This is my first project with cement. From my front door I want to build a 5' x 7' landing with 2 steps leading to my city side walk. The cement slab needs to be 14 inched above the ground to reach just under the front door. I live in Montreal, Quebec, Canada where the temperature in winter can go to - 30 Celsius, very very cold. Do I have to dig 4 feet down, that is the frost line, or will 10 inches do? Once I have my hole I will build a wood form to keep the cement in, add crushed stones, plastic sheet, wire mesh, then pur the cement in. The cement will be delivered by truck. The plastic sheet is to slow down the cement from drying to fast.