Foundations / Footings
Foundations, also known as footings or the substructure, are a solid base upon which a house is built, designed to support the walls and roof.
The kind of foundation will depend upon two distinct variables; the total weight of the house and the type and quality of the subsoil. Another factor to take into consideration is the weather conditions, which may reduce the effectiveness of foundation, depending on the type of soil. Frost heave, where water in the ground freezes and pushes upwards as it expands to ice, for example is common in sandy soils. The opposite problem is drought, causing shrinking of the land, which is common in clay soil types. Both of these conditions can affect the foundations with serious consequences, such as disturbance, and in more extreme cases, cracking of the foundations. However, with deep enough foundations, usually at least 750mm, these problems can be avoided.
Foundations are one of the most difficult elements of a build, and one of the most important. A specialist such as your architect or builder will advise you of the best foundations to choose. Below is an insight into what they are recommending.
Deep Strip foundations are used most frequently as they offer good strength and value for money. Firstly, a trench is dug out deep enough to avoid damage to the foundation caused by changes in the topsoil. The trench will usually be dug until clay is reached. 150mm of concrete is poured in the bottom of the trench with a width of 600mm. The concrete should have a minimum depth of 225mm. If the site sits on a slope, steps may be built into the concrete with forms that direct the depth. Once the foundations are completed, cavity walls are erected up to finished ground level, being either brick and block or, block and the trench and cavity are backfilled up to finished ground level.
Trenchfill foundations fill the excavation, almost to the top, with concrete. Although more expensive than a Deep Strip foundation this method gets you out of the ground quickly. The Trenchfill foundation is used when soil is loose or in areas with a high water table. In areas with heavy clay and in the presence of trees, Trenchfill foundations may be taken deeper, to a level where the moisture content of the subsoil is unaffected. It may also require the provision of mesh reinforcement, and the trench sides might need to be lined on one or both sides with a compressible material and / or a slip membrane.
Raft foundations cover an area at least the same size as the footprint of the building, usually used on soft compressible subsoil such as peat or soft clay. It is imperative that these are well reinforced to avoid the effect of ground movement and the raft should be constructed with an apron edge to avoid sliding.
Pile foundations are used to support buildings in subsoil types such as shrinkable clays, waste tips or infill, slopes and areas with a high water table in poorly drained regions. These foundations are used where other foundations would need to be so deep that it would not be cost effective. A basic pile foundation is a series of columns, or stilts, which rest on a solid, load bearing layer of the soil up to 4m below the surface. Beyond 4m the cost of pile foundations usually becomes excessive for small build projects; however there are self-builders that have completed a project to budget with piles to 10m below. Piles may be either precast of cast on site.
Pre-Insulated Foundation - Similar to Raft Foundations, this is a foundation system that offers excellent thermal performance in line with Part L regulations.
There are various pre-insulated foundation system which are well worth considering, as the high thermal performance values achieved can help the overall buildings energy efficiency required, without the need for an additional layer of insulation and screed.
When compared to traditional methods of foundation production, such as beam and block and raft foundations, they can be quicker to install, more environmentally friendly and offer excellent thermal performance.
Example pre-insulated foundation suppliers:-