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Building Regulations and Standards: garage foundations

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      CommentAuthordjh
    • CommentTimeMay 25th 2013
     
    So our foundations are awkward since we have both clay and trees. It's a pain for the house but I understand the reasoning. But I'm having trouble understanding the garage. It is a detached garage and is over the size for permitted development, so we are subject to building regs. For foundations that seems to mean NHBC standard, and they don't make any difference between a house and a garage. Even if the garage is just basically a timber shed.

    So we're looking at 2.5 m deep strip or trench foundations with a suspended concrete slab, or a raft with 1.25 m hardcore underneath it, or piles or .... Now that's an awful lot of concrete, muck away and money.

    I'm sure that will give me a garage that doesn't move, but why should I care? Even more, since the drive outside it *will* move, why would I want a garage that doesn't? Can anybody point me to somewhere that explains the rationale for what appears to be idiocy? Or better yet, tell me how to avoid it.
    • CommentAuthortony
    • CommentTimeMay 25th 2013
     
    how about building it on a raft?
    • CommentAuthorEd Davies
    • CommentTimeMay 25th 2013
     
    Posted By: djhand is over the size for permitted development, so we are subject to building regs.
    What's PD got to do with building regs? Genuine question - building regulations apply to some things which are permitted development and some things which require planning permission aren't covered by building regs so what linkage do you have in mind?
    • CommentAuthortony
    • CommentTimeMay 25th 2013
     
    How near to the boundary are you with it?
    •  
      CommentAuthordjh
    • CommentTimeMay 25th 2013
     
    Posted By: Ed Davies
    Posted By: djhand is over the size for permitted development, so we are subject to building regs.
    What's PD got to do with building regs? Genuine question - building regulations apply to some things which are permitted development and some things which require planning permission aren't covered by building regs so what linkage do you have in mind?

    Sorry, you're quite right. I got sloppy, there's no connection. There are two facts: we are subject to building regs, because the floor area is too big. We are subject to planning, because of the position, but we have that permission. Sorry for the confusion.
    •  
      CommentAuthordjh
    • CommentTimeMay 25th 2013
     
    Posted By: tonyhow about building it on a raft?

    That's the current theory. The 1.25 m hole to be dug and filled with hardcore is my problem, and because of NHBC rules, the hole has to be over twice the size of the garage.
    Posted By: tonyHow near to the boundary are you with it?

    Pretty close. About 0.5 m. That's another problem.
    • CommentAuthorJonti
    • CommentTimeMay 25th 2013
     
    djh,

    if it is a timber garage then a Tony asks, what about a raft? I would not think building regs will demand a certain type of build but rather that it is proven that the foundations meet the stipulated standards.

    Jonti
    •  
      CommentAuthordjh
    • CommentTimeMay 25th 2013
     
    Posted By: Jontiif it is a timber garage then a Tony asks, what about a raft?

    I thought I already answered that?
    • CommentAuthortony
    • CommentTimeMay 25th 2013
     
    I see no difference between timber or masonry on a raft may be different design of raft, some houses are built on rafts
    •  
      CommentAuthordjh
    • CommentTimeMay 25th 2013
     
    Posted By: tonyI see no difference between timber or masonry on a raft may be different design of raft, some houses are built on rafts

    Right, neither does NHBC. The difference to my mind is that if you tilt a timber frame building, it should still be standing if it's been properly braced, but masonry will topple. There's no question a raft is adequate; it's the volume of the hole that's required underneath it that is the problem.

    Does anybody have any other suggestions, or a link to the rationale for these standards?
    • CommentAuthorJonti
    • CommentTimeMay 26th 2013 edited
     
    Posted By: djh
    Posted By: Jontiif it is a timber garage then a Tony asks, what about a raft?

    I thought I already answered that?


    Started my reply before you posted yours to Tony so did not see yours. BRs are based on calculations of required standards as far as I am aware. If you can prove how you wish to build meets this then it has to be accepted.

    Jonti
    •  
      CommentAuthordjh
    • CommentTimeMay 26th 2013
     
    So the requirements stated in the Approved Document for part A say:

    Loading

    A1. (1) The building shall be constructed so that the combined dead,
    imposed and wind loads are sustained and transmitted by it to the
    ground:
    (a) safely; and
    (b) without causing such deflection or deformation of any
    part of the building, or such movement of the ground, as will impair
    the stability of any part of another building.
    (2) In assessing whether a building complies with sub paragraph
    (1) regard shall be had to the imposed and wind loads to which it is
    likely to be subjected in the ordinary course of its use for the purpose
    for which it is intended.

    Ground movement

    A2. The building shall be constructed so that ground movement
    caused by:
    (a) swelling, shrinkage or freezing of the subsoil; or
    (b) land-slip or subsidence (other than subsidence arising
    from shrinkage, in so far as the risk can be reasonably foreseen), will
    not impair the stability of any part of the building.

    As far as I can see there is no requirement there that the building shall not move. I can't see anything in the guidance either. I can see that it makes some sense for a house, but where does the idea that this is a reasonable goal for a detached garage come from?
  13.  
    I would have thought a decent slab with some rebar mesh thrown in would comply with the regs as quoted by djh especially if the wooden garage is bolted to the slab to stop it blowing away!!! If you use a light weight roof then I would have thought that a good sized car has a chance of weighing more than the building! (I am assuming that the garage is planned to be timber and light weight)
    • CommentAuthorPaulJ
    • CommentTimeMay 26th 2013
     
    The 2 issues are building regs and NHBC.

    It may be that NHBC are doing warranty and regs. Request to them that the garage is excluded from warranty cover, which they will agree to. It won't hurt resale and may reduce premium.

    WRT b regs, they have a square area beneath which the building is excluded. Nothing to stop you building another building of the same max size next to it, with a 50mm gap. Knock through at your leisure. Really, b regs are not that interested in outbuildings. Don't waste concrete. Do a slab with mesh or beam and block on a 1m deep strip footing.
    • CommentAuthorGotanewlife
    • CommentTimeMay 27th 2013 edited
     
    Less thickness of concrete, more mesh/rebar = less time time and hard work, meanwhile why do you need a 1m footing, seems excessive to me, a rough 30cm footing with 10cm slap if on well compacted hardcore that is blinded with sand or 15cm (max) straight onto hardcore. You guys have got to be more Italian about these things!

    Obviously fully Italian wouldn't bother with the strip footing at all and I would have to stand over them to ensure sufficient metal went in....except that I would do it myself.
    • CommentAuthorSteveZ
    • CommentTimeMay 27th 2013
     
    Hi djh - not that this is any consolation but it might make you feel that they are not picking on you. When I built my 20' x22' garage in Swindon, I assumed a strip foundation with a reinforced 6" slab would do nicely. Having ordered the sectional garage, I was pretty surprised to be told by BC that this lightweight construction needed the same foundations as a two storey house - because it was over the normal size for a garage!! Sound familiar? Had I realised earlier, I would have built a proper block garage and then not had all the leaks around the baseplate that the (well known) sectional building gave me - DOH! Still get annoyed about their stupid sealing method even after all these years - internal mortar flaunching where the wall meets the floor, if you're curious.
    •  
      CommentAuthordjh
    • CommentTimeMay 27th 2013
     
    Thanks all.

    NHBC are not providing a warranty, but their standard gets thrown around by people when discussing foundations. Regs are interested in outbuildings, except when they are excluded, usually by reasons of the small size exceptions. e.g. garages under 30 m² internal floor area are excluded. Mine is larger because it's a double and Lifetime Homes specifies minimum size over that. It isn't next to the house, so there are no considerations about knocking through.

    SteveZ's experience sounds all too familiar. I'm planning a timber garage with a metal roof, so it should be fairly light and strong in the sense of coping with movement.

    It sounds like I need to have a chat the the BCO and see what he will accept. I was just hoping to find some additional background first.

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