[Closed] shed/summerhouse concrete base confusion

Given this is STW and it's a summer shed, I'd recommend as a minimum:

Half a dozen piles down to bed rock.

2m deep concrete foundations with a minimum for 60 sheets of rebar.

DPM is to stop the moisture from the concrete draining into the ground which results in the concrete cracking. Concrete needs water to cure.

Even if there is not much load going onto it, I'd put a single layer of A142 mesh in it. Doesnt really cost much but will help if there is any differential settling in the sub base which would ordinaryily lead to a failure / crack.

Are you getting the concrete delivered or are you making it?

If the first, it'll be bloody expensive over a BH weekend, if the latter, hopefully you've got a mixer and ordered in and got the materials already.

Either way, will the 'shed' have a base, or is the concrete the floor?

will the ‘shed’ have a base, or is the concrete the floor?

This is important. If it has its own base the a concrete pad is probably overkill. Plenty of simpler, cheaper and more eco solutions. If it is the base the answer depends on how good you want it. Just dig, shutter, DPM and pour will probably work. Dig a bit deeper and add a bit of compacted mot and some sand first will be better

So maybe just clear and level, shutter, lay DPM and pour concrete?

I would have thought this is fine, but Lowey makes a good point re the mesh.

Plenty of simpler, cheaper and more eco solutions

please divulge...? emphasis on the 'cheaper'

I used the eco plastic mesh grid and 10mm pea gravel for both our shed and greenhouse.
They seem to be working really well and also drain well.
Was dead easy to lay but needed a lot of gravel shoveling

If you are using a timber building (Dunster House, etc), you dont need to get carried away. Mine (4m x 4m) sat quite happily for 10 years on 16 bits of paving slab laid on about 150mm of broken up masonry. Clay sub soil next to trees and hedges. Minimal topsoil strip.

emphasis on the ‘cheaper’

Mine is on a layer of recycled crushed rubble (free), a few bags of sand (£20), paving slabs from freecycle/facebook (free, and you aren't going to see them so they don't need to be pretty). Stable and solid, low cost, eco, also easy for the next owner to dig up if they don't want a shed there.

easy for the next owner to dig up

This - we have the base of an old garage in our garden. A concrete slab 6m x 4m. And at least a foot thick. Absolutely solid.

Impossible to really do anything with (without spending a lot of cash!). It would be lovely to get rid, and grass it, but really cannot justify the cost

Given this is STW and it’s a summer shed, I’d recommend as a minimum:

Half a dozen piles down to bed rock.

2m deep concrete foundations with a minimum for 60 sheets of rebar.

😀 Minimum.

we are planning 6mx3m, bespoke timber build, insulated with pitched tiled roof and 2nd hand doors / window. its got to be slab or piles really.

This – we have the base of an old garage in our garden. A concrete slab 6m x 4m. And at least a foot thick. Absolutely solid.

Impossible to really do anything with (without spending a lot of cash!). It would be lovely to get rid, and grass it, but really cannot justify the cost

We had a mixed bag which is really what makes me think of it. The shed was on mot and slabs so I dug all that up and re-laid in a new location. Very simple and very little cost. The path was about a foot thick (yet still got cracked and lifted by the cherry tree). That was a right pain to dig up. With a lot of effort it became sub-base for the new patio. The original patio had also been laid on mot so the slabs lifted nicely and all of it could be re-used. I will admit my workshop is on a steel reinforced slab so that will be a job someone if they do want rid but its block built and will be around for a while.

I don't think you need a concrete base/slab. I'm certainly not going to bother with the expense and time to lay a concrete pad for the 6m x 2.5m shed/summerhouse I'm planning to build.

Plenty of youtube video of people using decking blocks (expensive), building block piers (waste of time) but I think this approach is the simplest and most straightforward:

I would think it depends on the size and weight of the structure along with sub soil. I'm finishing off a 5x4m SIP panelled 'shed', we went for a skim dig to create rough level base, shutter, DPM and steel mesh. Pin piles were considered but due to unknown sub soil we went for the slab. Cement was delivered; which for the volume was worth paying for: trying to mix, transport and spread the required volume would be a massive pita (and a back breaker).

On the flip side, for the shed (~2x3m) I went for a simple sand/breeze block/DPM foundation which has worked fine.

One thing for certain; I will not be the one to be knocking out the slab if my decides its in the wrong spot!

Hole
Shuttering
Stone
Sand
Membrane
Concrete (spotmix, after mixing a couple of cubed metres in the past went for the delivered. Although you still have to move quick hence the cobbled together tamper)

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Delivered or carry the bags of cement/sand/ballast yourself ?

If you are mixing yourself then doing the job correctly/overkill will be fairly minor compared to the carrying and mixing!

The shed/summerhouse will have a base but my experience of shed bases are they are not very structurally sound and need a pretty perfectly flat base. Could look into slabs but whenever I've laid on something less substantial over time and use the ground moves or is undermined by ants (we have lots of ants around here, they're a real PITA) or moisture or something and you start to get flex and movement so concrete base is the only way to be sure. The last slab base I laid just only lasted about 5 years before things started to move and the shed base developed soft spots and started flexing so really want something with a solid floor. Also I want the thing to be elevated a few inches above ground level. The lawn doesn't drain particularly well and gets pretty sodden in wet weather so want the base of the building well above ground level, also there will be some decking that will be in front of the structure eventually too.

I'm not concerned about the inconvenience of the next owner, can't imagine leaving this house any time soon (can't afford to) so a problem for a long time into the future. There is every chance in 15 years time I'll be ripping it out myself for some other project.

Was in B&Q last night and no problems with the materials I need. Was plenty of stock of timber, cement, hardcore etc, its not a big area so not a huge volume of concrete, but will be securing the materials tonight. Hopefully wont be an issue unless they're been a rush in the last 24hrs. Good point on cement mixer though, but not a large enough volume of concrete needed to justify a full on cement mixer to pipe in the concrete. Not essential I do it this weekend if I do struggle wit materials.

DPM is to stop the moisture from the concrete draining into the ground which results in the concrete cracking. Concrete needs water to cure.

Interesting. Always wondered what the purpose of the DPM was.

Plastic grids filled with ballast. Quicker, self draining, cheaper and less hassle. Put some sharp sand down first along with a weed barrier. Plonk the grids on top and fill with gravel, tamp (I used my stupendous 1930s mower because I'm an idiot) and build away. Have built three structures like that in the past.

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^ won't the base of the summer house get and stay damp and therefore eventually rot?

Was in B&Q last night and no problems with the materials I need.

Let's say you're doing a 5m * 4m slab, 100mm deep - that's 2 cubic metres already, ignoring the sub-base.

So the best part of 2 tonne bags of 'mix' and then whatever the cement needed (10-20 bags depending on strength needed). Even a 1/2 bag mixer will need in excess of 20 full loads.

And once you start...

If your going to half ass the concrete you'd be as well not to waste your money and spend it on making the shed floor out of proper timber and lay it on block piers

Fwiw I got a spot mix on a similar size pad 300mm deep (although with rebar and insulation) and also a 2.4m/8m /120mm with rebar shed base for 450 quid.

But by **** did I sweat moving it. Had 3 barrows and people and we were running(and to top it off was the only day of heavy rain in the whole project)

I think the same job out of bnq with me mixing would have taken many times as long and cost more.

won’t the base of the summer house get and stay damp and therefore eventually rot?

There will be an airflow from left to right and the base will drain well so no reason for it to stay damp. It won't get that wet in the first place either. Also looks like pressure treated timber for the bearers. The modern stuff can get and stay very wet without issue. If anything a concrete base would be far worse for the same timber structure as it will allow water to flow under and hold the moisture.

Let’s say you’re doing a 5m * 4m slab, 100mm deep – that’s 2 cubic metres already, ignoring the sub-base.

So the best part of 2 tonne bags of ‘mix’ and then whatever the cement needed (10-20 bags depending on strength needed). Even a 1/2 bag mixer will need in excess of 20 full loads.

^^^ THIS

You also got to carry it to the car/van.. drive it home and carry it and make as many trips as you need according to space and load ... and after all that then mix it.

If your going to half ass the concrete you’d be as well not to waste your money and spend it on making the shed floor out of proper timber and lay it on block piers

I considered recycled oak sleepers

I'm building a 8' x 10' shed this w/e onto a timber framed QuickJack kit with 11mm OSB on 45mm x 95mm timbers. Not the cheapest option, but far less work and no base laying required and as long as the galv components stand up to it, a base you can take with you or transfer onto another shed.

A cube of conc weighs 2.2 tonnes, these bulk bags contain about 800kg if your lucky ,so to make 2 cubic m of concrete is never gonny happen

A cube of conc weighs 2.2 tonnes, these bulk bags contain about 800kg if your lucky ,so to make 2 cubic m of concrete is never gonny happen

I wasn't going for perfect maths, I was just making the point that the materials ain't coming home in the boot of a car.

yeah, might have underestimated the amount of concrete needed!

🙂 I did the same once. Bought a mixer and thought how hard can it be. Did it but paid the price and was sore for a few days... Saved a few quid 😐

Just get it pumped. Our 'shed' base...

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He managed to get the whole building pumped as well...

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Mine wasn't a lot of effort and sounds like it could work for you.

I made 8 x (or 12 x? I can't recall) pads using 2x2ft paving slabs laid on hard compressed ground. On these I used the plastic levelling feet and on to those I laid my timber frame for the shed to sit on top of.

It sits nicely above the ground and should the ground settle I can jack up the frame and wind out the supports.

https://timberfixings.com/adjustable-decking-feet/

yeah, might have underestimated the amount of concrete needed!

I think you may have ...
Speaking from experience .... I woefully underestimated.
Bloody awful weekend doing nothing but forwards and backwards for cement/sand and ballast and carrying it ...
Promised myself next time (should there be one) I'd get it pre-mixed...

You could try my other idea that was oak sleepers.. however having 2 tonnes delivered for the borders and me and a mate carrying them through that was PITA as well!

yeah, it's about 48 bags of cement so that would get tedious very quickly. I guess cost aside having it ready mixed and pumped in makes the job even simpler.

Plastic grids filled with ballast. Quicker, self draining, cheaper and less hassle. Put some sharp sand down first along with a weed barrier.

What size is that?  I've got a similar 8x10 cabin on order.  Had an artificial lawn laid yesterday and got them to lay all but the turf (barrier - aggregate - granite sand - barrier) where the cabin will go so I have a level and 'solid' base to work with.

Am planning to use the plastic grid base for ease so would be interested how many bags of pea gravel was required? I've paid a bit extra for composite foundation beams rather than the pressure treated timber and will also lay a dpm under the floating floor so damp should not be an issue.

Also how easy was the build (I'm a competent DIYer).

shed/summerhouse concrete base confusion

???????????
Why a concrete base for a summerhouse?

DPM is to stop the moisture from the concrete draining into the ground which results in the concrete cracking

Actually the DPM acts as a slip membrane allowing the slab to slide as it shrinks, and prevents cracks that way. (A concrete slab will shrink as it dries out). If the DPM has big wrinkles in it the slab is likely to crack where the wrinkles are.

When I did mine I just laid-out and part-buried (and levelled, obviously) some 9" cinder blocks at regular intervals.

Quick, cheap and effective. Worked a treat. Seemed like a good idea at the time and still standing 20 years later with no issues.

48 bags of cement would give a medium strength mix of concrete but 4 cube of the stuff
600kg cement/ cube is probably the Queensferry bridge mix

@razorrazoo it was a 2.5x3m Rhein from dunster house. Went up really well and pretty good quality (apart from the doors which were cr@p).

I then went on to line it with superfoil insulation and some pine cladding. Was really cosy once I’d finished.

Not sure how much gravel was used as it was lying around the garden anyway

Regularly spaced paving slabs or blocks are all you need. A concrete slab is just over-engineering of a very basic project. I put a medium size cabin on slabs about 15 years ago. It's still perfectly stable and going nowhere.

I’ve just ordered a new shed (10ft x 6ft) and will be doing as thebees suggests - paving slab base. That’s what the company advised too, admittedly my shed is smaller than what the OP is planning.

@razorrzzzo

Am planning to use the plastic grid base for ease so would be interested how many bags of pea gravel was required?

My 9x12 foot base at 2" deep took just short of a tonne. (Prob 3/4 of a builders bag)

Thanks for the responses to my question.  Will weigh up the builders bag vs individual bag cost. Looks like I’d be left with a load of gravel if ordering a builders bag which I’ll have nowhere to shift.

I laid a base for our shed a few years ago and went for the paving slabs over type 1. That was probably overkill and if I did it again I'd go for the plastic mesh filled with gravel option. The shed hasn't moved though and as we get a lot of water being near the bottom of a hill, I made sure it had a slight fall on it so water didn't pool under the shed.

We've just laid a concrete slab base where we're hoping to put a hot tub. For that we went for 150mm concrete with 2x mesh inside, poured over compacted hardcore and type 1. That had a DPM under it and should be up to the job.

Regularly spaced paving slabs or blocks are all you need.

Hey, this is STW, what's need got to do with it? We're all over-biked, over-car'd, over housed, over coffee-grinder'd etc. why not be over-shed-base'd too?

Anyway I want to raise it up a few inches above ground level so by the time I've backfilled everything I might as well have just poured a base. Also we have an ant problem and they undermine slabs laid on sand...all my slabs down the side of the house are wobbling, so just easy and less hassle to do a concrete base.

Also its not a big shed/summerhouse. Only looking at about 3.5m x 2.5m.

Why a concrete base for a summerhouse?

I wondered that too. If it was a workshop with heavy machinery then yes. Otherwise just poir concrete pads and build a base frame from decent size timbers, put a dpm on it then osb flooring then flooring of your choice.
Every off the shelf / bespoke garden office ive seen uses this for the base.

Hey, this is STW, what’s need got to do with it? We’re all over-biked, over-car’d, over housed, over coffee-grinder’d etc. why not be over-shed-base’d too?

I was replacing a falling apart shed... it was like a Reverb..(in STW speak). I spent more time servicing it than using it.
I stuck in the concrete base and the EPDM roof as most of the damage came from those areas.. either invasive species (forgotten the name) or leaks in roof (or both)

Was going great until the neighbours built a house at the bottom of the garden .. now its wrecked with their waste chucked on the roof but I think if they hadn't thrown the scaffolding pole on the roof it would still be water tight (if covered in mortar they pour over).