Here, Professor Dr. Michael Benfield of Simply Self Build, a Benfield ATT Group company, discusses everything you need to know about foundations.
When it comes to buying your land, get a soil test done. Testing the soil of your land with boreholes can be a sensible precaution and will help you identify which foundation method may be suitable for your project.
Our advice is based upon experience of building hundreds of self-build homes, and is merely this:
• With foundations, you get what you pay for
• Scrimping on foundations will inevitably have its consequences
• Get a third-party check for level and squareness before your groundwork contractor leaves the site – once they have left your site, it will be almost impossible to get them back in a timely manner to correct any inaccuracies
• Plan access for your services – water, waste, gas, electricity and telephone ducts – at the site preparation stage.
Types of foundation
There are many types of foundation for homes, especially in the innovative self-build market, although there are a few key popular, tried-and-tested types that builders and architects will recommend.
Strip foundations for self-build projects
Strip foundations are the most common type of foundation used because of their simplicity and subsequent cost-effectiveness. There are three types of strip foundation – traditional shallow strip, wide strip, (for higher-load structures) and deep strip (which we call trench/fill foundations). These foundations have narrower trenches filled with concrete within a couple of brick courses of completed ground level.
Raft foundations for self-build projects
Raft foundations cover an area of (at least) the same size as the ground floor area of the building. They are used on soft, compressible subsoils such as soft clay and peat.
Pile foundations are typically the most accurate as they are mostly pre-engineered; these types of foundation tend to be much more precise and therefore reliable. The systems typically consist of stilts or columns which rest on a solid, load-bearing layer up to 10m (usually about 4m) below the surface. The piles may either be precast and drilled into the soil, or cast from ‘wet’ on the site (sometimes referred to as ‘pads’). The use of screw piles can be a very quick, accurate and cost-effective solution for many self-build projects.
It is important to note that, especially with modern methods of building a self-build home, accuracy is extremely important. The foundations are the most heavily-loaded structural element in a building. They are constructed in largely unseen conditions and their integrity is reliant on the quality of workmanship in construction. Recent research indicates that, on average in the UK, 20% of building projects are delayed by more than a month due to construction problems in the ground. Simply Self Build would suggest that on timber frame projects, this number is more like 40%; it is not unfamiliar to us to inspect foundations and for them to be out-of-level to an alarming, and, equally, concerning amount. Please do yourself a favour and do not cause unnecessary delay to your project by installing faulty or inaccurate foundations.