this post was submitted on 22 Feb 2026
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A normal sized greenhouse hardly weighs anything. Digging footings etc seems like a complete ball ache and overkill.
Personally I'd flatten the ground. Paving slab it then build off that. You can even get paving concrete in bags. Slap it on the ground nice and flat, put the slabs on it, then either water with a hose or wait for it to rain. It's basically postcrete for slabs.
However if you absolutely positively must have foundations then the standard mix will be more than enough. The standard mix is 1 2 3, 1 cement to 2 sand to 3 aggregate(gravel etc).
Hell, for something like a greenhouse you could probably get away with 1 cement to, what, 4 sand then add aggregate to suit.
Will postcrete work? For a standard sized greenhouse then, yeah, within reason.
Bit bodgy, but it's only a greenhouse.
It's not designed to be laid in long lines so it may crack.
If I had to use postcrete I would get a shit ton of aggregate, some sharp sand, and add it to the postcrete mix to add strength, flexibility, and try to mitigate cracks.
However it would probably be cheaper to buy a wheelbarrow and rent a cement mixer for the day and mixing up normal concrete, then barrow it through the house.
Alternatively you could dig holes and postcrete. Then build 'stilts' off of the postcrete. Then build off of the stilts, which, admittedly, will take longer but should be cheaper.
It's not really the vertical weight so much as the force when the wind's across it - hell even the empty frame moves around in the wind and needed pinning down.
Good point on the postcrete, I'd not thought of it cracking in longer lines, will give that a miss then.
Yeah, I just don't have the experience to know if a 1-2-3 or a 1-4-? mix is right, or not, but at least you've confirmed it's not something completely different...
Thanks
If you've got wind problems (😏) then the 1 to 4 with aggregate slung in to add strength will do the trick.
I live in Scotland's countryside. Very, very windy at times. As in it's difficult to walk at times type of gusts, and the rain can spend the day going almost sidewards not downwards.
And one of our sheds is built off paving slabs. Now, admittedly, I didn't build and I probably wouldn't have built it like it is, but it's passed the test of time all the same.
What they did was:
Plonk the slabs down. B Shuttered the sides.
Concreted over slabs to tie it all in and give it lateral strength.
Built the building and screed it down, then built around the bottom of the shed a little bit to stop any sideways sliding motion in the wind.
Wouldn't pass any building regs but it's been up for decades and it's still here.
Edit: you could probably do the above slab way with external self leveling compound, not concrete. Flatten, slab, shutter, self level, wait, build greenhouse and screw it down, put something around the edges of greenhouse to stop it from going sideways.
Good idea on the edging. I won't ignore the advice from a windy Scot!