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Green Building Bible, Fourth Edition
Green Building Bible, fourth edition (both books)
These two books are the perfect starting place to help you get to grips with one of the most vitally important aspects of our society - our homes and living environment.

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    • CommentAuthorzak99
    • CommentTimeMar 4th 2019 edited
     
    Hi,

    Looking to do a small side extension on the side of a 70's bungalow as part of other works going on there (makeover and loft). Its a brick n block cavity wall, and is all on a concrete raft. Will be looking to open up the load bearing wall between the existing and new with rsj etc.

    Assuming the extension was in timber frame, timber clad, probable flat roof, what are my options please for foundation. I understand (but welcome advice on) that I have to somehow tie together yet allow for movement between old (on the huge raft) and new extension.

    Thank you
    •  
      CommentAuthordjh
    • CommentTimeMar 5th 2019
     
    Posted By: zak99Its a brick n block cavity wall, and is all on a concrete raft

    How is the raft supported? e.g. are there foundations underneath it? You're sure it is a raft and not just a slab? All the walls are on the raft?

    If it is a raft from the 70s, then I think it almost certainly means the ground underneath is unstable. Possibly trees and/or clay heave risk or subsidence risk if you're in a mining area. I think you need to establish exactly what ground conditions you have - perhaps from old records or maybe a survey. Then you'll need a local engineer to design your foundations for the extension.
    • CommentAuthorzak99
    • CommentTimeMar 5th 2019 edited
     
    Hi DJH,

    Sorry maybe my terminology and understanding is wrong. My internal floors are all concrete with a lighter screed with plumbing pipes in. All the walls are on the concrete slab/raft or whatever it is called. (edit Mmm I think, but not sure now!!) Outside the top of concrete slab/raft is about 8 inches above ground and the bottom disappears into the ground. (edit actually I cant see the top of the slab outside, just the brickwork down to ground, but the door thresholds are a good 8 inches above ground) There are large trees fairly close and an open ditch running along one side of house about 20ft away. Years back there was also a large tree 30 ft to the front, now gone.

    (Theres also 10 what are now very large conifers within 25ft that the water board are just about to fully cut down, which is bit of a worry.)

    I did ask building control and they said they had no records.

    Can I ask who should I look for to do a survey and what sort of engineer should I be looking for also please?

    Thanks for the helpful advice as always
    •  
      CommentAuthordjh
    • CommentTimeMar 5th 2019
     
    Posted By: zak99Outside the top of concrete slab/raft is about 8 inches above ground and the bottom disappears into the ground. (edit actually I cant see the top of the slab outside, just the brickwork down to ground, but the door thresholds are a good 8 inches above ground)

    Hmm, that makes it sound like perhaps the outer wall is not on the slab/raft. I think it would be worth digging a hole alongside the wall to see what is below ground.

    A raft is basically a slab, usually reinforced with concrete, that is designed to stay whole and not break no matter where it is supported or not supported. As I said, they're usually specified where the ground is unstable. Slabs are a bit thinner and with less reinforcement and are designed to rely on good support from the underlying ground.

    If you have builders doing other work, it might be worth asking their opinion. They might also be able to point you at local firms who do soil surveys or structural engineering.
    • CommentAuthorphiledge
    • CommentTimeMar 5th 2019
     
    I think youll need to do your investigative digging on the inside. If you dig down at the outside the edge of strip foundations can look the same as the edge of a raft.

    Got the t shirt for that mistake when a builder said we were on a raft after exposing the outside and we paid alot of money for an extension raft to be designed. When we started work it turned out we had shallow strip foundations!
    • CommentAuthorzak99
    • CommentTimeMar 11th 2019
     
    Thanks for replies. Ew yuk, digging inside sounds fun, especially as the inside seems to be 8 inches of concrete higher that ground level outside. I guess I have to get through all that before even nearing slab/raft.

    Spoke with Building control today and he was saying if it is slab/raft it needs to have steel reinforcing inside to be suitable for additional load. But on the other hand he was saying I should dig outside. Confused.
    •  
      CommentAuthordjh
    • CommentTimeMar 11th 2019
     
    I would start by digging on the outside. You may get enough information from that. If you're unlucky like Phil then you may need to get a jackhammer out (or maybe a large drill?) and start inside as well. How do you know it's 8" of concrete by the way? Rather than say 4" of concrete on top of some hardcore.
    • CommentAuthorphiledge
    • CommentTimeMar 11th 2019
     
    Not that we built off a raft, but the design not only incorporated rebar in the new raft, but the rebar had to be tied into the existing slab to stop differential movement. There was going to be alot of drilling into the edge of the existing raft to bond the new rebar to it- glad we never made it that far!

    If you speak to the building control officer for your area he should have a good idea about the local ground conditions and give you a clue as to the type of foundations they will be expecting, unless one of the invertebrate types!
    • CommentAuthorphiledge
    • CommentTimeMar 11th 2019
     
    From earlier posts I took it that internal floor level was 8" above ground level rather than it being an 8" floor slab??
    • CommentAuthorMike1
    • CommentTimeMar 12th 2019 edited
     
    To save the internal floor, dig down externally until you hit concrete - which will either be the top of a strip foundation or the edge of the slab - then cut out the brick / blockwork and follow the concrete through the wall and beyond.

    A strip foundation will normally extend roughly (±100mm-ish) the same distance on either side of the wall, so if you follow the concrete back and it ends roughly where expected, you probably have a strip foundation, and you will probably be able to find an obviously separate floor slab higher up, cast on stone of some kind. If the concrete keeps going horizontally (or steps / slopes up and then continues horizontally) well beyond where you'd expect to find the edge of the strip the chances are you have a raft. Be aware that you may damage a damp proof membrane and that this would need patching as part of any reinstatement.
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