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    • CommentAuthordereke
    • CommentTimeJun 12th 2017
    I am planning to build an office in my garden using one of these SIP kits you can get.
    The ground here is clay (about 300mm below ground level). I've read that a raft foundation is suitable for this kind of soil but this seems a bit overkill for a 4x3m wooden structure.

    Does anyone have any experience with this kind of situation? What (non concrete or at least masses of it) options do I have?

    • CommentTimeJun 12th 2017
    'Clay' covers a multitude of sins. Ideally you would know how expansive it was, which depends on how pure the clay is and what type of clay it is. I expect there are articles on the intertubes explaining how to estimate these if you don't have a soil survey to hand.

    It depends a bit on what your requirements are for the office. Conventional wisdom says you need to remove all the topsoil (is that the 300 mm?) before anything else. A raft will certainly work, as long as you don't mind your building moving around a bit if there is any heave, but as you say, it may be a bit OTT.

    All I did for my garden shed was to take the surface off to make a level surface, then put a layer of sand as blinding and some paving slabs I happened to have as a 'foundation'. Then some fence posts laid horizontally and then the floor of the shed. The floor is a bit wobbly anyway since it's a cheap shed but if the ground does move (it's right under an oak) then I expect I'll need to bash a few wedges in to even things up.

    I presume a SIP kit is a bit more expensive so you might want to take it more seriously. What is the floor stucture - how is it supported?
    First the health warning - I am not a structural engineer - having said that a raft foundation covers a multitude of sins, pick one suitable for the occasion.
    As an office what sort of floor do you expect to put in? Presumably as an office it will need to be an all year around usage (=heating). What about a ring beam of suitable dimensions with insulation in the centre and 100mm - 120mm of concrete (with rebar mesh) over cast with the ring beam, a stable (I would say strong and stable but that is now a tainted phrase) structure on which you can put the floor of your choice.
    For how long do you expect to use the structure. This could/will determine how much you are willing to spend on a semi-permanent / short term (delete as appropriate) structure
    • CommentAuthordereke
    • CommentTimeJun 12th 2017
    I've no idea what kind of clay it is unfortunately. Yes 300mm of top soil then clay.

    The floor structure is a SIP so they are 150mm thick and I am pretty sure I need to keep them dry as well.

    I'm spending about £4k on the building + foundations so I'd prefer something that won't be err weak and wobbly.

    I've found a couple of different methods which sound like they might do the trick:

    ProBase sounds a bit like a fancy version of @djh paving stones: http://www.shedbase.com/probase/
    Swift Plinth sounds like a bit more work but also more forgiving (you can adjust it later) https://swiftfoundations.co.uk/swift-plinth-plus/

    I really don't want to do a big concrete raft!
    I have a SIP kit garden room, or supplier recommended supports. Does yours do the same?

    In the end ours has been constructed like decking, we have 4x4 posts sunk vertically into concrete and then the frame built suspended of the ground.
    How about a local steel fabricator to make you something like this:

    • CommentAuthordereke
    • CommentTimeJun 13th 2017
    @richarddelliot the supplier I am in discussions with doesn't seem to want to tell me what kind of foundations to use!

    I was thinking of doing piles similar to that but my concern is the clay.

    @Nick Parsons the floor is already structural. What would be the benefit of a steel frame? I actually have done a conservatory like this before and we fixed it to concrete piles - but again the clay question - is that appropriate?
    • CommentTimeJun 13th 2017 edited
    Will the floor span from edge to edge? i.e. could be installed clear of the (exposed) clay surface? then all you need is suitably supported edge/ground beams. How about a farmer sinking some gate posts (with his tractor mounted auger) as piles? Railway sleepers (or longer/heavier 'crossing sleepers') as ground beams.
    • CommentTimeJun 13th 2017
    I think you need to resolve your concern about the clay. The best solution is a soil survey but they cost money. You can do quite a lot yourself, though. Check how much clay there is in the soil (sausage test and/or suspending the soil in a jar). Check the expansivity ( http://www.engineeringcivil.com/determine-free-swell-index-of-soil.html ). Check drainage. How long have you had the site? If it's a reasonably long time, have you seen any signs of movement?

    What does the supplier say about foundations if they don't want to design them? Are there alternative suppliers?

    I like the look of the Swift Plinth system but how tall is it? If you take out 300 mm of topsoil and you want to keep the timber (I'm assuming the SIP has a timber shell?) the usual 150 mm above the ground, you'll need to fill the gap somehow. Hardcore in the hole is one option, I suppose.

    I don't like timber posts set in concrete. They're guaranteed to fail at the junction after a while.
    • CommentAuthorringi
    • CommentTimeJun 15th 2017
    Google "conservatory foundations screwpile" and see if any of the systems will work for you.
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