A strip foundation is a continuous strip of concrete cast in a trench beneath load-bearing walls, spreading the wall load over a wider area of soil. It is the most common foundation for houses in Ireland where firm natural ground is found at a reasonable depth.
In a strip foundation, a trench is excavated to a level where the soil can carry the building load, and a concrete strip is cast in the base of the trench. The load-bearing wall is then built up off the strip, usually in blockwork, to damp proof course level. The width and thickness of the strip are calculated from the wall load and the bearing capacity of the soil.
A variation called trench fill fills the entire trench with concrete rather than building blockwork up from a thin strip. Trench fill uses more concrete but less labour and is often preferred where trenches are deep or where the trench sides are unstable.