No matter how big or small a building is, it can only be as good as it’s foundation - and that’s a fact! It all starts from the footing and when it isn’t deep enough, unlike badly installed gutters or gapped hardwood flooring, can make the whole building collapse to the ground. Just like any code recommendations, the depth of the footings of your porch is a parameter that must be complied with and adjusted to suit your local code requirements.
As a rule of thumb, you should always dig below the frost line when excavating your footings. At this depth level, the moisture present in the soil will be frozen. Ultimately, this will help the ground to act as a barrier to insulate the soil below the footing from freezing during the cold months. As a result, in regions where the weather is almost always very cold the depth of your footings is expected to be at least 48 inches. But in areas featuring a warm climate, you may be allowed to install your footings on grade.
In addition, it’s compulsory that your footings must be installed at a minimum depth of 12 inches below unsettled soil. However, if you’re constructing the structure into a steep hill’s side or by the side of a retaining wall, it’s required that you maintain the least distance to grade both vertically and laterally. In the event whereby you need to achieve a good lateral barrier, you’ll need to have your foundation dug a lot deeper compared to if it’s on a flat surface. For a more accurate and updated information, you can consult your local building inspection department to get familiar with the requirements in your area.