Allotment greenhouse build options & considerations

[Forage420]

Hi everyone,

I'm looking for some advice about a greenhouse base which I need to build. Kindly I am having one donated. The size of the external sizes of the greenhouse base you see in the photos are 2metres x 2.67metres corner to corner. I would like to build a foundation for my greenhouse exactly like this one in the photos.

The width is 4inches (100m).

[Forage420]

These are close up photos of the foundations. As you can see if I have exposed some earth and stuck a trowel in underneath, and it feels as though there is bare earth.

The depth is 12 inches at max + 1-2 inches of concrete below ground, as per the photos...

[bario1]

Hi Forage, I've moved your thread to the 'Undercover Operations' section.

[Forage420]

Key considerations


How should I build the form work for the foundation?



What concrete mix should I use? i.e. how much sand/cement/water ratio?



Do I need to fit rebar/mesh ?




What other considerations are there ?

[Doncasterpaul]

If it’s a simple repair job probably 4 sand to one cement. If it’s a complete new base a 6-1 will be fine. But use a sand and stone (ballast) mix instead of just sand. With regards to water it is a case of just add it until the mix looks right

[lottie dolly]

does it have to be a base so thick,you could do the job a lot easier/cheaper and quicker,useing concret edgings,or some slabs cut up,maybe worth hiring a machine to do the cutting,unles you already have a grinder,
to add,mine stands on bare ground and is fine,if you are woried about the winds,it can still be drilled and pluged into the slab,also nothing wrong with putting a little cement mix to bed the slabbs on,

[Chris Hulme]

Hi.
Dig down as far as the one you're trying to replicate.
Then use ply wood to build the shape you want (shuttering). Pour concrete into the shape/mould you've made. 6 ballast (sharp sand, gravel mixed) to 1 cement. You can buy bags of ballast for wickes etc. Once the concrete has hardened removed the ply wood and you should have your base.

[Chris Hulme]

You could also use concrete blocks and build a one block high wall. Dig a trench and lay the blocks on a bed of sand and cement (building sand)

[SarrissUK]

You could also use concrete blocks and build a one block high wall. Dig a trench and lay the blocks on a bed of sand and cement (building sand)

I agree with that - greenhouses are always just a little too low along the lowest part of the roof. Raising it up a bit allows much more height on plants

[Chris Hulme]

Or build the blocks to suit your height.
Mine is on a 3 high block wall, which was luckily already there when I took over the garden. Just pulled the old greenhouse down above the wall and built on top again.

[4Shoes]

It is going to be easier to do a block wall than tying to shutter with boards. You'll have more opportunity to correct any errors in square, plumb and levels. You could use brick if you like.

[Snadger]

I built a breeze block base for a greenhouse last year. Best way is to erect the greenhouse, set up all four corners with blocks and level to suit your greenhouse size then simply fill in the gaps with breeze blocks levelling as you go.

A mortar bed of a couple of inches below breeze blocks should suffice.

[Forage420]

Thanks a lot.


So what I think I'll do is dig out each corner as per Snadger's method, and pour in a few inches of ballast/cement mix that Doncasterpaul & Chris Hulme specify (6:1).


I'll do one corner first and work from one to the other levelling as I go.


The last stage would be infilling the perimeter with blocks, but could I get away with using just compacted ballast or would it need cement ?


I'm trying to avoid using too much concrete in the foundation as the committee will have something to say about it.


Lastly, to save on cost, could I get away with using postcrete at the corners instead of ballast/cement mix ?

[Chris Hulme]

Don't use ballast, ballast is for concreting.
Use building sand and cement 4:1
You don't need a concrete foundation, just lay the mortar (sand and cement) in your dug out trench and put your blocks straight onto the mortar.

[Forage420]

Thanks again Chris,

Lastly, are those concrete blocks or breeze blocks in the photo? Is there a reason to use one over the other?

In your photos there's a mortar joint in between each block, is this easy for someone like me to do who has no skill or special tools in this area? I've never laid brick or block, but I do a fair amount of carpentry and other DIY related jobs.

Also, just to clarify, are we talking a good few inches of mortar, or just a splodge underneath each block ?