Exterior footings shall be placed not less than 12 inches (305 mm) below the undisturbed ground surface.
Furthermore, you must install footings at least 12” below unsettled soil. If you are building into the side of a steep hill or beside a retaining wall you will have to maintain the minimum distance to grade not only vertically but also laterally.
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).
Excavation should be to a depth that gives sufficient bearing and protection from frost damage. To avoid damage caused by frost, the depth of the foundation(s) in frost-susceptible ground should be at a minimum of 450mm below ground level.
Builder's tubes are designed for below-grade use, but may be poured to three feet above the ground line. They are not designed to be stripped.
Sonotube Builder's Tube concrete forms are designed for below-grade use, but may be poured up to three feet above backfill line.
Maintain the reveal. Make sure the top of the foundation is a minimum of 8 inches above grade. This prevents the exterior finish from absorbing groundwater, and it's key to the overall strategy of protecting the home from bulk-water intrusion.
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.
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.
Footing thickness - 8 to 12 inches.
Between your foundation walls and resting on top of the footings lies your foundation slab: the giant piece of concrete which makes up your basement floor or subfloor. Each of these pieces must be poured and cure one at a time: a process that can take up to a week for each step.
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).
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 minimum depth of foundation for the load-bearing wall is 900 mm.
The usual footing dimensions range from 1.0x1. 0 m to 3.0x3. 0 m although sometimes they may be larger and their height varies between 0.5 and 1.0 m for flexible spread footings and between 0.7 and 2.0 m for rigid spread footings.
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.
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 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.
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.
You can find out by digging a hole next to the foundation and see how deep they go.
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.
Acceptable house leveling percentages can vary depending on factors such as the type of foundation, the severity of the settlement, and structural engineering standards. Generally, acceptable house leveling percentages fall within the range of 0.1% to 1%.
To fix or improve the grading, you can add soil next to the foundation and slope away from the house, however, you should have at least four inches of your foundation (concrete, block, or stone) showing above the soil. The soil and vegetation should not be in contact with the siding or any wood.
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.
Under 1/8" is typical (even on much larger buildings), but it's just a shed, so @batsplatsterson has an answer I'd agree with. If using sheet goods (plywood, OSB) for roof and floor you'll have some extra waste to trim due to the out-of-square condition. Effect on the walls should be minimal.