I've had a builder build me a small concrete block raised bed (internal dimensions 3m x .8m x .5m) with a soil base. It's not reinforced but built out of cemented standard engineering concrete block and rendered on 2 faces.

I've had the genius idea (;)) of making it a small water feature with pump and little water feature. I'll obviously use liner.

edit - the side wall water depth will be only 35cm

My DIY brain says that the walls will easily hold the water as it's only 50cm high but now I'm doubting my fag packet logic - before filling it.

Obvious question coming - are the concrete walls going to be strong enough? If not, can I reinforce them retrospectively? Any other sage advice would be much appreciated.

  • How about a picture.
    – JACK
    Commented May 29, 2021 at 11:51

3 Answers 3


Not sure what it might be called where you are, but there's a "reinforced stucco" (might be called reinforced render in where you are) that can be used to drystack blocks and then bond them together as a unit - parging a layer of that around the inside would provide some structural benefits. If you can't find the product as a product, you might be able to find loose reinforcing fibers for cementitious products and add them to a render/stucco coat for the same effect. "Surface bonding cement" seems to be another name for it. Third possibility - the render coat you have might already be reinforced?

As to whether or you need to do anything, partly depends on frost movement or lack thereof as a concern - you are correct that it's not very deep, so the pressure is quite low, but unreinforced masonry has very little tensile strength. If the water was not drained and it froze, you can just about guarantee that it will crack, unless you shape it to have tapered sides. Even emptied, movement from frost in the ground will usually crack unreinforced walls. If frost is unheard of in your location, that might be a reason not to bother doing anything. Since you are using a liner, tapering the sides should be as easy as building up sand against the wall before placing the liner.


Based on Ecnerwal's idea of parging with reinforced stucco, you could apply a truck bed liner coating such as Rhino Liner to the inside of the walls. In fact, if you compact the soil on the bottom and pour a concrete slab, then Rhino the whole interior, you can eliminate the pond liner.


build it dry (with no cement) on you patio, line it with a garbage bag or plastic tarp and see if it will hold water.

I suspect that friction alone will be enough to keep the walls in-place as it is a very shallow pond.

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