A building foundation is designed to support the main portion of the house and transmit its load to the ground. In most residential structures, the foundation consists of walls that rest on an enlarged base called a footing. The footing spreads the transmitted load directly to the supporting soil. In areas with freezing temperatures, the footings must be located below the frost line, otherwise they become vulnerable to frost heaving resulting from the freezing of soil moisture. Columns, pilasters and piers are also used to support the main structure.
Normally, foundation walls are designed to support the vertical loads from the house and to resist the horizontal forces resulting from the earth’s pressure. In most areas across the country, the house must be anchored to the foundation in order to provide resistance to wind and/or earthquake forces. If a structure is not properly anchored, high wind forces or rapid ground movement can cause it to lift, shift or rotate slightly. If it is properly anchored, the resistance to this movement produces tresses that are within the foundation wall, so it is essential that the foundation wall is designed to withstand these forces. There are also cases in which the combined horizontal and vertical forces acting on the foundation wall are great enough to require additional bracing and stiffening of the walls, so plasters are often used for additional support.
Foundation Inspection Methods – Factors Affecting Foundations
Most houses built in the last 70 years have foundation walls of poured concrete, concrete blocks or cider blocks, while older houses typically have stone or brick foundation walls. During a stone or brick foundation inspection, particularly in houses built before or around 1900, pay particular attention to the mortar joints. Some joints might have holes, some may be filled with soft, crumbling mortar that can be easily raked out, and in some cases the joints have been filled with earth or mud water instead of mortar.
Any deteriorated mortar joints in foundation walls should be repointed, because they can allow water to penetrate into the basement and enable mice and other pests to infest the area. In some older homes with brick foundation walls, the bricks might be soft, crubled and flaking, so they should be replaced. This condition is usually caused by using underburned bricks in the construction of the wall. If you find this condition during a foundation inspection, record it on your worksheet for future masonry rehabilitation.
Cracks Along Foundation Walls
Cracks in poured concrete or concrete-block foundation walls may be a result of shrinkage, differential settlement, lateral pressure on the wall by the soil, or poor-quality workmanship. Short cracks along foundation walls are not uncommon, and they are usually of no structural concern, however all cracks should be sealed as a precautionary measure against water penetration into the basement If they are not sealed, in the case of a hydrostatic pressure buildup in the soil against the foundation water will seep through the cracks.
If you find long narrow cracks along foundation walls and both sides of the cracks line up so that there is no noticeable differential settlement, the condition is usually not serious and can be controlled by sealing the cracks. On the other hand, when both sides of the cracks do not line up or there are long open cracks along foundation walls, there is a more serious condition of differential settlement. A single foundation inspection cannot determine whether the differential settlement is active or dormant, so the wall should be checked again for incremental movement over a period of time (a few months usually).
After some time, in most cases the differential settlement stabilizes, with little effect on the house. However, if you suspect a more serious condition of the foundation wall, you should request a professional foundation inspection.
Another type of cracks that raise concerns are long, open horizontal foundation cracks, especially if the foundation wall shows signs of bowing. This condition is mostly the result of an excessive horizontal pressure being exerted on the foundation wall by the earth backfill and indicated that the wall cannot properly withstand these external lateral forces. Record this condition on your foundation inspection worksheet as a condition that requires further investigation. Depending on the extent of bowing, it may require reinforcement or replacement.