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).
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.
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.
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.
Footing thickness - 8 to 12 inches. Footing depth - varies based on frost line and soil strength (some footings can be shallow, while other must be deep)
The depth of concrete you need will depend on the use: footings for extensions, for example, will need to be at least 200mm thick, while around 100mm should be deep enough for a shed base.
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.
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.
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).
As per Cl. 7.2, IS:1904, All foundations shall extend to a depth of at least 50 cm below the natural ground level. 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 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.
To determine the size of the footing, a simple formula is used: Structural Load (lbs) / Soil Capacity (psf) = Footing Surface Area Required (sq. ft.)
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.
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).
In reinforced and plain concrete footings, the thickness at the edge shall be not less than 150 mm for footings on soils.
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.
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. The minimum width of footings shall be 12 inches (305 mm).
Use compaction whenever the soil is disturbed during construction or when it's used for fill. Compact disturbed soil beneath footings, slabs, basement floors, patios, garage floors, driveways, concrete steps and sidewalks. If a sand, gravel, or crushed stone base is used beneath a slab, compact it too.
Exploratory test pits. The only way to confirm the foundation detail is to physically check by digging exploratory test pits.
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.
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.
Footings need to be 600mm wide x 1m deep minimum,in some cases depending on ground quality/type,proximity of large trees footings are required to be up to 2m deep.