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We are wanting to use a structural ridge beam to put a new roof on our house. I have been talking with an engineer who said that we need to determine how big the footer is under our slab. However, when I dug out under the slab I discovered that there is not footer - it is a floating slab. We need some way to reinforce the slab to support the weight of the upright posts and roof, but we can't just pour piers that would go below the frost line because it would move separately from the rest of the slab. Is there any other way to reinforce the slab to hold the weight of the roof?

-EDIT-

Apparently I used incorrect terminology. What we have is a slab that appears to have no footer whatsoever. I thought that was called a "floating slab" but apparently a floating slab still has a footer. It appears that we just have a 4-6 inch slab of concrete sitting on top of hard clay. There is no footer at all visible at the perimeter of the slab. I dug down 2 feet and dug 1 foot under the edge and there is nothing there.

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  • I'm not sure your terminology is incorrect - simply a matter of several different things, varying with time and region, all of which are, one way or another, a floating slab. The main issue you need to find out about the slab you have is whether it was done with adequate reenforcing steel for your proposed loads, or not. Unfortunately, that's a bit hard to tell after the fact. It's been carrying the (wall-distributed) load of your house since it was built, which is a good sign that there's at least some reenforcing in there. – Ecnerwal Jul 23 '14 at 17:16
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First of all, the ground directly under the center of your house is unlikely to freeze, because the house is sitting on top of it protecting it from direct exposure to the cold. Freezing under the foundation is mainly a concern at the perimeter.

In order to support the ridge beam of your roof, which means the weight of the entire roof, you definitely need to carry that load all the way from the beam down to a footer.

So - directly below where you want the support to connect to the beam, using a plum line mark the foundation. Cut through the foundation and down to a depth of at least 2 feet (whatever code requires). Install a new footer in the ground and level the new concrete with the level of the slab.

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    Ridge beam is only half the weight of the roof - the other half is supported by the walls. – Ecnerwal Jul 23 '14 at 14:17
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    I'm a fan of over-engineering. ;) – The Evil Greebo Jul 23 '14 at 14:26
  • Thanks for the feedback. Please see my edit in the question. Two of the posts would set right on the edge of the slab (just inside the outside walls). The issue is that we can't just pour a footer and tie it in with the existing slab because it will cause it to crack. – BWDesign Jul 23 '14 at 15:43
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Difficult to be sure, but it will depend in part on what your "floating slab" construction is - around here, that's a "thick-edged" reinforced construction and would typically support the load - the edge of the slab is thicker than the center, and considerable reinforcing steel is placed to transfer loads so that the whole slab supports the structure - the entire thing is "the footer" and it results in very low ground pressure as a result. Ours are also insulated to deal with "not being very deep" in country where the frost goes 4 feet down - between insulation and heating the building, they don't freeze.

If you cannot find details about how your slab is constructed (make a significant effort to find that information), you may need to have an engineer investigate it to get some idea of what it might support. Or you may decide that sticking with a trussed roof that does not require central support is the better option, all costs considered.

As a generic approach, you could potentially build a structure on top of the slab to spread the column load, if the slab reinforcement is not, itself, up to that task - that will of course be "architecturally inconvenient" unless it's something like a steel beam that you can replace the bottom of a wall with so it's hidden. Your engineer would need to size that structure. In new construction that would simply be built into the slab with extra reinforcement and possibly a thicker part of the slab - that is hard to retrofit in an economically sensible fashion.

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  • Please see my edit to the question. Apparently I misunderstood what a "floating slab" is. Our slab has no footer at all that I can find. – BWDesign Jul 23 '14 at 15:44
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Properly supported loads are supported by foundations. A slab is not a foundation. A footing is a very simple type of minimal foundation.

  1. Using sound engineering practice calculate the required loads per the Building Code. These include but are not limited to: dead, live, snow, seismic, and wind loads.

  2. Determine the bearing capacity of the soil. Consult a geotechnical engineer if necessary.

  3. Using sound engineering practice determine the size and location of the footing as well as the design of all connections between components in accord with the loads.

  4. Saw cut the existing slab and excavate for the new footing.

  5. Drilling and epoxying dowels into the existing slab is a common way of monolithically joining a new footing to existing flatwork.

Construction is messy and expensive. Using buildings no more qualifies one to build than using the web qualifies one to write a shortest path algorithm.

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  • I don't think you understand my question. I am not asking what a footer does or how to add one under my slab. I am asking is there any way to pour piers within the slab that won't cause the slab to crack because the slab is not anchored below the frost line. All I want to do is support a ridge beam for the roof - not tear half the house down to add a footer under the slab. – BWDesign Jul 23 '14 at 16:43
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    I understand the question. After more than two decades in AEC, I've seen the idea a few times...ok having once worked as a building plans examiner more than a few. Anyway, piling concrete atop a slab never doth a foundation make. Concrete saws are among the standard tools of construction. Home Depot rents them. My recommendation is don't change the structural system if installing foundations is problematic. – user23752 Jul 23 '14 at 18:21

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