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How to prepare an existing concrete slab for a prefabricated shed?

I'm wanting to built a 3x3 prefabricated shed on an existing concrete slab. The slab is much larger than required for the shed (approximately 4.2x3.4) so I'm worried about potential flooding.

The slab is not level due to a a series of cracks - likely caused by a tree root - and so I'm wondering what my options are.

I figure pouring a rebate will be an expensive and time-consuming project that, given I'm not rectifying the cause of the crack in the current slab, may degrade quickly.

I'm interested in fixing the shed to the existing slab and building a floor as per @MitchellMc guide how to install a shed floor.

However, I want to ensure the part of the slab the shed sits on is level. Could I build up a 3x3 form and pour self-leveling compound to achieve this as suggested in @MitchellMc post how to level a cracked outdoor slab?

Thanks.

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Comments

Akanksha

Many thanks for your question, @doberon. Trust you've been well and have been finding the site a handy resource for all your D.I.Y. projects.

I'm sure @MitchellMc will be happy to answer your questions on preparing your concrete slab as soon as he can. Let me also tag our ever-helpful members @Jewelleryrescue, @DIYGnome, @jimohalloran and @riziqbl in case they have any thoughts.

Thanks for your patience in the meantime.

Akanksha

Jewelleryrescue

Hi @doberon

Given you may expect more floor movement I might suggest you build your shed on treated pine sleepers 200mm x50mm laid flat all the way around the slab using packers for any hollows then bolting that to the slab. This will give you an easy shed assembly screwing into timber,

Have the timber cut so the outer shed walls sits on the outer timber frame.

As a huge bonus You can then lay a great raised 50mm height yellow tongue flooring (as you have seen in how to install a shed floor.) by simply adding some extra floor treated slabs to support the floor.

Any future cement floor movement simply undo floor bolts partialy and repack under sleepers leveling shed sides again at need or desire.

Ho[e this works for you. 🙂

jimohalloran

Self levelling compound might be an option, but it depends how thick you'd need to pour it to get level. See if any leveling compound you're looking at has a "maximum pour depth". I know epoxies do, and I thought levelling compound is epoxy based. Might need to pour it in several stages depending on the depth you need if it does. More tree movement underneath would quickly undo the work you've done to get the slab level though.

Does the slab need to be level, if the floor is level and the shed is straight?

My first thought would be to build a frame for your shed out of treated timber (basically like the form you'd make for the levelling compond), level that up and build the shed on top. But, then you're going to build a floor inside the shed, maybe make the floor slightly bigger and build the shed on top of that? Basically building.a level platform for the shed, that's also the floor inside. Make sure everything that's exposed to the weather (or water potentially getting underneath) is treated and you should be fine.

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/33416#Comment_33416

Some excellent ideas here from our members @doberon.

Given the state of the current slab, I'm not sure if levelling the area is the best option. Does water pool in this area or run off freely? I like the idea of elevating the floor, but if water pools here, it means that water will get under the shed. That will create quite a humid environment inside. It might be an idea to do something similar to this guide: How to waterproof a shed floor. That way, it will prevent water from creeping under the shed wall, which you'll also seal down to the concrete.

Please let us know if you have any questions.

Mitchell

doberon

Thanks for the input @Jewelleryrescue @jimohalloran and @MitchellMc a wooden base sounds like the best option.

I'm leaning toward @jimohalloran suggestion of a base and floor and have a few questions about materials and configuration.

Would H3-treated pine be sufficient?

I'm thinking of building a frame using 70 x 45 mm H3 treated pine for the rails (2 x 3000mm), joists (6 x 2910mm) noggings (6x), and 18mm plywood for the flooring sheets.
What ply sheets would you recommend? And screws to fix it all together?

Thanks!

MitchellMc

I'd tend to follow the guide How to install a shed floor @doberon. I've selected chipboard flooring as it has moisture-resistant properties, whereas plywood doesn't. You could go for marine-grade plywood, but the additional cost over the chipboard would likely not be warranted. It would be cheaper to use the chipboard and then, if you wanted a plywood finish, skin over it with 3mm plywood.

Standard timber screws as per those recommended in the guide would suit.

Mitchell

doberon

https://www.workshop.bunnings.com.au/discussion/comment/33479#Comment_33479

Thanks, Mitchell.

I'll be sure to follow the guide and let you know how it turns out.

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/79053#Comment_79053

Keep us updated @doberon, and reach out if you have further questions.

Mitchell

doberon

https://www.workshop.bunnings.com.au/discussion/comment/33479#Comment_33479

I thought I’d update you on my progress. The flooring frame is complete. I’ve spaced joists at the recommend 450mm spacings for the 19mm yellow tongue flooring but wonder whether I should add noggins every 800mm where the boards will meet?

The structaply technical documents doesn’t mention this but I wonder if there might be some flex.

I’ve built the frame on the timber’s

short end - considering it will be carrying the weight of the shed - and have decided to use Builders Edge Decking Feet to lift and level it from the slab. I plan to then anchor each corner into the concrete through the rails of the frame.

Your thoughts on noggins and the new approach are much appreciated!

MitchellMc

https://www.workshop.bunnings.com.au/discussion/comment/79200#Comment_79200

Hi @doberon,

Typically, you'd run the board perpendicular to the joists. The tongued side needs no extra support, but the ends should finish midway on a joist. If this is impossible due to your configuration, a support or noggin must be installed under the non-tongued join, or it will flex.

Great work so far!

Mitchell

doberon

https://www.workshop.bunnings.com.au/discussion/comment/103942#Comment_103942

Thanks, @MitchellMc

I planned on running the boards perpendicular to the joists and cutting them to size so they end on the joist - the base is only 3050mm wide so only one board wide.

I’ve added noggins halfway down the joists to account for some warped joists (approximately 1500mm) but it’s good to know I won’t have to add another two rows.

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