Selfbuild Foundations

Whether you’ve found your ideal plot, designed your dream house and got the planning permission or, maybe you are just starting to consider your options, the

process of design and build is a challenge - but don’t be daunted by the technicalities, there are hundreds of professionals and experts who will be available to advise you, or undertake the work, at every stage of your build project. It can be a frustrating time when you are sinking money into a hole with no visible rewards but, planning and foresight should help everything to run smoothly.

It is important to investigate the ground quality of a plot before you purchase. Check records and ask around locally to make sure that it doesn’t have an unsavoury history. For example, an infill site may be an expensive option because it is like that it will require piles. A proper survey with test bores holes - to assess the type and quality of the ground - is sensible precaution and should ensure that there are no surprises when the digging starts.

As you think about the subsoil you should also look at the landscape of the plot. Trees close to a substructure can affect foundations, while a slope may mean additional work in levelling off the site, or a decision to build with more expensive stepped foundations. Tree roots may damage foundations as they grow. Certain species are potentially worse than others. The enormous amount of water that a tree takes out of the land around it can change the balance of the subsoil. If it is necessary to fell a large tree before you build, you should allow at least one year for the land to settle before you start work on the foundations. Where possible, try not to remove established trees and shrubs - they will add enormously to the character of your garden once the house is complete. You may find that you are not allowed to remove them anyway, as many are protected by Tree Preservation Orders controlled by your local planners.

Don’t forget to plan access for your services - water, waste (foul and surface) gas, electric and telephone ducts - before you start work..

Types of Foundation

Foundations, sometimes called footings or the substructure, are designed to support the walls and the roof and to provide a solid base on which to build the house. The type of foundation chosen will depend on two distinct variables: the total building load, ie, the weight of the house, and the type and quality of the subsoil. In addition, different weather conditions and soil types may reduce the effectiveness of foundation, depending on the type of soil. Frost heave, is common in sandy soils where water in the ground freezes and pushes upwards as it expands into ice. The reverse problem is drought, which causes shrinking of the land and is most common in clay soils. Both conditions can seriously affect the foundations, causing disturbance and, in extreme cases, cracking of the foundations. They are, however, easily avoided if you dig deep enough - usually to at least two-thirds of a metre.

Foundations are one of the most important and difficult elements of a build and, it is likely that an expert - probably your architect, or builder - will advise you of the best type to choose. The following should provide an insight to what they are recommending.

Strip

Strip foundations are the most common type used because they offer good strength for their cost. They are created by excavating a trench deep enough to avoid damage to the foundation caused by changes in the topsoil. Usually the trench has to be dug until clay is reached. The bottom of the trench is covered with at least 150mm of concrete. If the site slopes, one or more steps may be built into the concrete with forms that guide the depth. Once the footings are complete, cavity walls are built up to finished ground level, either in brick and block or, block and the trench and cavity are backfilled up to finished ground level.

There are three types of strip foundation - traditional shallow strip, as described above, wide strip, used when the load of the structure is high relative to the weight bearing capacity of the subsoil and deep strip (also known as trenchfill foundations). Wide strip foundations are laid in exactly the same way as standard strip foundations except that the trench is much wider and reinforcement is frequently used to economise on concrete. The time that it takes to dig these trenches and build the walls means that wet weather may hamper the process - if the trench floods, its walls may collapse and so it is advisable to try and complete the work during a dry spell!

Deep strip or trenchfill foundations have narrower trenches filled with concrete to within two brick courses of finished ground level. Accuracy is important if walls are to be central and although materials for this type of foundation are more expensive, it is much easier and takes considerably less time to construct. Deep strip foundations can also be reinforced and they are accepted as a suitable alternative to wide strip foundations in soft clay subsoil. They should be at least 400mm wide and 900mm deep.

Raft

Raft foundations cover an area at least the same sizes as the base of the building. They are used on soft compressible subsoil such as soft clay or peat. However, it is important that they are well reinforced to resist the effect of ground movement and the raft should also be constructed with an apron edge so that it doesn’t slide.

On a recent self build I visited, it was calculated that on the raft foundation alone there was a weight of 400 Tons only counting the crushed limestone and concrete.

Pile

Pile foundations are used to support buildings in subsoil conditions such as shrinkable clays, infill or waste tips, slopes and sites with a high water table in a poorly drained region. They are a substitute where conventional foundations would need to be so deep that they would be uneconomical. In effect, a basic pile foundation is a series of stilts or columns, which rest on a solid, load bearing layer of the soil up to 4m below the surface. Beyond this depth, the expense usually becomes prohibitive for small build projects although there are self-builders that have completed a project to budget with piles going down 10m. The piles may either be precast, or cast in situ, ie: on the site.

Site Preperation

So you've bought your plot and are about to embark on your selfbuild. But where do you begin?
The first thing you can do is clear the plot. After all, you don't have to have much expertise to tidy up the site.
Before felling any large trees, make sure it is okay with the local planners. Some are protected. Also try to understand some of the "locals around your plot". If you upset them or they find out that you are going to cut down some of the trees, there will be a Tree Preservation Order TPO on them very quickly. An acquaintance told me that he had told a friendly neighbour that he was going to put in for planning permission in his garden and before he could apply there was a TPO put on a tree which made planning permission impossible.
Remember, a chainsaw is a potentially lethal tool, capable of inflicting horrible injuries. If you have never used one, it is essential you undergo some training.
For large trees it is probably advisable to call in a tree surgeon. If you have a log burner or open fire, the money can probably be recouped by getting him to log up the felled timber.
Don't cut large trees down to ground level. Leaving a couple of feet of stump exposed makes it easier to pull out the roots with a JCB.
The removal of trees can create problems in clay soil. Leave it as long as you can before laying your foundations to allow the ground to come to a new equilibrium. If that's not possible you may have to dig deeper foundations to allow for what is known in the trade as "heave".
If there is debris on your site, it will be well worth hiring a skip. These vary considerably in price so it is worth shopping round. If you live close to a refuse site, it may be worth investing in a tow hitch for your car. You will congratulate yourself later when it comes to buying materials.
Asbestos was a common building material for old sheds and outbuildings and this needs special treatment if you find it on your plot. Talk with your local council for disposing of it correctly.

Before you call in the builders or order materials, make sure you have planned where you will put everything during the build. Is there proper access to your plot? What size lorry can deliver on site? Allow plenty of room for lorries to manoeuvre for unloading If you don't yet have a proper driveway, it may be worth putting down some hardcore, particularly if you have clay soil. This is essential in winter. You can do this yourself, either by hand or by hiring a mini- digger from a plant hire shop (around £50 a day). When organising your site, don't forget to allow about a metre around the footprint for scaffolding. Bricks and blocks, particularly on smaller sites, should be delivered early and stacked at the back of the plot. The front will be cluttered enough later on.
The rear of the plot can also be used for 'spoil' - unwanted soil and rubble. This can be used later when you are landscaping your garden. Remember though, that a JCB will still need to have access to get to the rear later - unless you want to move it all by hand!
Another thing you can do to make life easier before things get too hectic is to erect a sign so that all lorries know exactly where your plot is. Once again, it sounds like common sense but it is amazing how many deliveries end up at the wrong address. And let's face it, shifting a couple of thousand bricks is no fun. When building my house I always faxed a site location plan to the supplier, it makes sense to have the delivery driver "happy" when he arrives on site, he will then tend to put materials where you would like them to go.
One of the best investments a selfbuilder can make is in a cement mixer. For around £200-£300, you have access to all the cement/concrete/mortar you will ever need. Besides, most bricklayers will expect you to supply one and the fuel for it (petrol or diesel).
If you don't plan to live on site, security is another thing to take into consideration. An on-site lock-up is essential. This can be a purpose-built shed (for lots of gear), or a simple garden shed which can be used later. You may even opt to build the garage first. Depending on your involvement in the build, you may want an on-site office with a telephone if you plan to manage the build yourself. This could incorporate a lock-up. You will have less of a need for a lock-up if you intend living on site. The most popular choice for on-site living is a static caravan. Providing your plot can accommodate it, without impeding the builders, this is a cheap answer to both your accommodation and site security problems.
Of course, to have a van on site you will need some services - usually at the request of the local council who will be concerned about what you plan to do with your waste in particular. They will require to know this before granting planning permission for an on-site van. A septic tank may be one of the first things to sort out.

You will also have to allow for other services, such as water and gas pipes, electric and telephone cables, and soakaways to be installed. Don't go blocking their routes with bricks, topsoil etc.
It is probably best to wait till your shell is complete before getting water and electricity laid on. Working on your site over winter can bog you down - literally- so don't be in too great a hurry to get things on the go. Even a JCB will make a mess of your plot in the wet. I bogged down my JCB the first day that I bought it. Wait a few months till the spring and life will be a great deal easier, and cheaper in the long run. A JCB is usually essential to prepare your ground for the foundations. You can hire a JCB and driver for under £20 an hour (cheaper if you shop around). They can strip off the topsoil and level off the ground. By taking the topsoil off the whole site and putting it to one side, it will mean your garden will be free of building debris when your build is over.
You should now be in a position to mark the corners of your house with corner pegs. The house should be placed in the exact position stipulated by the planners(They can later insist that the house is pulled down and rebuilt if it is not). And, of course, the layout of the house should be in accordance with the approved plan.
Marking the footprint of the house with corner pegs may seem perfectly sensible but when the digger comes to excavate the foundation trenches the pegs will be ripped out. Builders solve this problem with profile boards. These are boards fixed a metre or more away from the position of the trenches, on which the line of the wall can be marked.