Concrete pillars with footings are the most stable type of foundation you can use. The advantage of sills is that they solidly stabilize the concrete piers that are attached to them. However, this technique is rarely used because of the extensive excavation work involved. A large area must be excavated in order to place the formwork below the frost line. This work requires the use of a mechanical digger.
Use prefabricated circular bases that adjust to 8″, 10″ or 12″ diameter pier tubes into which concrete is poured. These may be inserted into the holes without excavating the entire perimeter of the deck, but since the diameter of the hole to be dug must be at least 18″ a mechanical digger would nonetheless be required.
1.1. Dig out 4″ of soil inside the perimeter.
1.2. Calculate the weight of the terrace in order to decide on the number of pillars required. If you have drawn your own plan, you may need to consult an expert to determine that. Then, determine the location of the pillars. If plans call for the terrace to be built at the same time as the house, have both foundations dug at the same time. That will make it easier to install the formwork (54″ deep) and pour the concrete.
1.3. Dig 54″-deep holes using a posthole digger (In such cases use pre-fabricated circular footings which adapt to circular forms of varying diameters and in which you will pour the concrete or dig holes large enough across to house the 24″ forms that will be built.
1.4. Build 24″ square forms using 2″ × 8″ boards. Lay in a reinforcing rod (diagonally) and add wire mesh. Forms are ready for concrete
1.5. Stand an L-shaped reinforcing rod in the centre of the frame placing the short end of it under the diagonal reinforcing rod. The L-shaped rod will serve as the link between the footing and the pillar.
1.6. Pour concrete and hold L-shaped rod erect as the concrete is spread. Be sure to comply with the concrete manufacturer’s instructions on curing time.
1.7. Place tubular forms on the centre of the footings when concrete has cured and hold them up with pieces of wood. Use a level to ensure the tubes are vertically leveled and the tops are even.
1.8. Nail the tube to the footing at a 45° from the outside of the tube into the footing.
1.9. Backfill the footing using the original soil. Use a long narrow object to slowly stir the concrete to the bottom of the tube to make sure the concrete remains homogenous and free of air bubbles.
1.10. Insert in the centre of the pillar the post saddles. These are for the treated posts which will be attached later. Do this before the concrete sets.