How to correct a poured slab with no footing
I have an existing patio slab adjacent to my house. How do I go about adding footings for a patio cover?Answer:
The answer is dependent on the type of footings you are trying to put in. If you are in an area that is subjected to freezing temperature, the footings will need to be below frost line anywhere from 30 to 48 inches below finished grade. You need to call your local building official to find out the requirements in your area.
If you need footings for posts, you can cut out portions of the slab where the post will be located, excavate down to the desired depth, install rebars and pour the footings. In some cases you may need to extend the footings upward to be just above the slab. This can be accomplished by using sonotubes or formwork. Sometimes it is cheaper to just pour solid concrete all the way to the top of slab. This will save a lot of labor for doing formwork.
If you need continuous footings, the procedures will be the same as mentioned above except you are cutting a continuous line to match the footing layout. Saw cutting and demo of existing slabs are not cheap. It is important to consider all the cost related with saw cutting, demo, and pour back versus removing the entire slab and start over because there is no easy way of installing footings below an existing slab.
For instance, if you have a 10ft x 10ft slab and you are going to remove 2 to 3 feet from each side to install your footings, you are better of demo the entire slabs and start over. The cost of saw cutting, drill and epoxy dowls for pour back on top of the increased difficulty you are going to face in trying to avoid damaging the slab need to be considered. When trying to save a slab, heavy machinery necessary for concrete demo sometimes cannot be used which will increase the difficulty of the work.
Keep in mind when cutting out slab for footings, make sure to cut at least 6 inches wider the the footing size called for. This will make excavation much easier.
Another thing to consider are the construction joints from the pour backs that will be visible if the slab is exposed. If you choose to demo the entire slab and start over, you will have a more uniform finished slab at the end.
These are some of the things to be considered if you are trying to install footings at existing slab.