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https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl DIY Forum >> Building Questions >> concrete base for prefab garage https://www.askthetrades.co.uk/cgi-bin/yabb/YaBB.pl?num=1425766400 Message started by Natedog on Mar 7^th, 2015, 10:13pm

Title: concrete base for prefab garage
Post by Natedog on Mar 7^th, 2015, 10:13pm

I've currently got a corrugated steel garage on a wooden frame. The floor inside it is just compacted gravelly stuff (don't know if it's strictly what you would call hardcore) it was built by my father in law as a temporary thing in the mid 60s.

As it is now well past saving (doors long since gone, roof partly missing and timbers rotting) I'm going to replace it with a 2nd hand prefab that next door conveniently want rid of.

I've never put one of these up before. Last one I had was new and the price included putting it up and putting the base down.

How thick does the concrete need to be, and what mix? 6:1 gravel* to cement ok?

*I know gravel isn't the right word, but I can't for the life of me think what it's really called. 1/2" to dust stuff

I'm assuming that it would be sensible to put a layer of plastic membrane under the concrete to help fend off damp. If this is the case, what do I do with the plastic at the edges to make it look nice?

Cheers guys

Title: Re: concrete base for prefab garage
Post by woodsmith on Mar 8^th, 2015, 9:45am

Are you planning to put a concrete base down over the entire garage area or just a perimeter support for the walls?

To do a proper job you will probably need to dig out the existing base. Put a layer of hardcore and compact it. You may well be better off getting ready mixed concrete delivered rather than try and mix it yourself. Concrete is All in Ballast (a mixture of sharp sand and gravel) not inch to dust (sometimes called MOT or quarry dust) which is what you could use as a hardcore or you can use separate sharp sand and aggregate. Ratios are 1:4 for all in and 1:2:4(cement, sand, aggregate) for separates.

Depending on the ground conditions you may want to add a metal mesh in the concrete to stop it cracking and it is common to not put a membrane in a simple garage base.

As for depth of concrete, again it depends on ground conditions but 100mm would be the minimum thickness you could use, with it thicker at the edge to support the walls.

Hope that helps

Title: Re: concrete base for prefab garage
Post by CWatters on Mar 9^th, 2015, 12:05am

Good site for this sort of thing.

http://www.pavingexpert.com/concrete.htm

Quote:

Concrete paths should be 75-100mm thick, whereas drives, garage bases or hardstandings should be at least 100mm thick. For heavier use, such as large vans, use a 150-200mm thick concrete slab. Concrete slabs intended for exceptional loads, such as commercial yards, lorry parks etc., will be at least 200mm thick and should be specifically designed as they will probably require a sub-base of at least 100mm thickness, and steel reinforcing mesh or a fibre-reinforced concrete.

Title: Re: concrete base for prefab garage
Post by Natedog on Mar 10^th, 2015, 11:27am

Ballast! That's the word I was looking for!

The ground is very firm (I know this having had to dig out and replace a sewerage pipe last year) there's about 4 inch of gravelly sandy stuff that is compacted (don't know if by machine or by time) then 8 to 10 inch of dense clay, then more clay with a lot of very large lumps of stone in.

If I go for 6 inch (150mm) of concrete with reinforcing mesh that should be ok then? I doubt it will ever have a car parked in it!

I will go for mixing it myself. Partly as I need the excercise and partly I actually enjoy it. Yes, I know I'm weird, but it's a nice change from my day job that isn't even slightly physical
Thanks guys. I'll update later in the year when it gets built.