I would like to take my existing patio 12'6" x 17'x 3.5" that goes from my house to the garage and add footings under to help support the roof above that covers the patio that is connected to the house and garage frame.It use to be a 2 nd floor deck that was converted to a roof.What I need to know is can I use my post hole digger and dig along the side of the slab and go under it 6-8 inches wide and about 30 inches deep ?Also I need about 2 inches extra of width for alignment with the roof overhang when the wall would be constructed.