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).
Depth of Footings
Footings should extend to a minimum depth of 12 inches below previously undisturbed soil. Footings must also extend at least 12 inches below the frost line (the depth to which the ground freezes in winter) or be frost-protected.
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.
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).
Depending on your local building code and whether you have a basement, that might be 5-10 feet below grade. Footings are required to be built on undisturbed soil.
First off: Why you need accurate footings calculations
If you make your foundations too shallow or too deep, there is a risk to the integrity of the build, and a danger that any reinforcement, be it mesh panels or rebar rods, is positioned incorrectly, leading to reduced tensile strength and load-bearing capacity.
Deck pier blocks are used in deck building as an alternative foundation to traditional footings. They are placed directly on the ground, and do not require digging or pouring. They come pre-formed with holes for attaching the posts of a deck, making it easy to install.
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.
In reinforced and plain concrete footings, the thickness at the edge shall be not less than 150 mm for footings on soils.
Even the best contractors occasionally have to build on a sloppy footing. Ian Geisler, a noted ICF expert who has worked on hundreds of projects across the continent, says that realistically a perfect footing is anything within ¼” of level. “If it's within ½”, then it's workable,” he writes.
Deep foundations are the go-to for heavy structures or in situations where the upper soil layers are too weak or inconsistent to bear the structural load. In such cases, the foundation needs to bypass these layers and transfer the load to deeper, more competent strata.
For single-storey extension foundations, a concrete foundational depth of 150mm will suffice.
To determine the size of the footing, a simple formula is used: Structural Load (lbs) / Soil Capacity (psf) = Footing Surface Area Required (sq. ft.)
Foundations should be placed at least 450mm below the finished ground level to avoid the risk of the bearing strata freezing and causing damaging movement.
The thickness of the footings should be not less than 200mm and is usually mass concrete only, ie. no reinforcement. The depth below ground level to the base of the footing should be not less than 300mm or to rock, whichever occurs first, which allows for a minimum of 100mm of ground cover to the pad.
The minimum depth of footings below the surface of undisturbed soil, compacted fill material or controlled low strength material (CLSM) shall be 12 inches (305 mm). Where applicable, the requirements of CBC Section 1809.5 shall also be satisfied.
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).
Strip Wall Footings are constructed with typical slab depths between 6”-8” (15-20 cm), footing widths of 12”-24” (30-61 cm), and lengths that vary as needed. Strip Wall Footings are commonly paired with column widths of 6”, 8”, 10” (15, 20, 25 cm).
Footings are structural supports that are used in addition to a foundation. They help transfer the load's weight from the foundation into the soil. Footers are typically in direct contact with the ground, while the foundation is in contact with the footing.
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.
Exploratory test pits. The only way to confirm the foundation detail is to physically check by digging exploratory test pits.
Concrete block foundations, or Cinder Masonry Units (CMUs), are made from stacked cinder blocks. Using the CMU method, the footings are coated with mortar—a mixture of sand, water, and fillers to bind masonry—then cinder blocks are layered evenly on top. The process is repeated until a foundation wall is formed.
J&W Lumber recommends using QUIKRETE to build your concrete footings. Take a look at this step-by-step informational video on how to build the footings for your deck.
Gravel is an ideal material for use under decks. Gravel's usefulness in deck construction is not limited to drainage. It can also help stop soil from being washed away and act as a deterrent against weeds. Additionally, it's easy to install, comes in various sizes and colors, and offers numerous other benefits.