Concrete Bed for Shed - Advice Needed

almostover

Hi all - have purchased an insulated steel shed from a local business. Paid deposit in April and expecting installation in September. They have just sent me the drawing for the base and I want to get on with building it so that it has ample time to dry and cure before the install. Also, ground conditions will be at their driest during the summer.

The shed is a 5m x 3m galvanised steel frame shed with 40mm insulated steel cladding. Have seen their demo units and they look to be of excellent quality. The base that is recommended is attached. 5in concrete with a small rebate for the French doors that I'm having fitted. The shed is being installed in the back garden of my semi-d house. It will be used as a general hobby space, home gym, small bit of DIY, storage of golf clubs, lawnmower, tools etc. I went for insulated as I may in time move my home office out there. The shed vendor is recommending a 5" slab above ground level and they will overlap the cladding over this by 3" to prevent water ingress. I prefer this method too over the 2-part shed base used by Steeltech as it allows me to fully finish the slab prior to shed install.

Will be doing the base DIY for a number of reasons. Hard to get building contractors for small jobs, cost, and I am handy DIY wise and my old fella is retired and looking for some summer projects. He would have plenty of construction experience but more so in welding and putting up steel frame buildings.

I have a few questions that I'm hoping others who have done similar projects might be able to advise on:

1. Should I insulate the floor slab? If so how would that be done?
2. The floor slab will be above ground level - how can I stop moisture soaking through the few inches of exposed slab on the outside? Are there sprays / paints than can be used to seal the concrete?
3. What depth hardcore and what hardcore grade should I use? There is a lot of rubble present under the topsoil of my garden, as I found out to my detriment when trying to install a clothes line.
4. Is steel re-bar necessary in a slab like this? The old lad doesn't seem to think so, but he's a bit laissez faire with these sort of details!
5. What grade concrete should I use for the slab? 20N?

Any advice is appreciated. The shed vendor gave some general guidelines but I'm an engineer myself (mechanical) and I'm keen to do the best job possible that will last as long as possible.

mickdw

You could use a bituthene type product to seal the face of the concrete joined down to a dpm that you would run under the concrete slab and leave sticking out.

The cladding then running down partially over the front face of the conc. Could be finished at bottom with a base board or something.

The bituthene is a tar base with sticky tar backed membrane that will bond to concrete.

If bothering with floor insulation, I'd go for a different build type and probably a screed to bring the build more in line with habitable standards. I wouldn't bother in this case.

Just dig out vegetation. Wouldn't be too worried about depths of stone. It's a steel shed after all. Clear vegetation, place compacted stone to desired level to suit garden then prep for slab.

You could place 1 layer of light mesh if you wished.

20N would suffice for what it is.

jack of all

I have a Shanette shed and the contractor I used did the 2 part base, I wanted insulation under the floor- this is a must in my opinion if you plan to heat it or use it as a workspace. A single slab with exposed perimeter face is difficult to weather in terms a traditional DPM as you have no way of turning it up at the edges. No real need for mesh in the floor at that size,but I did opt for some rebar around the edge in my mini "strip" foundations as the ground was poor. Steel and insulation will add significantly to costs at today's prices but you only get one chance to do it.

almostover

Thanks for the responses. Very helpful stuff. Looks like the procedure is:

1. Dig out vegetation and some topsoil to 4" depth.
2. Backfill with 804 stone and compact with a whacker plate.
3. Place timber formwork on top of stone. Lay DPM under formwork.
4. Place light steel mesh in formwork on some spacers.
5. Fill with 20N readymix.
6. Remove formwork and seal concrete with bitumen sealant once concrete is fully cured.
7. Add a timber baseboard after shed install for aesthetics if desired.

I'm gravitating away from the 2 pour method with a screed added after shed install. It's likely that this shed won't be installed until Oct/Nov and I want all the concrete done during the summer as I'm badly caught for winter storage. With that in mind I have thought of 2 ideas for insulating the floor.

1. Put 50mm insulation under the slab. This will leave 2" of exposed concrete where there is no insulation as the insulated sheeting of the shed overlaps the 5" slab by 3". I would still have bitumen sealing on the edge of the slab to prevent moisture ingress and a 2" timber base board outside that. The heat conduction through that 2" shouldn't be excessive? I have attached drawings of this idea.
2. Build per steps 1-7 and add an 2" insulation on top plus 3/4" OSB. Would leave a nice finish to the floor but would result in a small loss of headroom. The shed will have a 7' eaves so it's not a massive loss to bring that to 6' 9".

What are the pro's and con's of these 2 options?

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jack of all

Others might chime in with other options, but the way my contractor did it was:

Dig to formation level, place hardcore and compact

Place outer forms, pour strip/ground beam with rebar. He the laid a DPM on blinded hardcore, placed floor insulation, placed 6x2 on top of already poured perimeter strip and laid floor slab/ screed to be level with top of these. When timbers were removed a 2" rebate was left around the perimeter to allow for shed to be fitted and bolted down. So in essence the vast bulk of concrete work could be finished weeks in advance of the shed arriving. When installed the rebate is made good with concrete which is only 2" thick and would be dry in a month or less, depending on conditions.

The exposed edge single slab pour you are suggesting is likely to wick moisture into the interior space and I would be relying on any paint on sealer to keep it out.

almostover

Thanks for your advice - my only reservation with that method is that it will lose me 6" on headroom inside the shed given that the shed will be bolted to the strip foundation and then a 6" screed will be poured inside that. I'm leaning towards building the slab in solid concrete with DPM underneath to control moisture and then adding a floating insulated floor with OSB on top of insulation boards similar to this video. The inside surfaces of the shed sheeting are flat so it would make installing a floating floor easy enough. I'd lose less than 3" of headroom that way. Plus have a nice finish to the floor.

https://www.youtube.com/watch?v=I7GD0MB8SzY

But I am open to more input on this, the more opinions the better.

jack of all

The method I'm suggesting means you'll lose approx 2" of head room, but there are other options of course

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almostover

https://www.boards.ie/discussion/comment/119212933#Comment_119212933

That does look a neat job, I'll have to consider that!

gabbo is coming

Hi, would that be sufficient quality for a home office floor?

gabbo is coming

https://www.boards.ie/discussion/comment/119211126#Comment_119211126

Was that 50mm insulation?

FortunaMan

Screenshot_20230215_095949_Samsung Internet.jpg

I used the one pour method and two coats of attached sealer on exposed edges and it has preformed perfectly over the last 2 years. Also used steel mesh and dpm.

Supercell

I wonder would this method work for a shed base?, looks a lot easier than mixing the wet stuff. At the end of it they show a bigger one they did a year earlier for a chicken coop which is far bigger and there's no cracks, quite impressive.

https://youtu.be/GC0j2Ey5NNk

There's a few things wrong with that video.

The thing is, concrete weighs about 1.5 tonnes a cubic metre when set. This stuff in Goodwins cost €6.30 for 20kg, so you'd need 75 bags costing total €470 for a cubic metre. You'd probably be able to get pre-mixed concrete delivered & pumped in for less than that; when I got it a couple of years ago it was €300 for the pump + €100 per cubic metre. And for (say) a 4m x 2.5m shed, 1 cubic metre would only give you a 10cm slab.

Yes the drymix would be much easier than mixing it on-site. (Which is a curse, and which I regret doing for part of my own project- cost the same, was a crappier job, and wrecked my back).

But getting it pumped in would probably be cheaper than the drymix anyway, and definitely easier than either option. So why put yourself to additional expense & effort with no benefit?

https://www.goodwins.ie/products/flo-mix-40n-rapid-strong-concrete-drymix-20kg-bag-add-water-4000011.html

The Continental Op

https://www.boards.ie/discussion/comment/120308225#Comment_120308225

I've done that for spot bedding foundations (blocks set in grass for supporting a wooden shed) dry stone wall foundations, paths etc, and even without watering like they do in the video it goes off rock solid.

Supercell

https://www.boards.ie/discussion/comment/120308451#Comment_120308451

Good points, getting it pumped in would be the way to go if you have site access. My back garden is far too far back from the main road to be accessible by a cement truck and has a very narrow side passageway. My shed is falling apart with the rot so Im trying to figure out what to replace it with. This whole thread is full of useful info.

https://www.boards.ie/discussion/comment/120308518#Comment_120308518

If you have side access, you could recruit some friends & family and barrow it in from the truck. Obviously more work than getting it pumped, but you'd save a few hundred quid for getting the pump on-site.

When I got it pumped, the truck was on the road and they pumped it at least 20 metres from there. The guy wasn't exactly stretching the hose to get it that far either. So it's definitely worth contacting someone who does this to find out how far they can pump it. Or, like I say, just grab a few strong backs and barrow it in.

The Continental Op

You don't need to buy ready mixed bags of concrete you can still do a dry mix in a mixer.

Supercell

https://www.boards.ie/discussion/comment/120308583#Comment_120308583

Would that not make clouds of cement which is probably not great for your chest? After hiring a mixer would you save much over buying the ready mixed stuff say for cubic meter'ish of cement? Just curious, anything that saves labour/cost and i can have a go myself is a win win in my book.

The Continental Op

https://www.boards.ie/discussion/comment/120309296#Comment_120309296

No there will nearly always be enough moisture in the sand for the cement to bind to it. I have a mixer so obviously didn't give the price of hiring one a thought.

https://www.boards.ie/discussion/comment/120309534#Comment_120309534

Is there much difference in the effort involved as compared to a wet mix? You're still hauling cement and aggregate and throwing it into the mixer- just not adding water. And still hauling tipping the mixer over to get the mix in place.

Sounds like mixing it dry (as compared to buying premixed) would save money, but not a whole lot of effort.

Anyway, it'd definitely be worth making enquiries about getting it pumped and bulk deliveries of dry mixed stuff. At least then you'd know for sure what all your options are.

Looking online, there's cheaper options for the drymix- €4 for 25kg bags, so half the price of Goodwins. Which would change my thinking at least!

10-10-20

No way would I even consider dry-pouring a shed base. You've no control over how it all cures and it's likely that the exterior surfaces will adsorb the moisture initially and lastly the inner-most, likely leading to cracking everywhere as the slab cures at different rates. It's not an issue when you're bedding blocks around the garden where the volume of sand/cement are low and the cure duration is not critical, but look at any online guidance and nowhere will it suggest that you can do this and end up with a good quality slab.

For my shed I barrowed in about 3 or 4 tonnes off a mixer truck for the slab and then I hired a mini-mixer for the floor-screed and put in another 1.2T of concrete using that. It's a fair bit of work but I only had 25m to barrow the concrete. I think it was 45 to 50 barrows.

I looked at a pump for another job but was put off as the pump needed to be flushed out afterwards and the guy was saying that the washings needed to be dumped on site. So that's another consideration if you're thinking of pumping.

I'd suggest the option of getting some friends around and doing a barrow-brigade is probably best for you long-term, or going for a wooden shed-base instead.

lkkev80

https://www.boards.ie/discussion/comment/120312424#Comment_120312424

Where are you seeing the bags for €4? I can't find anywhere cheaper than Goodwins