Hi, im hoping for some friendly expert advice! I'm planning to build a raised bed (for planting purposes etc) which i would like to be approx 400 above ground level, capped with coping stone, and with the outside rendered. My current thinking (and most from reading other threads) is: Build a concrete foundation depth: 150 width: 300 Set the foundation strip 150 below ground level Build the wall using 7N blockwork Put coping on Render it Does this sound right? I have a couple of questions which im not sure about also: 1. Should i line the inside of raised bed with some kind of moisture resistant barrier? if so what? 2. Do i need any drainage holes in the wall? 3. Would the wall be strong enough to take a wallplate for the edge of my decking that i plan to butt up against the wall, or should i put posts in and simply not attach to the wall.? Thanks in advance for any help/advice Phil
how big are you aiming this to be ? your dimensions imply strip founds - if you have a loose infill down to ground (for soak away) you wont need any drainage or barrier - if you are aiming for a solid found, so no easy route for moisture to drain then line with 1200 EDPM - any builders merchant will supply if you don't line it, and allow a route for moisture to drain it will get water logged and the rendering will become discoloured very quickly if you build it properly it will be more than strong enough to sit the decking edge on - remembering that the size of the founds will depend on the composition of what is below ground
Hi Sean, Thanks for advice, the dimension would be approx H500 (above ground, i plan to build 3 block high i.e (3 x 215mm)) x L3m x 400 (gap, so outside to external to external would be 700). So based on that I think i would have a 200 gap in the middle that i could leave between the strip foundations, for drainage or would it be best to simply concrete the whole base as you mentioned? (just thinking i could drill some drainage holes down through the concrete perhaps for drainage, and or happy to get the EDPM. I live in north east england (County Durham) and i have no idea what kind of soil we have here, but if i need to go deeper with the foundations i will!
drilling a few holes for drainage isn't a good plan - they'll soon get clogged up I'd do the founds as strips and fill the bottom 200mm with 20mm chips, stone, hardcore etc for good drainage - maybe line the inside walls with a DPC to stop the wall getting saturated from the moisture within
Your foundation depths sound okay, but make sure you are on some solid ground. Render is a waste of time and money on a retaining wall, it will blow, the same as paint, either leave it block face or use a semi engineer brick face. Same as, no need for any sort of dpc etc behind wall. Ensure good drainage, ie dig a bit lower than finish concrete level and chuck some P-Grit in. Build some plastic pipe into the lower perp joints every 3 blocks to aid drainage. The wall will be sound enough to sit the decking on.
but its a planter, holding a few cube of loose soil - its not retaining anything that's likely to cause any render to blow, provided it's applied properly
Hi sorry i have one last question i wanted to check if its possible? I was planning on building the blockwork on the 440 x 100 face (i.e the most economical way) of the block. Is this correct? (i was wondering if the 440 x 215 face might be better, but of course i would need more blocks) Thanks Phil
if you really were building a retaining wall then laid flat would be a stronger bond - laying the block the 'normal' way for your planter will be fine
Hi Phil, My husband and I have done this a few times at different properties so have some experience of how blocks or bricks react when holding moisture. Firstly I would say definitely put some drainage holes in the first/bottom course of blocks at the top of the layer of rubble/hardcore mentioned later. I suggest one little hole in the mortar joint ever 2 blocks along the length of your raised bed. Lay a 100mm bed of say 40mm hardcore/rubble (nothing small that could block the drain holes) Cover the hardcore with a permeable quality geotex fabric (this will prevent your growing media/soil from blocking the drainage holes. Then, on top of the geotex lay a sheet of capillary matting (a good one will hold about 5ltrs of water per sq mtr and will mean less watering. Next, place a piece of PVC pipe vertically in the the bed so it is buried in the drainage rubble and extends 2-3 inches above the level of the soil (the soil will support it) Use this pipe to water the bed from below and when water comes out of the drainage holes that is your cue to stop watering. This method of watering from beneath is much better that sprinkling it on top as it prevent evaporation and the plants will be able to take just what they need. The drainage holes will also prevent water logging. This is a great method for s raised bed as it has its own water reservoir and means you can go on holiday with the knowledge that your plants will have sufficient water in you absence. Now for your rendering (should have mentioned this first) I would paint some tanking paint on the inside of the raised bed. Let this, together with the mortar go off for as long a possible as this will give your render a better chance of adhesion. You may get good advice on this in the plasterers forum. Hope this helps and happy gardening.