Garage Foundations - General Advice

Discussion in 'Builders' Talk' started by Uncle Spud, May 11, 2010.

  1. Uncle Spud

    Uncle Spud New Member

    Hi All.....Appologies, this may seem a little long winded but just trying to give as much info as I can....

    I am in the process of clearing a site at the bottom of my garden on which I am going to build a single garage.

    Dimensions of which are 5.1m Long, 3.7m Wide, overall height of approx 3.2m (Pitched Roof). Just awaiting the confirmation of the planning application from the council.

    Due to the size of the buildings footprint, it falls outside the scope of building control and as such I am not getting much assistance from my BCO in terms of guidance with regards to foundations.

    I am proposing to use the trench foundation method, BCO has advised to go to a depth of 750mm which seems a standard depth so happy with that. Im planning on bringing the concrete to approx 150mm below ground height which I think is also a kind of standard from what I have read on the internet. This means the actual depth of the concrete will be 500mm deep.

    What I am struggling to find information on is how wide should the footings be??

    My Dad suggested 16 inches wide (hes not a builder though), that would give 6 inches either side of a concrete block, does that sound about right.

    Any thoughts, much appreciated??
  2. ian anderson

    ian anderson New Member

    Hi Spud,

    It all depends on the ground you see. It actually impossible to say 'how deep' should one go unless you are familiar with the soil once you get down there. Depths on a drawing will be an ideal, 'subject to approval'.

    Generally, it sounds daft I know, but it doesn't really matter what you build regarding foundations. A garage or an extension are going to be pretty similar because it is the load bearing structure of the soil that counts. Once you hit decent soil you can build anything on it. If you don't hit good soil then it is unlikely to cope with even the lightest loads.

    Thats why you often see little garden walls in trouble because people assume that you don't need to go very deep.

    IMHO all walls etc should be built on the load bearing subsoil, which around my old stamping grounds,(Newark, Nottinghamshire) is around the 1 meter mark, but always assess the soil once you start digging.

    RE the width, well what you are describing is often called trench fill' and sometimes this was narrowed down to 450mm (down from the 'standard' 600mm).

    In good soil a garage will generally be ok with 450mm wide trenches going down 2'6" to 3'. The trench fill certainly saves a bit of back ache!

    Of course you could always build off a raft.........

    Hope that helps, if not call back here.....
  3. ian anderson

    ian anderson New Member

    Of course I don't mean 'anything', I mean domestically..........Not a hotel etc.........
  4. ecm

    ecm New Member

    The width of your foundations will be dependant on the wall construction you use for your garage. What type of wall construction are you going to use? 9" solid, single skin, 100mm cavity etc.
  5. ian anderson

    ian anderson New Member

    True, I assumed that it would be single skin brickwork...?
  6. Uncle Spud

    Uncle Spud New Member

    Thank you all for your replies.

    Ian - Your assumption was correct. It is going to be a single skinned brick wall.

    Going back to your earlier comment regarding foundation depths, how will I be able to tell when I have come across decent soil as you put it.

    Ive only done minor works in the garden so far digging down a foot at most and to that depth it just seems like nice soil, no stones or anything.

    What should I be looking for in a soil to decide whether its sound enough to build on.

    Sorry if this is a dumb question, but this is all new territory to me.


  7. ian anderson

    ian anderson New Member

    You should notice the change once you get to the bottom of your topsoil. This can be from a few inches to 18 inches depending on where you are and how much the original developers messed about with the site!

    Difficult to advise as each area is different. But generally, the subsoil that sits underneath the topsoil will look and feel different to the soft blackish topsoil.

    I would take a look at this <u>leaflet about foundations etc</u> first then I would start your dig and go down about 750-1000mm and see if you hit a third subsoil layer. This third layer is usually the one to go for. If you hit clay then you want to be going at least a meter deep anyway.

    Where are, we have 12" of topsoil, then 18-24" of firm subsoil and then we are on straight ballast which is a sweet base to build off (600mm wide trenches tho)

    I am really surprised that the local building control officer can't give you a suitable depth, as they will have seen digs on just about every street going!

    It is largely common sense though, I am sure that you will be fine, you need to aim for firm underfoot and solid to the squeeze!

    Dont forget to keep the sides nice and plumb and its very important to make sure that the trench is square bottomed.
    This <u>pic shows a fair trench</u> (although steel is not always necessary)

    Hope that helps
  8. ian anderson

    ian anderson New Member

    just looking at the pic of a trench again, you can really see the subsoil under the top soil and then again you can see the darker/damper looking 'third' level in the bottom few inches of trench.

    One other thing. Try not to prolong the dig, i.e. leaving open/exposed trenches. Its best to get them dug and concreted soonish. Open trenches can both dry out and flood dep on weather!

    You want the concrete in place within a day or three of hitting the bottom! (no pressure then!)
  9. doing a bit

    doing a bit New Member

    you need a strip foundation , say 450mm wide , i wouldnt trench fill, it puts the concrete too close to the surface, plus the cost of concrete and the restraints of getting it to the site, if you have any. my local bco advised at least 600mm deep trench on sandy soil and 1000mm on clay , oh and 150mm thick concrete. i built a extension on a house with sandy soil and 600mm foundations, but the rest of the house had no foundations except a small brick one and it was only 200mm below the ground level. this was a 1930's house

Share This Page