Phase 1: Footings
There are several types and sizes of footings to choose from and the one that you decide to use depends on the soil conditions, and how far down you need to go below the ground to protect from frost. Note that you can provide protection against frost by also using good drainage around the foundation, which guides the water away from the building.
Selecting and placing the footings for your home is essential because the footings receive the house load through posts or foundations walls, which is then transmitted to the soil. When we placed the footings, we made sure that the distance between the footing base and the finished grade was at least the depth of any anticipated frost penetration. See the diagram below for the minimum depths for different soil conditions.
Building code requirements for your region should be consulted when preparing to place footings. Before we started with our footings we talked to our local building official to discuss local soil conditions; that way we could plan what we needed more accurately.
Side forms should be used for footings, unless the soil conditions and design allows for sharply cut trenches. When measuring the distance for wall footing placement, it is important to make sure that the footings project beyond each side of the wall at least 4 in. (100 mm), with a thickness of no less than the projection beyond the wall. Footings should never be less than 4 in. (100 mm) thick. In cases where a preserved wood foundation is being used, continuous wood footings are usually more practical and economical.
Before going ahead with finalizing the footings, you should check to see that the excavation is even. If you find that it is not, and that in some places the excavation is too deep, a compacted granular mat can be used to level it. Note that excavated material should never be used as a base.
In our case, the soil had a low load-bearing capacity and so we had to use wider reinforced footings. Using a key at the top of the footings was recommended by our builder. Keys help the foundation wall resist lateral pressures from the earth pushing against it. Bring the idea up to your builders to see how it can work into the plan. Any pipe tranches that are directly under wall footings need to be backfilled with concrete.
These footings can vary in size depending on the allowable soil pressure and the load they support. Footings should be placed so that the members they support are centered. Common sized include:
- One story house: 4.3 sq.ft. (0.4 m2) (about 25 x 24 in. (640 x 640 mm))
- Two story house: 8 sq.ft. (0.75 m2)(34 x 34 in.)(870 x 870 mm)).
** A minimum thickness of 4 in. (100 mm).
Stepped footings are needed when there is unstable soil, where there is a steeply sloping site, and in some cases they are used with split-level houses. The vertical part of the step should be placed at the same time as the footing. The bottom is always placed on undisturbed soil, or on compacted granular fill. Sometimes, where there is a very steep slope, more than one step is needed. The vertical connection between footings at the step should be of concrete at least 6 in. (150 mm) thick, and the same width as the footings.