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  1. #1
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    Default What do I do here? (moved)

    Hi all,
    Hoping people can give me some suggestions on slab set down / drainage beside a slab.

    Background:
    I'm planning a reno that includes demolishing a timber framed floor, and then rebuilding on a slab that is approx 650mm below the existing floor level.
    I've tried to draw it to make more sense.

    The building is built about 100mm from the side boundary, and at some point over the last 90 years the neighbour has built a wall between the brick piers of my place, and created bit of a retaining wall that extends about 100mm under my place! We have no access down this side of the house, so have never been too concerned.

    What I want to do is build the reno so that the side wall stays as close to the boundary as it is now. The wall will be a timber framed weatherboard wall. When I build it on the new slab, it will be about 2-300 mm below the neighbours yard.

    So the first thing I'll have to do is move the 'retaining wall' back to the boundary where it belongs. but then......

    What sort of distance do I need from the retaining wall to the wall of the house, and should I widen the slab to build the retaining wall on it? if so, should I have an extra set down for the retaining wall?

    Any thoughts/ideas much appreciated!
    Cheers,
    Jim
    Last edited by jimbo_jones; 18th Aug 2008 at 09:28 AM. Reason: clarity

  2. #2
    Senior Member Batpig's Avatar
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    Dear Jim,

    Don't build the retaining wall on the boundary, because for starters - your drainage would have to go into his property. Smartest thing you could do by far would be to build the retaining wall along the edge of the proposed extension, so that the retaining wall ends up as part of the new wall for the extension. It will have to be properly "tanked" for waterproofing purposes, but once built, you can just use shorter wall frames that sit on top of the retaining wall, rather than longer ones that sit beside it, if you get my drift. Also, you wont end up with an odd little very-narrow alley between the retaining wall and your new wall frames that you wont be able to get into anyway in order to do any maintenance. Your builder and/or his engineer will know what to do with the slab to get it to behave like a retaining wall footing; bugger all I'd expect given the very low height of the soil to be retained.

    Best Wishes,
    Batpig.

  3. #3
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    Thanks Batpig,
    That makes a lot of sense. Do you just use brick for the tanked part of the wall, or is there a better way?

    Thanks again,
    Jim

  4. #4
    Senior Member Batpig's Avatar
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    Dear Jimbo,

    Whilst it would indeed be possible to construct the Retaining-Wall in brick, it would not however be practical because it would have to include a reinforced grout cavity between two leaves of brick, which would end up being thicker than just a single leaf of reinforced concrete block...

    So the only reason you might go for the brick option would be perhaps if the external walls of the extension were either Brick Veneer or Cavity Brick, and due to the fall of the land, the use of brick for the Retaining Wall therefore allowed a seamless transition - from a "number and height of courses" point of view - between the retaining, and the non-retaining, sections of the wall, that would be a fair bit harder to do if you were instead using brick over and block to retain. But since your walls will be Weatherboard, this wont apply to you - so go for reinforced concrete block. Don't be tempted to not reinforce it either - you wont be able to get at it to replace it in the event of some kind of failure if it's unreinforced, and the reo will help with the waterproofing situation by minimising cracking anyway. Line up the inside face of the block with the inside edge of your studs, so that your internal finish (VJ's or Plasterboard, etc) will run neatly down past the blockwork to the floor.

    Once again, make sure you waterproof properly.

    Best Wishes,
    Batpig.

  5. #5
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    Could you form up the tanked part of the wall and pour it as part of the slab, instead of the reinforced concrete blocks? Sorry for all of the newby questions. Cheers,
    Jim

  6. #6
    Senior Member Batpig's Avatar
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    Quote Originally Posted by jimbo_jones View Post
    Could you form up the tanked part of the wall and pour it as part of the slab, instead of the reinforced concrete blocks?
    Theoretically you could Jimbo, but I'd be surprised if it was anywhere near as practical to do as separate blockwork on top of a concrete footing. Otherwise you'd see a lot of concrete "stemmed" Retaining Walls out and about - and the fact of the matter is, you just don't see any at all...

    What I have seen are concrete "upstands" that effectively retain soil around the edge of houses that have been restumped and raised, and then concreted-in underneath. Such upstands, however, have been practical to do in those cases because the formwork for them has been fixed to the side of the house's existing concrete stumps. You wont have the luxury of any existing stumps in your case - the formwork for yours would instead all have to be propped from the ground if you were trying to do the thing in a single pour in order to eliminate an extra trade. Don't even think about keeping your brick piers so that you could fix some formwork to them - that would truly be a case of "the tail wagging the dog..." And in your case, since you will still need to waterproof the rear-face of whatever you come up with, you will therefore have to form a rear face to the stem - out and away from the soil face. That means that you will have to set up both front and rear formwork - both propped from the ground - in order to construct the wall in concrete. This just wouldn't be practical or cost-effective at all...

    No, methinks blockwork will still be your best solution...

    Best Wishes,
    Batpig.

  7. #7
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    Thanks Batpig,
    I really appreciate the advice.

    Yeah - I hadn't really thought about how I was going to build the formwork for that wall.

    I'll go away now and check out the blocks.

    thanks again,
    Jim

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