Foundations – just because you can’t see them doesn’t mean they’re not important. In fact, getting your foundations right is key to the success of any building project. They transfer the weight of the building into the ground so if they’re not planned and executed correctly, you could be in trouble. We take a quick look.
Planning your foundations
Since the weight of the building rests on the soil it’s important to be sure the soil has the right properties to support the weight of the building you have planned. Your foundations need to be built in good, strong soil. Depending on your site, you might have to go down two metres or more, which can be expensive and, potentially, dangerous.
So how can you find out what sort of foundations you need? That depends on your project. For smaller extensions, you can make an initial assessment yourself. Look out for things like large trees or boggy ground that might indicate potential problems. Conversations with neighbours or a local building firm might reveal useful information. You can also seek advice from your local building control officer – they can offer advice on the type and depth of foundations you’ll need.
If you’re considering a larger project, or are working on looser soil types, you may need to consider a professional ground survey, conducted by a structural engineer. By digging holes around the site, they’ll be able to plan the best type of foundation. They do tend to be cautious, often recommending engineered solutions which may be more expensive than traditional foundations.
Types of foundations
The right type of foundation for your project depends on:
- The nature of the building being supported
- The ground conditions
- The presence of water
- Sensitivity to noise and vibration.
Foundations are broadly divided into two types: shallow foundations (as little as 1m) and deep foundations (from 20-65m).
Shallow foundations are fine where the soil at that level is capable of supporting the load. These are also known as spread footings or open footings as all the earth is excavated to the bottom of the footing before the foundations are built. There are several common types of shallow foundations which all use concrete as their base:
- Strip foundations: these are linear and support the weight of the entire wall.
- Individual or pad foundations: these support columns which carry the load of the building. Each column will have its own footing.
- Raft foundations: these support the whole structure and can be helpful where the soil is weak as they spread the weight of the load evenly.
The most common type of deep foundations are pile foundations. These are long cylinders of strong material pushed into the ground. The structure is supported on top of it. This method is often used where there is particularly weak soil or where the building is very heavy. It will usually involve the skill of a structural engineer to determine the best solution. Piles are a good choice for waterside homes too as they allow water to travel underneath the house should any flooding occur.
Laying your foundations
When you’re building your foundations it’s vital that you stick to the plans which have been agreed. Dimensions will have been stipulated on the building contractor’s drawings to the architect. Your building control officer needs to be informed at certain stages of your build – usually your building contractor will take care of organising this. They will come and check that you’re not deviating from the plans.
The ramifications of not sticking to the plans can be very serious so take the time to get it spot on. Build in the wrong place and your completed house might have to be completely demolished.
Before you start to dig, you should mark your trenches (assuming strip foundations) using a spray chalk line down the centre of the foundation. How you dig depends on the scale of your job, and on access to the site. For a domestic extension it’s a good idea to employ a mini-digger. You can dig shallow foundations by hand, but it’s hard work!
When laying simple strip foundations, you have a couple of choices. You can either pour as little solid concreate as possible into the trenches (minimum of 250mm) then build up in blockwork to ground level. Or you can fill the whole trench with concrete to just below ground level. The latter saves labour and time, but the concreate does cost more. Just above ground level, the footings are topped with a damp-proof course and then the ground floor is fixed.
A final word of warning: you never know what’s under the ground until you start digging. This can mean unexpected surprises along the way. Hold back contingency funds in case you come across a problem which could push up your costs.