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Moving slab

tubby | Posted in General Discussion on March 13, 2009 06:26am

All,

I’ve got a cabin thats built on a floating slab. There’s a porch over the door that’s on it’s own slab(see pic). The slab under the cabin won’t move as it’s a heated slab so I’m not concerned about it freezing under there. The slab for the porch is a different story though.It tends to rise in the winter and settle in the spring. The problem is the posts holding up the porch sit on the slab so this causes the porch to move up and down an inch or so. I’d like to stop this movement so I have a couple of ideas.

1. Inexpensive fix – insulate around the edge of the porch slab and try to get 2″pink foam down 2-3 feet around the perimeter to hopefully prevent it from freezing underneath due to the heat from the cabin slab.

2. more expensive fix – Brace porch roof up and jackhammer out slab, put tubes in below frost line (6′) pour new footings for posts holding up porch, then pour new porch slab.

3. Most expensive fix – Brace up porch roof, jackhammer out slab, dig complete footings below frost level and pour new footings for complete slab. 

I’m guessing option 2 is the best bet but I’d really like to be able to make option 1 work or get any other good ideas.

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Replies

  1. FastEddie | Mar 13, 2009 07:28pm | #1

    Sorry, I don't see a porch over the door.

    "Put your creed in your deed."   Emerson

    "When asked if you can do something, tell'em "Why certainly I can", then get busy and find a way to do it."  T. Roosevelt

  2. MikeSmith | Mar 13, 2009 07:28pm | #2

    #1 seems iffy...
    i like #2

    Mike Hussein Smith Rhode Island : Design / Build / Repair / Restore
    1. robert | Mar 14, 2009 06:38pm | #7

      I'd go # 2 also,

      Of course, When I was ripping out the patio under the deck on my last house to put in poured concrete patio...............................

      I wanted to isolate the existing deck footings from the slab.

      Two of the guys who bid the pour for me told me I was crazy and that 4" of stone, 4" of concrete and deck posts on the slab was "Just Fine".

      Needless to say neither did the job.

  3. brad805 | Mar 13, 2009 08:12pm | #3

    I am guessing the cabin is near the lake.  Do you have a granular soil or a clay soil? If you can excavate to a granular layer, you could effectively drain the subsoil causing the heave.   

    #1 can be an effective solution, but in addition to what you are thinking you need to extend the insulation away from the building horziontally.  A frost bulb can still get around the vertical insulation and heave the edge of the slab, but by adding horizontal insulation you will reduce this possibility.  How far and how much insulation depends where you are.

    If it were my cabin, and I could live with a bit of trial and error, I would try the easier fix first since the others are quite a bit more intense.

    If you do go with the guaranteed permanent solution, you might want to think about casting the slab as a suspended slab design.  Add a bit more rebar and span the slab from the exterior line to the existing foundation.  Put a void form below and this will isolate the slab from future heaving soil issues.  A benefit of this, is the slab will perform much better long term (less cracking).

    1. tubby | Mar 13, 2009 08:53pm | #4

      The cabin is in NE MN and the soil is very heavy, lots of clay. If I did option 1 I would also extend 2" insulation out 2-4' perpendicular to the slab. I already have the 2" out 4' perpendicular to the cabin so I could easily do it around the slab, maybe even go to 4" thick. I've got good drainage for the surface water but the ground up there is so thick and heavy that if I put foam perpendicular to it I can maybe steer more water away from underneath it.

  4. User avater
    Dam_inspector | Mar 13, 2009 10:30pm | #5

    Tear the porch off. Problem solved.

    1. Shoemaker1 | Mar 14, 2009 05:18pm | #6

      I live a heavy clay zone in western Canada. Try # 1 horizontal foam works like a charm. Deflects water and lessens the drying of the clay under the slab.
      was told by an enginner clay from dry to wet will expand 30%. when it expands it has 11 times the lateral force of ice. the basement repair guys are busy round here. 8 inch walls give like putty and block walls you can watch them move.

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