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Plans for greenhouse foundation

Postby Jaydeew » Sun May 14, 2023 9:33 pm


I’m currently working on foundations for a 6’ x 14’ aluminium and toughened glass greenhouse. We’re placing our greenhouse in the same site as the previous owners of our house and reusing some of the previous foundation.

They had a plinth of what looked like pavement edging blocks, set on a 2 concrete block deep foundation on a stone footing (seems like overkill for a greenhouse to me?!). I’ve removed the plinth as we couldn’t find exactly the same footprint greenhouse. I can use three sides of the existing block foundation, but will need to add a new foundation for the fourth side due to our greenhouse being about a foot narrower (see red line in image for placement of new foundation).

For the new foundation, I was planning to just pour a concrete fill trench foundation that’s 250mm wide and 150mm deep below ground level. Does this sound sufficient for a greenhouse? From research I’ve done so far looks like this would be fine?

Then from there lay a single course of solid engineering bricks all the way around my new foundation and other three sides of existing as a small plinth to sit the greenhouse on.

My other questions are:

- the previous owner’s block foundation is unfortunately not level. The high point is at corner marked “A” on the image. The rest of the foundation all slopes away from this point. Would it be best to level it out when I build the brick plinth I.e. by compensating through varying the thickness of base mortar I use (this is what previous owner did)? Rough estimate is I could be using 2- 3 inch mortar at lowest points. Or, given I’ll be pouring concrete anyway, could I put forms alongside existing block foundation and poor a thin layer of concrete on top of the blocks at the same time as pouring the fourth side trench fill foundation to level it all out in one go prior to building the plinth?

- Should I drill steel bars into the blocks where they’ll meet the ends of my new trench fill foundation to connect them properly (or overkill for greenhouse)?

- If I do pour concrete over the existing block foundation to level it out, should I be applying a bonding agent to the top of the blocks before pouring the concrete over them? If so, is there a bonding agent anyone can recommend (in the UK)?

Finally, should I use a full length of horizontal rebar in my new trench fill foundation, or again, for a greenhouse, is this overkill?

Thanks very much for any help on this. Much appreciated.

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Simply Build It

Postby stoneyboy » Mon May 15, 2023 11:04 pm

Hi jaydeew,
It all depends on the subsoil and from your description where one end of the existing foundation has sunk by 2-3” the subsoil is not very compacted Your proposed 250x150 foundation should be more than adequate provided it sits on a very firm base. No need for reinforcement or tying into the existing ones. Suggest you lift the existing slabs and relay them to give a level base to work from.
Regards S
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