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Extending an existing slab

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  1. #1
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    Default Extending an existing slab

    I'm extend my front porch out about 1.8m. The existing porch is just a tad over 3m wide and I'm wondering if/how to connect the existing slab to the new one.
    I had thought of drilling horiz holes in the end of the existing slab and poking some reo in there or putting in some long dyna bolts and leaving them stick a 100mm or maybe some chemset bolts. Just to tye it ot the new slab. Not sure if I need to do this or if I;m on the right track.
    Anyone have any ides?

    Cheers

  2. #2
    Novice Dion N's Avatar
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    It depends on what is going to be sitting on your slab... Are you extending out your veranda or patio roof? Is your house brick veneer or timber clad?

    If you move the veranda posts out to the new slab to extend your roof, you'll have to think about the possibility of the two slabs sinking or rising at different rates unless the underslab compaction is exactly the same. Timber and steel frames can flex a bit, masonry doesn't flex. So if you intend having a structure that is connected to two different slabs, you want to be pretty sure that the two slabs are not going to move at different rates and cause excessive flexing of the structure.

    For a structural slab, I think Rebar chem-set into horizontal holes in the existing slab is the way to go, but I'm not a qualified strucutral engineer...
    "If something is really worth doing, it is worth doing badly." - GK Chesterton

  3. #3
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    The house is brick veneer and I will be putting a timber deck over both slabs.

    Just running battens and then some 90x19 Merbau decking

  4. #4
    Lumberlubber Bleedin Thumb's Avatar
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    If you're running a deck over why waste the money? Is it a height issue with the door threshold?

    Even if thats the case wouldn't it be the same amount of trouble to Dyna bolt a bearer plate to the front of the slab and run joists ontop to the same height of the battens?
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  5. #5
    Old Goat
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    1.8m is a substantial cantilever for any size joists. Without knowing the properties of the existing slab, it's hard to tell whether flexural continuity can be achieved. I'd guess the best function of the slab extension would be to provide a foundation for the battens and protect them from ground contact. Tying slabs together to avoid vertical misalignment is best done with reo for the new slab embedded about 200mm into epoxy-filled holes in the existing slab. A crack will likely develop along the top of the interface; to prevent corrosion of the reo, form a slot about 12mm x 12mm along the top edge of the new slab at the junction, thoroughly clean all surfaces of the slot, and fill with a flexible sealant such as silicone or polyurethane.

    Joe
    Of course truth is stranger than fiction.
    Fiction has to make sense. - Mark Twain

  6. #6
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    Thanks Joe,
    I'll get myself some reo today and start drilling.

    I originally planned to quickset a few stirups into the ground and put some bearers in but trying to get 3 to line up level with the existing slab I thought might be too hard so I decided to just put a slab down. That way i can just get the formwork all level and screed the concrete to suit.

  7. #7
    Old Goat
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    G'day, Tubby.
    If the existing slab slopes outward, just follow that slope with the new slab. If the old slab is level, set the outer edge about 25mm lower to assure drainage; You can shim the battens there to make everything level. Shim spacing can be about 4 times the depth of the battens.

    Joe
    Of course truth is stranger than fiction.
    Fiction has to make sense. - Mark Twain

  8. #8
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    All done. Thanks for the help guys, Now I'm just going to wait a week before I start fixing battens to it.
    Screeding concrete is a bastard of a job. I'm glad I did a small section first as I was going to do some more out the back at the same time. Lucky I didnt.

    Thanks again

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