The term "spread foundations" is commonly used to refer to shallow foundations that are too closely positioned to allow a slab foundation to be installed beneath a building. Strip foundations are a form of spread or shallow foundations and are commonly installed as unbroken walls beneath the entire perimeter of a building.
Foundations and footings are installed under a building to provide stability for the building throughout its lifespan. Foundations are constructed, taking into consideration the soil conditions and environment around the construction site; shallow or spread footings are usually used in areas with predominantly dry and firm soil conditions around the building. Continuous spread footings that encompass the perimeter of a building are commonly constructed to between 16 to 24 inches wide and 6 to 16 inches thick. Common construction materials for both spread and strip footings include reinforced concrete rated between 2,000 to 5,000 square inches of compression.
Strip footings are installed to allow the load of interior and exterior walls to be placed on the strip footings on the exterior boundary of the building. Strip footings are often used in areas where a building is placed on sloping ground and can be constructed by simple excavation and concrete pouring techniques; two concrete pours are usually required when installing strip foundations. Continuous spread foundations use strip footing to create a perimeter foundation where the load of exterior walls is positioned on the exterior of the walls.
Spread and strip footings include an area of the foundation being buried beneath the ground frost line to avoid damage to the foundations, usually less than 9-1/2 feet in depth, according to the University of West England Bristol. Unlike strip foundations, a continuous spread footing system can include the use of interior pillars and piers to increase the strength and possible settling of interior walls. Spread footings can include the use of various types of footings, in conjunction with exterior foundation walls; foundation walls are often constructed at the same width as exterior walls.
The supports of both spread and strip footings are typically constructed from reinforced concrete; continuous spread footings support solid brick, veneer, concrete block and stonewall constructions. Strip foundations provide a line of support where columns or load-bearing walls are in need of support. Strip foundations can be constructed from stepped or plain reinforced concrete footing designs.