Step 2 Excavate and Prepare the Formwork. Excavate to a depth of about 10 inches. Deposit a base layer of compacted gravel about 6 inches in thickness; the pad itself is typically 4 to 6 inches thickand with a height slightly higher than the grade. Use lumber equal in width to the height of the pad (1x4 or 1x6 will do) to build the form.