Footings Overview

To transmit the weight of the house to the soil, without allowing the house to sink. (In any home, some settlement is typical within the first few years after construction.) Footings are located below the foundation walls and posts. The horizontal surface area of the footing is larger than the foundation, so that the load of the house can be spread out over a wide area. Footings are typically sixteen to twenty-four inches wide and six inches to sixteen inches thick. Footings carry the house loads below the frost line in climates where the soil freezes. The heavier the building and the weaker the soil, the larger the footing should be.

Concrete, brick, stone. In modern construction, almost all footings are poured concrete.

When the footings fail, the entire house moves. This is often a very serious problem. It is almost always expensive, and sometimes impossible, to correct. Since the footings are located below the soil and the basement floor, they cannot be seen. It is often difficult to know why they have failed.

Sometimes they will fail in one area only, and in most cases the failure is not uniform, (i.e. the building does not sink straight down but leans to one side or another). Often, one part of the house will pull away from the rest. This leads to cracking of interior and exterior wall surfaces.

Soils which are prone to compaction or movement do not support footings well. This includes recently disturbed soil. For example, if a basement is dug too deep, then backfilled to the correct depth, the disturbed soil under the footing is likely to compact over the first few years, resulting in building settlement.

This is not common on professionally built houses, but may occur in cottage construction as well as on porches and amateurish additions. Some homes were built on "mud sills". These are simply beams laid on the ground with walls built on top of the beams. These mud sills have to be replaced with a proper foundation and footing system as the sills rot, heave or settle.

Some clay soils which expand and contract significantly with different moisture contents may also result in failure. These expansive soils can heave floors and foundations when they get wet. When they dry, they shrink and allow the building to drop. This is one of the most significant causes of house structure problems.

Tree roots can affect the moisture content of soils noticeably. Most soils have strengths which change with different moisture contents. Some change dramatically. These are poor building soils. Silts are also poor building soils, in many cases much weaker than clay.

These may erode or weaken soil below the footings, causing severe building settlement. It is, of course, very difficult to locate and trace underground streams. They often flow only at certain times of the year.

This may be the result of poor design, or an additional load that has been added. For example, when a second floor is added to a bungalow, the weight may cause the footings to sink. The additional weight of a chimney can also cause localized footing failure.

The footing must be strong enough not to break apart under a load, and must be able to stand up to continuous exposure to damp soil.

If the basement floor is lowered, there is the risk that the footings will be broken off on the inside or will lose their support. Even if excavation is not done below the footings but down to the bottom of them, the lateral support for the footing may be lost, and the footing and foundation wall may move inward.


   Types of Cracks:

When a basement floor is lowered, the footings should be underpinned (lowered and, in some cases, enlarged). Alternatively, only the central section of the basement should be lowered, to avoid disturbing any of the soil near the footings. Depending upon how much the basement floor is to be lowered, the required clearance from the footings varies. A soils engineer is often consulted and a concrete curb (or Dutch wall) may be needed around the inside edge of the footings to ensure they are not compromised. One of the dangers in lowering basement floors is a greater risk of basement leakage. Notice in Figure 5 how the drainage tile outside is no longer in the correct location once the floor is lowered. It is too high to be effective.

When excavation is done on the exterior, (e.g. for an addition or swimming pool) the footings can be damaged or undermined in a similar fashion.

Houses built on or close to slopes may be subject to failures as a result of soil moving down the slope. This may be a slow steady process or a sudden event triggered by heavy rains for example. This can be extremely costly to correct.

Houses built on sloping lots may be more prone to footing and foundation failures. The chances of building on disturbed soil are increased on lots such as these. Efforts made to level and terrace the lot may result in soil being cut out of the hill to form a level terrace under the back half of the house. This soil is then used as fill in the adjacent area where the front half of the house is to stand.

On sloping lots, large lateral earth thrust and hydrostatic pressure can be built up on the high side, as there is a tall column of soil against the upper basement wall. Water running down the slope tends to be blocked by the building and accumulate here.

                         Lowering Basement Floors

On the downhill side, the footings may not be deep enough in cold climates. Frost heave can result where the footings are less than the regional frost line. The side of the house with the lower grade often has a walk-out basement, and chances of a footing being too shallow are greatest here.

The other danger is that the downhill half of the house is built on fill which may not be well compacted or may not be able to stay in place and support the house.

This may compromise the footings. In order to be effective, the footings in cold climates must be below the frost level. When an exterior basement stairwell is added, the stairwell opening effectively lowers the exterior grade level, and also lowers the depth to which frost can penetrate. After the stairwell is in place, the frost can theoretically go down as much as four feet below the bottom of the stairwell opening. This can lead to localized frost heaving of the footings and the foundations.

A properly added exterior stairwell will include deepened foundations, or a completely insulated approach, to prevent frost penetration below the building footings.

During an inspection, the results of footing failure can usually be seen. It is, however, difficult to know whether the building is still moving, and if so, at what rate. It is often necessary to monitor the building over a period of months or even years, to know whether the problem will warrant repair. A great many footing failures are not severe enough to warrant repairs.

The usual corrective action is to underpin the footings. This means digging under the existing footing, usually from inside the house, and adding a new footing wider and/or deeper than the original. This has to be done in small sections since one cannot excavate under the entire house at one time. Usually two to four foot sections are done at a time. This is, of course, very expensive work.

In some cases, where the soils are moving or are likely to move, underpinning is not appropriate. Piles driven deep into the ground are an alternative, but often so costly as to not warrant this type of repair to an existing building.

Where the problem is localized, the footings can sometimes be bridged across a weak area. For example, where there is a spring, the footing can be deleted above the spring, and a beam can be used above the weak area. This is an unusual and expensive approach.

In some cases, it is necessary to demolish the house. Depending on the soil conditions, it may or may not be cost-effective to rebuild on that lot.

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                      What does the building code say ?

            All exterior walls shall be supported on continuous solid or fully grouted masonry or concrete footings, wood foundations, or other approved structural systems which shall be of sufficient design to accommodate all loads according to the adopted code and to transmit the resulting loads to the soil within the limitations as determined from the character of the soil. Footings shall be supported on undisturbed natural soils or engineered fill.

A minimum footing width of 12 inches (305 mm) is required so that footings can span over weak locations in the soil and to allow for minor misalignment of foundations. The code  permits solid or fully grouted 12-inch-nominal concrete masonry block to be used for this minimum width of footing. The code specifies the minimum thickness of footings. These provisions provide for the transfer of loads to the supporting soil without exceeding the capacity of the materials used to construct the footings.
If any of the following conditions exist in the area of the foundation, conventional spread footings should not be used, and a designed foundationmay be necessary:

_ Filled ground, except when properly compacted.

_ Foundation soils subject to subsidence.

_ Expansive soils such as those having a plasticity ndex greater than allowed.

_ Highly compressive clays.

_ Unconfined sands and silts.

                               Minimum size.
        Minimum sizes for concrete and masonry footings shall be as set forth in adopted buildingcode. The footing width, W, shall be based on the load-bearing value of the soil in accordance with the residential code. Spread footings shall be at least 6 inches (152 mm) thick. Footing projections, P, shall be at least 2 inches (51 mm) and shall not exceed the thickness of the footing. The size of footings supporting piers and columns shall be based on the tributary load and allowable soil pressure in accordance with the code. 

                                      Slope.
The top surface of footings shall be level. The bottom surface of footings shall not have a slope exceeding one unit vertical in 10 units horizontal (10-percent slope). Footings shall be stepped where it is necessary to change the elevation of the top surface of the footings or where the slope of the bottom surface of the footings will exceed one unit vertical in ten units horizontal (10-percent slope).

PLEASE REFER TO THE LATEST EDITION OF THE APPLICABLE BUILDING CODE FOR YOUR AREA. Copies of the International CodeSeries may be obtained by visiting the web site of the International Code Council.