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How to build a concrete slab for shed?

Finding My Feet

How to build a concrete slab for shed?

Looking for some advice and information on getting a concrete slab down for a 3x3 or maybe even a 4x4 shed.


After using a couple calculators found on the web they say i will need approx 100-150 x 20kg bags of concrete, is this correct? or does it seem excessive for a 3x3-4x4 100mm slab, this means it will be $1000-$1400 worth of just concrete bags, at this point i feel like maybe just buying some already made smaller concrete slabs, or even paving would be much much cheaper.....


does the slab really need to be 100mm thick? could i not get away with 50mm ? it will only be used as a small workshop for small welding projects and wont even have anything heavy in it.


My other issue is i will be doing this job on my own, this means the typical way of pouring the concrete might be an issue, i cant see my self mixing and pouring 100-150 bags worth of concrete and also prepping and making it flat before it dries in the pad area, would it be ok to dry pour it? even though ive watched about 30 youtube videos of successful dry pours of pads much bigger than what i need, for some reason i am still very sceptical about the process...


the other option would be splitting the pad into 4 parts, this means i could try to pour the slab the usual way as each part would be smaller and split into 4 jobs, using metal mesh on to "connect" the 4 pads into one.


keep in mind my yard and house is not set up in a way where a truck can come and pour the concrete, paying someone to deliver and pour it is not really an option as either way they will need to cart everything over from the street into the backyard and would cost a LOT in labour just for this part. i dont want a slab for a small shed to turn into a $5,000 project.

Any advice appreciated.

Bunnings Team Member
Bunnings Team Member

Re: Advice / Info - Concrete slab for shed

Hi @Flakesz,


Thank you for your question and welcome to the Bunnings Workshop community, it is fantastic to have you join us.


If you want a slab that meets the generally accepted standard for shed slabs, unfortunately, I'd suggest this might be a job best left to the professionals. They would likely use a pump truck to transfer the concrete from the front of your property out to the back. 


A 3x3 or 4x4 slab might not seem that large when it is laid out on the ground but as you've found a 3x3 slab is 0.9 cubic metres and would require 90 bags and a 4x4 slab is 1.6 cubic metres and would require 160.  


The amount of labour and skill required to prepare your formwork, mix, pour and finish what works out to be over 3 tonnes of concrete in a limited timeframe is not something I'd suggest can be done alone. Even smaller slabs such as this will usually have 2-3 guys working on them.


I have also seen many dry pour slabs online of late, and they do look similar to a concrete slab, but I note that none of them are carried out by concreting professionals. I am extremely sceptical of their quality and would not suggest they are used in any kind of structural application.


My suggestion, which would be both strong and well anchored in place, would be to build a timber floor structure such as this Garden shed on timber base from our member @Malcolm.


The cost to construct this would be far less than you would need to pay a professional crew to pour you a slab, but it will be extremely solid and capable of supporting a shed.


The only thing I could see being a slight issue with copying this project is the fact you are planning to weld inside the shed. 


You could build the frame in the same manner, but I'd suggest you use a fibre cement flooring material such as this James Hardie 2700 x 600 x 19mm Secura Exterior Flooring instead of the timber boards.


Allow me to tag some of our helpful members to see if they have any suggestions, @Dave-1, @TedBear, @DIYGnome, @Jewelleryrescue.


Let me know what you think and if you have any further questions.




Re: Advice / Info - Concrete slab for shed

Hi thanks for the reply. it does feel like of a big concrete job for 1 person ( especially someone who has not done it before ), the timber floor idea actually looks rather nice. something to consider for sure, i will ponder a while more before i decide to do anything. 

And i agree with the dry pour , there is a lot of inconsistency in the outcomes with these kinds of jobs, some turn out good some turn out bad but at the same time the ones that turn out good might only be good for the length of the youtube video and then who knows , the may even crumble to pieces the next day. so i will just avoid that idea for now. 


thanks again

Home Improvement Guru

Re: Advice / Info - Concrete slab for shed

Evening @Flakesz 

Ive seen the same "dry pours" and generally even tho they sound possible I put them into the same catorgory as "Dodgy Bro Inc" :smile: Tho id be tempted to maybe make up a small stepping stone style pad and test it out and then once "cured" id drill a hole or two in it just to see.


I really like a raised deck to put the shed on. Its off the ground, the piers can be soild as you want and it can be bolte down. @JacobZ suggestion of fibre sheets for the floor sounds liek an idea with the welding.


I have lugged home a Ton of concrete bags for my deck, that was hard enough on me plus the springs in my car. Just something to factor in if you do go down that path. Also scrap one wheelbarrow as it was plastic and stood up until I dropped the last bag of concrete into it to mix and it went "CRACK"



Kind of a Big Deal

Re: Advice / Info - Concrete slab for shed

Hi @Flakesz the concrete floor won't be feasible as one person, bag by bag job. The concrete would start to go off in sections before you are finished and wouldn't be one cohesive surface. Have you rang a contractor to get a quote? It may not be as bad as it sounds, money wise, with the right team doing it. 

Bunnings Team Member
Bunnings Team Member

Re: How to build a concrete slab for shed?

Hi @Flakesz,


I'd agree that a D.I.Y. slab of that size wouldn't be possible for one person. You'd need a helper to mix the bags as you spread at a minimum. You'd also need to hire a cement mixer. If you did want to give it a go, or for the benefit of others coming across this discussion, we have a helpful step-by-step guide: How to pour a concrete slab.




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