Foundations for a single-story building strip will normally be 450 mm wide and at least 200 mm deep, while foundations for two-story buildings will be 600 mm wide and 200 mm deep. The depth of foundation walls is based on the amount of soil pressure that can be tolerated before the wall begins to show signs of distress.
The requirement for deeper foundations is based on the fact that most buildings produce some amount of groundwater flow under them. If this flow is not taken into account when designing the site, the potential exists for flooding due to rising water levels in the surrounding area. Sites that lie over rock or other impermeable material cannot produce such flows and therefore do not require deep foundations.
Groundwater flow under buildings comes in three principal types: seepage, surface runoff, and infiltration. Seepage occurs when rain or melting snow causes ground water to flow under the building through open cracks or holes in the subgrade. This type of flow tends to be slow (over time scales of years to decades) and rarely rises above the floor level of the building. Surface runoff happens when precipitation strikes the top of the roof and runs off toward the street or some other lower point where it can drain away from the building. It can cause problems if the storm drains are not sized properly or if there are any breaks in the drainage system.
Residential foundation walls are normally built with units 7-5/8 inches high by 15-5/8 inches long, with a 3/8-inch margin for mortar joint width. Nominal 8-inch-thick concrete masonry units are widely available in residential construction. They are delivered to a building site ready mixed and then poured as needed into forms made from scaffolding or steel plates. The number-one cause of foundation failure is inadequate footings, which can be caused by several factors including poor site conditions, adverse soil composition, or lack of depth. Foundation walls should be high enough to cover any lower walls that may exist on neighboring properties or below existing floors. The height depends on many factors such as the type of foundation you are designing and the load it will have to support. For example, if your house will have a first floor wall that serves as a basement wall, then the foundation should be high enough to allow for an inch of headroom above the bottom of the first floor wall.
The length of a foundation wall unit is determined by the thickness of the concrete required to provide adequate strength. Concrete is usually pumped into forms measuring 2 feet by 4 feet, for example, so that each face of a four-foot-high wall contains a slab about 24 inches wide.
The width of a foundation wall unit is based on two factors: the type of foundation being designed and the design requirements for the project.
Strip foundations are a sort of shallow foundation used to give a continuous, level (or occasionally stepped) strip of support to a linear building such as a wall or closely spaced rows of columns erected centrally above them. The main advantage of a strip foundation over other types of foundations is its ease of construction. A strip foundation can be easily laid out with the aid of string and stakes, then filled with gravel or concrete to make up the rest of the foundation.
The word "strip" comes from the fact that these foundations are built by first laying out a long narrow band of material, which is then taken away from the house as it is built upon it. Thus, the term "strip foundation."
The length of the strip depends on how high you want your wall to be. The thicker the strip, the more stable it will be. Thicker strips are also easier to fill with concrete. The thickness of a strip foundation should be at least as thick as a single floor joist. The depth of a strip foundation is equal to the height of the wall it supports. So if your wall is 4 feet high, the strip foundation should be at least 4 inches thick.
There are two types of strip foundations: horizontal and vertical. In the case of a horizontal strip foundation, the strip itself is always horizontal. Walls built using this method usually have flat roofs.