Strip Foundation 

Strip foundations (also called strip footings) are continuous linear strips, often made from concrete. They are shallow foundations designed to support light to moderate load (for example linear walls used to build a low- to medium-rise residential building). The aim is to spread the load over a wide enough area, preventing excess stress on a single point of soil.

Strip foundations are one of the most commonly used foundation types in the UK and are suitable for most soil types.

Wide strip foundation 

Wide strip foundations are used when strip foundations are too wide to increase their thickness in line with the 45° rule. The 45° rule mitigates the risk of shear failure. Instead, wide strip foundations utilise high-tensile steel rods to reinforce the concrete, minimising the risk.

Wide strip foundations are simple to construct and manufacture, providing a cost-effective solution. 

The 45° rule, used for traditional strip foundations, refers to the foundation width generally being three times the width of the supported wall – spreading the load across 45° from the wall’s base to the soil. As mentioned above, this mitigates the risk of shear failure. 

Deep strip foundation 

Also known as trench fill or trench foundations, these are used to reduce labour costs and mitigate the risk of damage on heavy clay soils. The trench width usually matches a standard bucket width for the excavator. Site setting out and excavation must be accurate to keep the walls over the foundations. In clay, the depth should be at least 0.75m to ensure that the trench bottoms do not dry out and shrink, but often more to avoid frost action and achieve good bearing. The concrete mix would be the same as for a strip foundation.

Short board piles

Short board piles were developed for weak soils or where movement may occur. The piles support reinforced concrete beams, off which load-bearing walls are built, transferring the load above ground to the deep rock and soil layers below. A mechanical auger is used to bore the piles into the ground.

This foundation is durable, suitable for soils with low bearing capacity, and resists corrosion and deterioration for long-term stability. The installation of the piles is complex, requiring specialised equipment and labour, which increases the relative cost. However, their durability is an advantage. 

Raft foundations 

Raft foundations are formed by a reinforced concrete slab (of uniform thickness) and cover a wide area, often the entire footprint of a building. They spread the load of wall columns built across the entire area of the foundation and sit on top of the ground. These are mainly used where the subsoil has a poor load-bearing capacity. There are two main types which are flat slab raft and wide toe raft foundations. 

Stepped foundations

When constructing on a slope, stepped foundations can be used. Often made from concrete, the foundation comprises a series of horizontal steps following the slope of the ground. The main function of the foundation is to spread the load of the structure evenly across the slope and to provide a level surface for the construction of the building.

Helpful links: Building regulations: Foundations - Outbuildings - Planning Portal

Approved documents A: Section 2E Approved Document A - Part A - Structure - Planning Portal