Concrete Base for Shed - Need Advice

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.

Shed Base.jpg

Seve OB

Good luck

I laid a foundation last year and only finished in September, with other stuff going on it’s still sitting there waiting to have a shed built on it 😱

but the good news for you is that I was close to your size @ 4m*3m and my plans for use of it are also similar to your own.

And even better news for you is that I think I pretty well documented the progress on a thread on boards

I happened to get a start with a builders so picker their brains for advice as I was getting stuck in. I probably went slightly overboard on spec but at the same time I did cut down on some of their recommendations.

Yes to insulation, a few quid, maybe a bit of extra digging, but maybe not to bad as you say you will be above ground level. It will be money and time well spent.

yes to rebar, I was told I would probably get away without it, but it’s not that expensive and it’s easy.

I dug a lot, you might not have to. My garden is already slightly elevated where the shed is going and with short fences, I just didn’t want extra height to avoid annoying neighbours.

it was hard work, get a mini digger. €150 for a weekend.

if you can, get help, especially when you are pouring concrete and get it delivered readymix. I did 2 pours, both by hand, help with first and on my own with second. Help would have been better second time around but I was stood up lol

make sure you have a decent bull float, I rented one for about €30 I think

ill see can I find thread now to link. I’ll be starting the actual build on shed in coming weeks so this is quite a timely reminder

Seve OB

Wow, almost a year to the day when I started the thread

https://www.boards.ie/discussion/2058188701/my-well-built-shed-project/p1

almostover

https://www.boards.ie/discussion/comment/119120975#Comment_119120975

Some useful info in your thread - looking forward to seeing the finished article. I gave up on the idea of constructing the shed myself for various reasons, time being the main one. I did the sums and the saving in cost for a build-it-yourself shed for me didn't equate well with the time and effort I'd have to invest in doing it. But kudos to you, I'm sure at the end of this summer you'll have a fantastic space! I need something ready to go before the clocks go back and I don't want to have to dedicate my free time all summer to it. In a new house so plenty of other DIY jobs to be done!

You thread has been a great source of info, especially the Ali Dymock video on the different floor options. I think I have 2 options for my own:

1. 50mm insulation under the concrete slab. I have drawn this up and attached it. More difficult to do but preserves head space in the shed. My shed is coming with a 7' eaves level which I really don't want to eat into too much.
2. Uninsulated slab with DPM + 50mm insulation placed on top and floored with 18mm OSB. Easier to make the slab but more expensive with the OSB. Nicer floor surface though. Will cost me 68mm / 2.75" of headroom.

Would like to hear the pro's & cons of each option? I know Seve OB you went with the rising wall option but that's both more difficult and more expensive, even if the best option. But I think it's overkill for my steel shed.

Also, is 100mm of 804 hardcore sufficient under a 5" concrete slab? My garden is soft enough on top, but like I said before, there's plenty of rubble under that.

I'll go with the re-bar, it's not a big expense or hassle for peace of mind. What concrete grade did you use on your own base? I'm going to hire a micro digger if at all possible, the digging would be a great workout but I've hobbies too at the weekends!

Thanks for your input so far, super stuff!

Base Detail Section DWG.PNG

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The Ritz

Mod Comment

Moved here from Home Entertainment .

Ritz.

almostover

https://www.boards.ie/discussion/comment/119125915#Comment_119125915

Could you move this to the standard DIY forum for me please? Expect I'll get more responses there. My question is more about specifications rather than quotes / prices.

The Ritz

https://www.boards.ie/discussion/comment/119135285#Comment_119135285

Hi,

I’m not a Mod on this forum so can’t assist unfortunately. Perhaps you could pm one of the Mods here to move it for you.