The footing is the thing upon which the home rests. Where the home meets the soil. The first member of the load bearing structural systems of the home.
The footing is also an integral part of the structures foundation . . . that upon which the first floor is built.
For purposed of discussion, we talk about the footing and the foundation separately.
Review
The structural elements of the home are those which carry the weight or load of the home to the earth on which it rests. Hence, they are often referred to as load-bearing elements or load-bearing systems.
They include the Footings, Foundation Walls, Floor, Walls, Ceiling, and Roof. It is important that these elements be properly designed and constructed. For they must not only support their own weight, but a portion of that from above. For example, the walls must be strong enough to support the weight of the ceiling and the roof.
The Footing
The footings are placed under all load-bearing parts of the foundation, i.e. piers, columns, foundation walls, etc.Concrete Footings
Footings are almost always concrete. The footing is usually formed by concrete poured into a trench and constrained by some kind of forms.Most homes are constructed on 2500 psi (pounds per square inch of compressive strength when fully cured at 28 days) concrete footings.
In some areas, steel reinforcing rods, also called re-bars, are required in the footings. This makes the concrete much tougher and less likely to fail.Steel reinforcing rods are required in the footings in areas where the existing soil does not present a good bearing capacity, such as sand. When soils move around more drastic solutions are requires. See "Active Soils" below.
Dimensions
The dimensions of the footings will vary from place to place, again depending on the bearing capacity of the soils present.The concrete footing details for your home may need to be designed by an engineer - depending on the type of soil on your lot.
Your building department and your footing/foundation subcontractor will be able to advise you on thid.In many areas a footing that is 8" deep and 16" wide is used. When the width is increased, the code requires an increased thickness.
You can imagine that if it is too wide and not deep enough, the weight of the house bearing down on it could snap it in half like a saltine cracker. The projection of the footing on either side of the wall is supposed to be no greater than the depth of the footing.
The bottom of the footing should be at least 12" below the finished grade line (surface of the ground). It may need to be deeper. It must be below the frost line. Water expands when it freezes.If the bottom of the footing is not below the frost line, it may be exposed to the upward pressure of the freezing ground water, which may cause structural damage to the home.Reinforcing the footing with steel rebars is helpful in strengthening the footing.
Your subcontractor will (or should) know what will be required on your lot. Talk with him about the design of footings. Also talk with your building official about this and about the design of the foundation walls.Here are some typical footing dimensions from the CABO One- and Two- Family Dwelling Code; 1995.
Load Bearing Capacity
So how do you know what the load bearing capacity of your soil is? You could have a soils engineer take a sample and test it.That probably won't be necessary, unless the building official requires it.
In all likelihood your subcontractor and/or the building official will know what kinds of soils are typical in your area and what kinds of footings and foundations will be acceptable.
Active Soils
Active soils are very fine particle clay soils, e.g. Bentonite, that expand when they absorb moisture. If active soils are present in your area, your footing and foundation system must be designed by a structural engineer who is familiar and experienced with these conditions.
The engineer may require the home to rest on concrete piers, called caissons, dug down to bedrock, and the use of special "void materials" to keep the expansive soils from causing the foundation to move.Do not try to design this system yourself. It requires special knowledge and testing of the soils present on your lot.
Draining
Footings should be well drained to prevent the damage that water pressure and freezing water can cause. This is accomplished with sand, gravel, and drain pipes.
Keys
A footing key is a groove of some king formed on the top of the footing, down the center lengthwise.It's purpose is to keep a concrete foundation wall poured on top of the footing from shifting from side to side.
Rebar sticking up out of the footing is also a good solution. Note that chances of such movement is pretty remote unless you are in an earthquake or active soils area, in which case you will be taking other, more extreme measures to keep your home from falling down!
Stepping
When installing trenched, flat, formed and poured in place footings on a it is usually helpful to "step" the footings down the hill.
Gravel Footings
Considering a wood foudation for your new home? Homes with wood foundations are often buillt on crushed stone or gravel foundations.Never heard of such a thing? Click here to see the housing industry's official take on this building system.
Slabs
Slab homes can have a traditional footing and foundation wall. Or they can have a "turned down" area at the perimeter which becomes the footing for the home. The latter is commonly used in warmer climates where the frost line is not a concern. In other words, you don't have to worry about freezing ground water causing your house to heave upward.
