Foundations for a single skin garage

28 Jan 2011
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United Kingdom
I am planning on building a 3 m x 4 m single skin garage, i.e. within permitted development.

The ground is think clay - and i'm trying to work out what type of foundation I should put in and what depth/thickness. Its a detached garage so i'm not really bothered about damp, but would like it to be relatively dry.

Option 1

Raft Foundation - i was think of digging a trench an appropriate width around the perimeter and then dig the middle out 250 mm. I was then going to spread 150 mm hardcore over the middle and fill the entire thing in a single pour of concrete. This is only a rough idea and don't know required depth and width etc. - was thinking something like this...


Option 2

Trench fill foundation

Dig trench - appropriate depth and fill to within 2 courses of ground level. come out of trench with foundation blocks.

Im confused - whats easiest? Given that I have a drive way that i need the finished floor to finish level with.

Basically, whats the most straight forward way to establish foundations that are easy to build off.

I was considering a raft as its 2 birds with 1 stone and negates the need to lay the first course in a trench, even if its only a very shallow depth, i.e. from the top of the concrete
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Raft is fiddly, expensive and unnecessary unless ground conditions dictate. Strip foundation cheap (est) and simple.
thanks jeds

The thing is i'm not that confident laying blocks in a trench so was trying to avoid this hence the possibility of a trench fill foundation

but then i thought if i'm gonna do a trench fill, then build up a course or two then pour slab, might aswell pour the slab and strip together - i might be being naive about the work involved though.....
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Don't make the same mistakes as in the link, keep it simple.

As Jeds says do a strip footing and not a slab or raft or you will regret it.

Whilst I admire the noble effort the guy has put into his garage there are a few things that are either unnecessary and a few things that should have been done differently.
Can anyone suggest a suitable a depth? I was thinking that a width of 300 mm would suffice for single skin block work wall - if I were to do a trench fill foundation that is.

Thanks so far I appreciate your replies. Just out of interest why are monolithic slabs considered difficult to do or at least more difficult than trench foundations.
Bear in mind you will need piers which would push the width of wall out, would be more normal to go for 450mm wide and about 1m deep.

Also if you ever plan to put a car in the garage you may want to make it longer.. a ford focus is 4.4m long..

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