Concrete base for a concrete garage

4 Sep 2007
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United Kingdom
Hi, i am required a 4 inch deep concrete base before i can have a concrete garage erected. I have a tarmac driveway which is reasonably level and i was wondering it i could put a concrete base directly on top of the tarmac?

I was thinking of placing the correct size shuttering directly on top of the tarmac making sure its level and square. Then put some sand down inside the shuttering followed by a sheet of polythene followed by the concrete, leveling off and tampering down.

Would this be ok?

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Most likely not, You'd be best diigging tarmac up, laying about 100-150mm of wackered hardcore, DP Membrane, insulation if needed, another DPM above the insulation, then a wet pour of again 100-150mm concrete with reinforced steel placed mid-way in the pour.
I assume it's a brick/block build.
Thanks for the replies. No its not a brick block/build its one of those pre-cast ones so there is not much weight in the structure.

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I'd still dig the tarmac up and make a sound sub-base, doesn't need to be quite as big as the above reply I made.
but wack some hardcore down, DPM and about 100mm of concrete pour, if you have the base raised a bit should prevent water running in.
Tarmac is not an inert product it gives off gases that could be detrimental to the structure.

Other things to avoid are sulphates as these attack the concrete.

Dig up the tar and use mot type 1.

Stop being stingy.
Not much weight ? You're joking right,those concrete sections are heavy as f**k !
You need a proper slab laying.....not like the botch job I had done.
Get it hard cored and make sure they use mesh in the concrete
Take up the tarmac and see. There should be a hardcore base underneath already, so you may be able to lay the concrete on top of the existing sub base.

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