On most soils, all footings should be at least 150mm (6") deep, with the bottom 350-400mm (14-16") below ground level. However, thicker and deeper footings should be employed in clay soil. Footings in clay soil should be 450mm (18") deep or more.
Thicker footings provide stability against frost heaving and other seismic activity. They also help prevent erosion by reducing the amount of water that flows through the soil on its way to the river. Thicker footings are also useful when building on wet sites because they reduce the likelihood that you will encounter problems with drainage or flooding.
The depth of footings depends on many factors such as the type of soil, how much weight you expect to put on the foundation, etc. It is important to choose a depth that is sufficient to meet your needs while keeping in mind that footings need to be at least 150mm (6") deep to protect against earthquake damage.
Footings should reach at least 12 inches below previously undisturbed soil. Footings must also be at least 12 inches below the frost line (the depth at which the ground freezes during the winter) or frost-protected. Footings should be a minimum of 12 inches wide. Larger footings are more stable.
The depth and width of footings depend on several factors such as soil type, how much weight you expect to put on the foundation, etc.
In general, footings should be at least as deep and wide as the lowest floor or platform that they support. For example, if your house has two floors, each with its own basement, then the footing under each floor needs to be at least as deep and wide as the floor it supports. The purpose of the footing is to distribute load across a large area of the soil and to keep it there. Loads from buildings above a footing spread out over a wide area of soil instead tend to cause localized damage to the soil's surface.
Footings should be constructed of strong material. Concrete is the most common material for footings, but stone, dirt, grass clippings, and recycled materials can be used as well. The footing should be of a size and strength capable of supporting any load that might be placed upon it from the structure it supports. In other words, the footing must be able to carry its share of the burden.
Footings must be built in conformity with the following specifications: 1.4 Soil pressure according to Table R401.2, 2. Wall thickness not less than 8 inches nor more than 12 inches, 3. Cuts or fills between footings shall be sloped so that water will drain away from the house foundation.
The depth of footings depends on many factors such as soil type, load requirements, etc. The required depth of footing is usually specified in the building code for each type of structure. In general, footings should be at least as deep as the highest point within the footprint of the structure. Footing depths should be checked against local codes before construction begins.
In areas where frost heaves are a concern, footings should be dug deeper than usual to allow for settlement. Also, if you plan to use concrete instead of dirt for the bottom layer of the footing, you'll need to add another inch or two to its depth. Concrete weighs more than dirt and it takes time and force to pour a solid foundation, so make sure to give it enough space to properly set up before continuing with other aspects of the project.
Footings Depth Footings should reach at least 12 inches below previously undisturbed soil. Where freezing temperatures are not a problem, footings can be placed any distance down into the soil and need only reach a depth of six inches.
The goal is to provide support for the weight of the building. The deeper the better as far as stability is concerned. However, make sure you don't go deeper than necessary because you'll be wasting money.
Generally, the deeper the foundation, the greater the cost of construction. But that's not always the case - sometimes a shallow foundation works fine if you use sufficient reinforcing materials. Consult an architect or engineer before you start work on your house to make sure that your choice of foundation depth will meet with approval.
You should also consider how much rain falls in your area when choosing the depth of your foundation. If you expect many storms with heavy rainfall, you may want to choose a deeper foundation than if you live in an area that experiences some flooding but no major storms.
Finally, think about what kind of soil you have where you plan to build.