12" thick concrete slab - sagging !

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Old 06-14-05, 09:48 AM
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12" thick concrete slab - sagging !

Our 40 year old house has a 10'x12' screened porch built upon a monstrous poured concrete slab. The slab is falling away from the house as the extreme end keeps going down about 1/2" every year. There is now a gap showing at the interface to the house, seen as you walk onto the porch looking down.
I want to rebuild the porch into a glass-enclosed sunroom.
However, I have 2 options:
1) Build an attached wooden floor with 2x12's that will go on top, BUT NOT RELY UPON, the concrete slab. This option would require some sort of intermediate support that may require breaking thru the concrete with a Hiltie Hammer to make 3 holes large enough for 4x4" verticle supports about 6 foot from the edge of the house-interface. Tough work.

2) Dig under the sagging edge of the slab enough to get 3 large floor jacks under there. Then, dig 3 deep holes, fill with gravel and concrete. This will then act as a "new" support for the sagging slab.

Which is better ? Do I have any other options ? i.e. jack-hammer the old concrete slab and remove it completely ?
 
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Old 06-14-05, 10:41 AM
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If it were me I would take option 4. Take the concrete out of the picture.It was not meant to support anything I would remove only the last 3 ft or so of the concrete and put in some real footings and then just add a beam and the joists . Attach all joists to the house with joist hangers lay the deck and then the GEPorch. I may double up on the joists on the sides just in case.you may have to remove all the concrete only if you need the height

I assume there are steps down to the concrete patio now.You will need like 2' or so. or you will have to use more joists of a smaller size and a wider beam. I don't think you need 2 support beams.


Is this a kit or are you buying the materials yourself? If it is a kit they will let you know what you need to do. Is the roof glass?
 
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Old 06-14-05, 07:05 PM
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Thanks, but...

the slab is exactly 9 1/2" below the French door's bottom (the door is the entrance to the porch). Thus, floor joists would need to be reduced to 10" lumber, otherwise the concrete floor must go entirely.
Also, the concrete foundation is exposed at the house boundary and thus would requite a header to be made and tap-conned into the foundation.
Given the plan is for a marble or porcelain-slate floor, I wonder if the 10" even if doubled-up would be enough to support this floor ?
Walls will be all glass/polycarbonate....ceiling plan calls for skylights in the existing roof, some fixed, a few motorized to allow hot air to escape.
 
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Old 06-14-05, 07:18 PM
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you may want to post in the tile forum for help with the base for the tile. the base for the tile will be like 1 1/2 " + the tile and thinset. Might need 2 by 8 like 12" on center or closer.
 
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Old 06-15-05, 10:08 AM
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Thanks...good idea !

and that sounds about right.....I'll have to reduce to 8" lumber to accomodate the 3/4" plywood and 1/2" rockboard, and wow, this floor is getting very "heavy" even before the tiling !
I wonder if I increase the length of the porch to 16' if even a doubled-up 8" lumber will be enough to prevent sagging.

Big problem: 12" centers will require cutting a TON of 24" insulation...a really ugly job. The floor must be warm. And I'm even considering using the new electric grids to keep it that way in the winter.
 
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