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.
Best Answer
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.