Raft foundation for garage

Discussion in 'Builders' Talk' started by Halifax 1, Aug 30, 2005.

  1. Halifax 1

    Halifax 1 New Member

    I'm helping a neighbour out by drawing up plans for a double garage, double skin block construction size 6m wide x 7.2m deep.
    When doing previous digging out he has found that the ground is very rocky and so was thinking of using a shallow raft instead of traditional strip foundations.

    I appreaciate that raft foundations are a complex thing and should really be left to Structural Engineers, however is there a standard raft construction he could copy which would meet building controls requirements?
    I've found some standard details for a conservatory raft foundation on www.nrwas.org (national home improvement advisory service?) but i'm not sure if copying this would be sufficient for BCO or if they would still want calcs?

    Any help greatly appreaciated, if you need more info just ask.
  2. jakeofalltrades

    jakeofalltrades New Member

    It's been quite a while since i put in a raft
    At one time i did not no what the spec's were
    But the BCO did advise me so at the time I did not need
    Calcs etc.
    It may be diiferent now
    Ring the BCO, Don't tell them who you are etc, just that you want advice
    They are Always on about how their here to help
    So let them help.
    Having said that there will be someone on the forum,who will Know ,
    and if they don't
    they will Know a man who can.
  3. building control

    building control New Member

    or better yet tell the BCO where you are put the kettle on and he will tell you.

    I would suggest a 150mm - 200mm flat slab with 2 layers of A193 mesh, on a compacted stone sub base
  4. jakeofalltrades

    jakeofalltrades New Member

    It's only the fact that some bco's can be a right pain in the ar** even if and when your truthfull, and up front with them
    But I do find the one's that I have meet have been O.K
    I do urge caution though.
    Thre's no harm in sussing out the enemy first
  5. Halifax 1

    Halifax 1 New Member

    Thanks all for your comments so far.

    Have spoken to BCO who seems to be quite relaxed and i think will probably allow a flat slab.

    However i think it would probably be wise to thicken up the edges of the slab, but if i do this do i also need to get cage steelwork fabricated to reinforce?
  6. building control

    building control New Member

    so what did he say?
    just thicken out to 250 and lay another layer of mesh.
  7. yozman

    yozman New Member

    just finished my garage extension i had exact same problem
    bc told me to put 2 layers of mesh 6mm gauge and thicken raft up round edges i put 150mm base/concrete and dug an extra 150mm deeper round edges making it 300mm deep about 450mm wide didnt put any extra mesh in just 2 layers straight across bc inspected said ok hope this helps
  8. Halifax 1

    Halifax 1 New Member

    yozman - exactly the info i was after

    - many thanks to all for your advice.
  9. Caveman

    Caveman New Member

    Having just done the ery same thing, here's how it went -

    Dug out approx 12" deep footings where walls were going then skimmed off soil in between so we ended up with a 6" slab sat on the 6" footings, lotsa steel in the footings and slab. All seems to have gone well EXCEPT - part of the slab is below existing ground level so we put a layer of DPC and a layer of damp proof membrane onto the slab then block walls on top. What I've found is the water seeps through between the slab and the dpm anyway! - what we needed to do was seal the membranes to the slab and each other. We've fixed it by pouring SBR mix 1 against the joint between the dpm and the slab, all is well now.
  10. limestone cowboy

    limestone cowboy New Member

    Halifax, if your building is double skinned you can get round the damp problem experienced by Caveman by stepping the edge of the raft down 150mm. You need a flat ledge 150mm wide all the way round except where the doors are. You build the outer block off this, and the inner at the edge of the top of the slab, then any water that gets through the outer wall can't get at the inner. The only problem is that it makes the formwork more tricky but I would say it's worth the trouble. Also consider using brick to DPC level on the outer.

    I think you may need a stonger raft than others have suggested. The last double skinned garage I did on a raft was 6m x 3m internal, and the raft was 225 thick in the middle, 450mm round the edges (except the step mentioned above), with 2 layers of mesh which I think were A193 and some B mesh and some straight and cut and bent rebar as well. I had calcs, drawings, rebar schedule done for this which I can find if you are interested.
  11. building control

    building control New Member

    we build houses on a 200mm double reinforced flat slab.

    I think you are going for overkill?
  12. limestone cowboy

    limestone cowboy New Member

    I thought it was a bit OTT and I questioned it with the structural engineer but he said it was the minimum design for the loading. Once it was approved I couldn't change the design. It didn't cost any more than strip foundations because there was a lot less spoil to excavate and get rid of, and most of what came out was top soil which I sold.

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