I need was wondering how deep footings have to be when adding an addition onto a house that sits on a slab?
Answers:
You need to check with the local building code authority. The depth of footing depends on what the frost depth is for your area. If the footing isn't below the frost line and the soil freezes your foundation can heave. Trust me this is bad.
If the rest of your house is on a slab, you can pour a monolithic footing/slab to save forming costs and do it in a single pour to save money.
Other answers:
That depends on your local building codes. Generally they must be below the frost line, in STL that is 30". But that is different in other areas. Some warmer climates just require solid soil depth. Just check with the local authorities.
That depends on your local building codes. Generally they must be below the frost line, in STL that is 30". But that is different in other areas. Some warmer climates just require solid soil depth. Just check with the local authorities.