Throughout the industry, a nominal six inch thickness of concrete (actual thickness is 5-1/2 inches) poured beneath columns is pretty well accepted as being adequate.
Simple Gravel and Concrete Footing Design
The minimum footing thickness should not be less than the distance the footing extends outward from the edge of the foundation wall, or 6 inches, whichever is greater.
Footing thickness - 8 to 12 inches.
1809.8 Plain concrete footings.
The edge thickness of plain concrete footings supporting walls of other than light-frame construction shall not be less than 8 inches (203 mm) where placed on soil or rock.
The minimum depth of footing on sandy soil is 0.8 - 1 m, for rocky soil, it is 0.05 - 0.5 m and for clayey soil, it is 0.9 - 1.6 m. The minimum depth of foundation for the load-bearing wall is 900 mm.
They are made not less than 1 1/2 times the projection of the slab beyond the face of the pier or column or the edge of the baseplate of a steel column. It should in no case be less than 150mm thick.
A minimum of 12”inches deep for all footings and the width will be determined by the amount of stories, 12” inches for a single story, 15” inches for a two story and 23” inches for a three story. This is based on a soil bearing capacity of 1500(psf).
Footings shall be a minimum of 8 inches by 24 inches (203 mm by 610 mm) for houses two and one-half stories and less. The footings for a three-story building shall be 10 inches by 24 inches (254 mm by 610 mm).
Gravel provides drainage, stabilizes the soil, prevents erosion, and helps distribute weight more evenly. Without it, you risk the potential damages listed above. If you're planning a concrete project, don't skip the gravel layer—it's a foundation that ensures your concrete work stands the test of time.
To determine the size of the footing, a simple formula is used: Structural Load (lbs) / Soil Capacity (psf) = Footing Surface Area Required (sq. ft.)
Each deck support post must be supported by concrete footings. The size of each footing is determined by the tributary load imposed on it. See the diagram below for an explanation of tributary load. Footing must be dug down into undisturbed soil and to a minimum depth of 12-in below the finished grade.
Fast-setting concrete is ideal for setting posts because there's no mixing—you simply pour the dry concrete from the bag right into the hole, then add water.
Simplest Concrete Footing Method
You can get away with usually a 15"x15" base and then at least 6" thick so you don't crack the deck footing. That is assuming you are using a 4x4 which is 3.5" square.
According to the rule of practice, the minimum allowable rebar diameter in a footing is Ø12 and the maximum acceptable spacing between the reinforcement bars is 150 mm. In certain occasions, the footing may have a reinforcement wire mesh both at its upper surface (see following figures).
It is not advisable to pour concrete directly on dirt. Even compacted soil—known as a subgrade—is not strong enough to maintain the form of concrete. You can, however, pour concrete over concrete as long as the overlay has proper bonding agents and the slab below is clear of damage.
Footings typically must extend below the frost line to prevent shifting during freeze-thaw cycles. Dig footing holes about 6 inches deeper than required. Fill the bottom of the hole with 6 inches of gravel and compact the gravel with a 2x4 or wood post.
Rebar, short for reinforcing bar, is a critical component in concrete footing for any structure. It is a steel rod or wire mesh added to concrete to enhance its strength and durability.
A guideline for the depth is 12 inches, plus one additional inch for every three inches of the column. For example, the footing for a six-inch column should be at least 14 inches deep.
The footing depth, on the other hand, is typically determined by the soil type and the extent to which you must dig before striking undisturbed soil. Still, we recommend that the depth of the footing extends to a minimum of 12 inches after you've hit undisturbed soil.
The most common types of footing in residential construction are Isolated and Continuous spread footings. They are both part of a shallow foundation construction hence why they are used in residential construction. In addition, the other five footing types are Raft, Strap, Strip, Pile, and Combined footings.
Required Footing Sizes
In most cases, a house should have a minimum footing of 12 inches wide by 12 inches deep, with foundations that are 8 to 10 inches thick. This should be enough to support house weight and resist lateral soil and water pressure.
For every 2 feet of adjustment to the width of the house, add or subtract 2 inches of footing width and 1 inch of footing thickness (but not less than 6 inches thick).
In reinforced and plain concrete footings, the thickness at the edge shall be not less than 150 mm for footings on soils.