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Groundwork & Foundations

Double garage foundation query

I'm building a 7 x 7m garage. I was making a cavity wall and was advised to build the foundations 600mm wide and 800mm deep using an ST3 concrete. I'm now doing a single block with piers.

Can anyone suggest what width and depth foundation I should use, I'm presuming it will be smaller. Also how thick should my hardcore be for the oversite?

10 Answers from MyBuilder Groundworkers

Best Answer

Hi Edwin,
If your garage is to be detached from the house, you dont have to have a seperate footing. It can be built on a reinforced concrete slab, form a shutter the size of you garage, put in 6" of crushed brick hardcore, blind with 2" of soft sand, lay a dpm of polythene over the sand and pour 4" of concrete over , maybe dish the edges so the conctete is a bit thicker where the walls go. Then you have killed 2 birds with one stone. slab and footings in one pour.


Answered 17th Feb 2011

Hi Edwin,
How can a simple question get 4 seperate answers ?
The truth is the question does not tell the whole story. Do you have trees or a hawthorn hedge neer to where you plan your garage. Do you have plans for the original garage design, Is the finished floor slab of the garage to be higher than the present ground height or are you going to remove the soil from the area. many questions i know but all these things will play a part in answering your initial question. However i can answer one part. 450mm is the answer for the width. You will need piers to strenghen the wall and the brick work needs to be placed along the center line of the concrete footing. The higher the concrete is in the footing, the less brick work you will need to get to floor slab level. Also the best price for concrete is based on a full 6m3 load. Mini mix charges are very expensive in comparison. Ring local suppliers and ask for rates. You'll find it suprising. The lorry and transport costs are roughly the same so have to be pro rata more expensive on a smaller mini mix wagon. One other thing on a 7m2 garage floor, i would advise the use of sheet re inforcing mesh in the floor slab. This will help prevent cracking and get concrete with fibres added.
Any questions or further clarification of your circumstances please feel free to email.
Dan Wade ACIOB


Answered 17th Feb 2011

HI there,

I would pour a 600mm x 200mm strip found, and you could add a layer of reinforcement mesh if need be. With regards to your hardcore 150mm well compacted would be sufficient with a 50mm sand covering to prevent any type 1 puncturing your damp proof membrane. Hope this helps


Answered 17th Feb 2011

minimum depth of footing is 150mm dependant on type of ground. width is usually thickness of brickwork plus 75 mm either side so in your case the thickness of the pillars plus 150mm.
minimum 150mm oversite and use 1200 gauge membrane and dont bother with a142 steel, just ask for fibres in the concrete for reinforcement.


Answered 7th Mar 2011

Dear Sir,
You will need to get it passed by Building Control, so please do not guess what you need to do, if in doubt have Building Regs prepared better safe then sorry.
It costs double or more if you have to take down and build.


Answered 17th Feb 2011

Hi. 450 wide concrete sufficiant but you must build on the centre . As for depth you need to be at least 1m deep from ground level for freeze protection as concrete will always contain some water and can fail in extreme cold. You do not however have to fill this all with concrete a minimum of.300mm is ok but obviously the more you fil the less below ground blockwork you need its a cost exersise realy , if you need to employ a bricklayer its cheaper to concrete but if you are doing the work it is cheaper to use more blocks. As for hardcore depth 150mm minimum,oh and put a sheet of A142 mesh in the top part of your floor slab for anti crack


Answered 18th Feb 2011

hi edwin i would reduce the width to 300mm. This will be more than enough. This will also reduce your concrete bill by half . With regards to your hardcore it really needs to be at least 4 inch thick.


Answered 17th Feb 2011

Footings always depend on your type of soil, whether the soil has been disturbed before or not. I would go 450 x 450 which gives you enough width for any piers needed on a single skin construction. As for the concrete, you can use mix-a-mate truck which mixes on site and you only use what you need so no waste. You might need a hand though so as one does the donkey work the other can be levelling and tamping.


Answered 19th Feb 2011

hi,, the sujested width is correct,, but please make sure that the wall is placed in the centre ,, the concrete needs to be 6" and 9" on the run in ie where the door starts,, this to be laid on 6" of compcted type 1 road stone , @ 2" layers dont forget the waterproof membrane....


Answered 17th Feb 2011

put in a slab 4'' in the centre and at the edges 4000mm wide by 6'' deep with reinforcing mesh on a good compacted base slab and footings all in one


Answered 18th Feb 2011

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