Allotment greenhouse build options & considerations

[SarrissUK]

I'd lay a line of mortar in a rectangle the shape of the greenhouse base. It must be fairly firm so that it doesn't just run out the sides when you put the blocks in. I find that it's easier to maintain level that way.

[Chris Hulme]

Concrete I believe will be stronger, light weight seem to detriate quicker.
Use the same mortar between the blocks, easy to do, just buy a cheap trowel from a diy store or screwfix , toolstation etc.
A bed of a few inches should be enough.

[Forage420]

Update

Hi everyone, so it's been just over a month since I posted this thread. I didn't quite proceed as I said I would here as I've had conversations with friends about how to go about building this and was lead to believe I would need to go quite deep as per the photo attached.

There are a few problems in my way that I don't know how to resolve at the moment, as follows :

1) the site slopes downwards, looking at the photo, from right to left. It is much shallower on the left side (about 250mm deep), the right hand side is about a foot and a half deep.

2) I've been told not to use concrete, however, I'm hoping to get around this by laying concrete blocks in the trench on a bed of lime cement which I'm told is not as strong. The reason it can't be cement is because there is an allotment policy that says that foundations should be easily removable, i.e. not concrete. I'm very stuck now about what to do next.

3) due to the slope I will need a significant number of brick courses just to bring everything up above ground level. I'm unsure how to actually build with brick, but it has to be brick because concrete blocks above ground level would not be acceptable to the committee here.


While researching more I found this video on youtube : https://www.youtube.com/watch?v=iZdIUx3NKtg . This is clearly a professional gardener. He has used blocks laid down on their sides and encased in cement. He has built quite a few courses above the concrete block, however, he has used aggregate in the mix that the blocks sit in. I was hoping I could achieve this but with a lime mix instead.

I think it's almost at the point now where I should think about calling in a landscape gardener to build the foundation and brick course and leave me with a timber sole plate that I can just screw the greenhouse to.

I am sorry for being hesitant and back tracking on advice already given me here, but I'm really not sure how to proceed now. Does anyone have any more advice they can share to help me move forward?

[veggiechicken]

If foundations should be easily removable and you can't have exposed concrete blocks, are you sure you're allowed to build with bricks?
What have other plot holders done?

[Forage420]

If foundations should be easily removable and you can't have exposed concrete blocks, are you sure you're allowed to build with bricks?
What have other plot holders done?

My friend on site poured a lime cement slab and built bricks onto it.

As per my suggested plan, I intend to finish the blocks below ground level.

Bricks are fine, but the bonding for them must be lime I'm told.

[Greenleaves]

However you construct the foundation it must be level, if it is not the glass/polycarbonate will never fit properly

[nickdub]

Don't worry too much about the lime stuff or whatever - builders use cement/concrete a lot because if a wall they build gets a crack in it they are in big trouble + they tend to be v conservative re how strong something needs to be - you're not building v high so you don't need that much in the way of footings. Best bet would be chippings/gravel or similar about 3+" thick banged down hard (commercially they use a motorised whacker plate) but a sledge hammer held vertically and bashed down will do the job. Finish it up by mixing up your lime with some sand and water and trowel smooth - check with a level.

You might want to check with the powers that be if you can use insulation blocks for the wall. they are much the easiest as they are quite durable, bigger than blocks but light to handle and you can cut them easily with an ordinary handsaw - I bought some off a bloke who had spares from an extension to do a similar job to you and paid about £1 each - massive work saver.

Get a long straight edged something BTW to act as a guide when building your walls, if you don't do much building work - the professionals use twine stretched tight, but its a bit of a bugger to set properly, if you're not used to it

[Stan79]

Have you thought about a timber base instead? Railway sleepers laid on their side would be 20cm high and if you're more used to carpentry that might be a simpler option? That's what i'm planning on for mine when i get around to putting it together!

Also, because they're long, it's easy to get the whole length level with sleepers. A gravel base in the bottom of the trench might help with drainage and should definitely help with levelling up. If you've got friends on the plot they can help you lift them!

[Forage420]

Don't worry too much about the lime stuff or whatever - builders use cement/concrete a lot because if a wall they build gets a crack in it they are in big trouble + they tend to be v conservative re how strong something needs to be - you're not building v high so you don't need that much in the way of footings. Best bet would be chippings/gravel or similar about 3+" thick banged down hard (commercially they use a motorised whacker plate) but a sledge hammer held vertically and bashed down will do the job. Finish it up by mixing up your lime with some sand and water and trowel smooth - check with a level.

You might want to check with the powers that be if you can use insulation blocks for the wall. they are much the easiest as they are quite durable, bigger than blocks but light to handle and you can cut them easily with an ordinary handsaw - I bought some off a bloke who had spares from an extension to do a similar job to you and paid about £1 each - massive work saver.

Get a long straight edged something BTW to act as a guide when building your walls, if you don't do much building work - the professionals use twine stretched tight, but its a bit of a bugger to set properly, if you're not used to it

Due to the slope I may need to build 3 or 4 brick courses high and intend to use bricks without frog as per the video I linked.

Chippings/gravel lying in the base of the trench makes sense, but this would mean I need to dig out more of the trench at the shallower side as it is only deep enough for one concrete block with a somewhat shallow bed of cement.

I've just had a look at the aerated concrete blocks you mentioned but a lot of the ones advertised through the main DIY stores have very poor reviews, with many people saying they are not suitable for foundations. They appear to be more than twice as expensive as ordinary concrete blocks.

I might get someone to build the brick work for me.

Any further thoughts?

[Forage420]

Have you thought about a timber base instead? Railway sleepers laid on their side would be 20cm high and if you're more used to carpentry that might be a simpler option? That's what i'm planning on for mine when i get around to putting it together!

Also, because they're long, it's easy to get the whole length level with sleepers. A gravel base in the bottom of the trench might help with drainage and should definitely help with levelling up. If you've got friends on the plot they can help you lift them!

I had thoughts about using creosote soaked sleepers but I don't like the idea of them sitting next to my plants.

I came across this website : https://www.uksleepers.co.uk/product...ilway_Sleepers

It's surprised me to see how many eco-options of sleeper there are now, it's a pity I've already dug the trench.

If sleepers were used wouldn't this mean I would have to level the site again and then re-dig a shallower trench to then fill with gravel and sit the sleepers on? I can't quite imagine how the end result would look unless the sleepers sat at ground level, see photo:

[nickdub]

For anyone who is thinking of going with a wood base, I'd say two things :- 1) get the gravel or whatever level first, and make sure its higher than the surrounding ground 2) put some sort of preservative on the underside of the wood before you lay it down - bitumen would be my choice.

[Forage420]

For anyone who is thinking of going with a wood base, I'd say two things :- 1) get the gravel or whatever level first, and make sure its higher than the surrounding ground 2) put some sort of preservative on the underside of the wood before you lay it down - bitumen would be my choice.

In this circumstance I would need to get in earth from somewhere to level the greenhouse area, i.e. refill the trench and buy in some top soil + the cost of the sleepers which I would need to source locally somewhere within the M25. I suppose I could steal earth from elsewhere on site but I don't think this is allowed or would go unnoticed.

Being an organic gardener I'm not sure how comfortable I am about treating the sleepers with preservative. The untreated ones sound exciting but as they are untreated would this mean they soak up more water ? And if they soak up more water would the constant soaking and drying cycle (moisture content fluctuations) cause enough shrinkage and expansion to cause the greenhouse frame to move putting unecessary strain on the glass, as mentioned previously ?

[Stan79]

I would use untreated sleepers. They're massive great big things... how long are you planning on having the allotment for? I'm pretty sure they'll see you out!

As for how they'd sit, you've got them lying flat in you pic, i would have them on their edge so you get more height from each one. At the upper end of the slope the lower one would be completely in the ground and the top of the second one (on top of the first) would be flush with ground level. As the slope comes down to the front, the lower one would gradually be visible above the ground level and the second one on top would be well clear of ground level at that end giving you more growing height at the lower end of the slope and less height at the upper end... or just drag some soil from inside the greenhouse at the upper end as you'd now have a retaining wall (of sorts) to be able to level the soil out inside the greenhouse.

In terms of wetting/drying, i don't think i will be a massive issue due to the size of the things.

[nickdub]

In this circumstance I would need to get in earth from somewhere to level the greenhouse area, i.e. refill the trench and buy in some top soil + the cost of the sleepers which I would need to source locally somewhere within the M25. I suppose I could steal earth from elsewhere on site but I don't think this is allowed or would go unnoticed.

Being an organic gardener I'm not sure how comfortable I am about treating the sleepers with preservative. The untreated ones sound exciting but as they are untreated would this mean they soak up more water ? And if they soak up more water would the constant soaking and drying cycle (moisture content fluctuations) cause enough shrinkage and expansion to cause the greenhouse frame to move putting unecessary strain on the glass, as mentioned previously ?

Only you can say whether trying to alter what you have done already is worth the effort - I was wring more for the generality of people who may fond themselves with similar problems to those you have rather than trying to tailor my thoughts to your precise e circumstances.

As for the wood and preservative, I'm used to working with recycled timer and it generally has a very limited usable life if you don't look after it. Bitumen is sort of natural and more or less inert - indeed I believe at one time it was used for coating the insides of drinking water tanks. Now with the wood you have in mind things may well be different - perhaps you could ask the suppliers how long they think it will last in close contact with damp ground ?

[Lardman]

I should post a link to my foundation build but its on a different site so I wont. I did mine with class b engineering bricks, the ground sloped 1 in 8 over the width.

If this in on the plot and not your garden I think you've overthinking it. If you're allowed blocks onsite, get some lightweight aerated concrete ones (£1 each even at B&Q) put these in your trench dry (no mix at all) on edge to about 6 inches, level them off. Get yourself some treated 1x4 put this on top of the blocks and screw through the wood into each block. Solid footing done, no mortar, wet concrete or anything else. if you need to remove it or move it everything should come apart easily.

If you want it higher screw some 4x4 fence posts to the 1x4 frame, if you want to add some extra protection for the 1x4 put some DPM between the blocks and the wood but I wouldn't bother.