built in blockwork and rendered) will be built on top of my elevated patio at the garden edge  (there is a drop to the garden of 70cm and currently no barrier, so the raised bed will provide a barrier).

    So I will walk out of my house onto my new patio and after 2.5 m of flat patio there will a 45 cm high raised bed sitting on top of it marking the boundary between patio and garden. This will provide a barrier to the drop below.  

    I am thinking of getting the builder to lay the blockwork for the raised bad"on its flat" at the 70 cm point at the garden edge, to "step" it out a bit.

    This will mean that the raised bed will overhang the garden by 4 or 5 cm. This is so that the water can drain out and drop into the garden below and not muddy the wall.

    The patio flags will run up to the start of the raised bed when looking from the house (there will be no overhang of the patio flags as they will not be at the edge of the patio).

    Does that make sense?


  • DervilaDervila Posts: 19

    Thanks for the photo, Bob. No, my raised bed will be on top of the patio, which is on a 70 cm base.

    The 45cm bed will start at the 70 cm point, so that when looking from the garden you will see a wall of 70 + 45cm high, stepped out at the 70 cm point by about 5cm.

  • Dave HumbyDave Humby Posts: 1,145

    A different scenario but I had a wall built at my last place to separate the garden area from the decking / gravel /seating area at the bottom of the garden. It would have been a not too dissimilar style to yours (if I understand correctly) although a little higher, say 80cm or so. The builder built in 'weep' holes by putting some pvc pipe (c15mm) every of often along it's length in a mortar joint. I then back-filled the base of the 'planter wall'' with rubble and then shingle before adding the compost. Prior to that I applied a couple of coats of Jack Black bitumen paint to the inside of the brickwork and this stopped any form of bleed through.

  • DervilaDervila Posts: 19

    Thanks very much for the tip about the Bitumen paint, Dave! I am not sure I like the piping idea, as I don't want any piping to be visible from the garden when looking at the combined patio-and-planter wall. I am hoping drainage holes in the mortar will be enough.  (Did your piping project past the wall or was it flush with it?)

  • Dave HumbyDave Humby Posts: 1,145

    Hi Dervila,

    The piping is virtually invisible as it is flush to the pointing. All you can see is a small hole. The pipe just maintains the hole structure.

    Let me see if I can find some pics of the wall and will post them up. They won't be close-ups mind.

  • Dave HumbyDave Humby Posts: 1,145


    Here you go. 

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