Strip Foundation
/ strɪp faʊnˈdeɪʃ(ə)n /
Also known as: strip footing, trench fill foundation, continuous strip footing
Definition
A strip foundation is a continuous strip of concrete cast in a trench beneath a load-bearing wall. It spreads the concentrated line load of the wall over a wider area of ground, reducing the bearing pressure to within the capacity of the soil. Strip foundations are the most common foundation type for UK domestic construction - houses, extensions, garages - on stable ground with adequate bearing capacity.
In practice
Two forms are used. A traditional strip has a concrete pad at the base of the trench with blockwork built up from it to DPC level - the trench is then backfilled around the blockwork. A trench fill fills the entire trench with concrete to within 150-225mm of ground level, avoiding the need for underground blockwork. Trench fill is faster and now the more common approach for new dwellings.
The minimum depth is usually 1.0m from finished ground level to protect against frost heave and seasonal moisture changes in the soil. In clay soils, trees within 1.5x their mature height create a risk of root-induced heave and desiccation - in these cases depths of 2-3m are required, usually using piled or deep trench fill solutions. A blinding layer of 50mm C10 concrete is placed at the bottom of the trench before the reinforcement cage (if any) and main foundation concrete.
Building Regulations
Approved Document A Table 10 gives minimum strip foundation dimensions for domestic structures based on total load and subsoil type. These are prescriptive solutions that avoid the need for structural engineering calculations for straightforward situations. For non-standard situations - large loads, poor soils, trees, sloping sites, or existing structures nearby - a structural engineer must design the foundation. Building Control will inspect the foundation before concrete is poured.
Full Building Regulations guidanceSee also