Five different types of foundations for a self-build home

Foundations are there to ensure long-term structural stability, to prevent any uneven settling, and safely transfer the building’s weight to the soil. There are other benefits, like protection against ground moisture, pests, and soil movement from freezing and thawing, but in short, they are a very important part of your build. 

So what type of foundations should you go for in your self-build?

There are four common foundation options:

1. Standard strip

2. Trench fill

3. Deep trenchfill

4. Piled and ringbeam

5. Reinforced concrete raft

We’re here to talk you through what these are. 

Standard Strip

For most soil conditions, this is the best and most cost-effective way to construct your foundations. A trench 450-600mm wide is dug 1-1.2m deep beneath all external and loadbearing walls. In the bottom of this a layer of concrete, at least 225mm thick, is placed and upon this the foundation walls are built in blockwork with two skins to external walls and a single skin to internal loadbearing walls.

The costs for a building roughly 10x7m with one cross wall would be around £8,750, and it would take the average team of groundworkers a maximum of 10 working days to get to oversite level.

Trench Fill

Where the water table is high or the trench sides are unstable, it is often better to revert to this type of foundation. The trenches are essentially the same except that, instead of just putting concrete in the bottom, you fill them almost to the top.

This means that you can be out of the ground in one day, and it also means that the amount of below-damp proof course blockwork is reduced. The costs (for the same size building) rise to around £9,750 but the time taken for the job should be no longer and perhaps even shorter.

Deep Trenchfill

Where heavy clay is present in the vicinity of trees, it is necessary to lower the foundation below the level at which a tree's thirst for water is active. This can mean digging to a depth of 3m and filling the trenches with massive amounts of concrete. At these levels special care needs to be taken and shoring might be required.

Additionally, it is usually necessary to insulate the foundation from any ground movement by lining one or both sides of the trench with a compressible material plus a slip membrane.

The costs can rise to as high as £16,000, and it will take an extra week. At some stage it is no longer economically or physically possible to dig any deeper by conventional means. Additionally, the cost of sending spoil away becomes prohibitive and it is at this point that it becomes economically viable to switch to a piled foundation.

Piled and Ringbeam

This kind of foundation is used in situations where the top layers of ground have poor bearing capacity and good bearing can only be found at deeper levels. The piles are driven or bored into the ground to support a reinforced concrete or stainless steel ring beam (ground beam) spanning from cap to cap, which supports the house walls.

This ring beam can be cast in situ or delivered as a prefabricated concrete or steel unit. Systems are also available which combine the ring beam with the floor as a stable platform upon which the house can be built.

A piled system for a 10x7m home will cost £5,000-£25,000 depending on the depth at which bearing strata is found and the number of piles required. The time required for an in situ cast ground beam type is approximately three weeks, while for prefabricated systems, it is no longer than 10 working days, provided everything is properly coordinated.

A less expensive alternative to piling can be achieved using helical screws. If soil conditions and structural designs permit their use, these heavy steel screws can be driven into the ground to replace more expensive augured piles. Helical screws can also be used to underpin failing or inadequate foundations on existing properties.

Reinforced Concrete Raft

This system is used where the ground has good bearing but is inherently unstable due to geological or mining conditions far below the surface. A large hole is dug and filled with consolidated layers of hardcore. Upon this a specially designed reinforced-concrete raft is cast, and it is this raft or slab that supports the whole house.

It uses a lot of concrete, and it needs to be carefully designed, but in areas where it is common, most groundworkers see it as “standard”. The time taken is 10-15 working days for projects valued between £15,000 and £25,000.

In most cases homes built with either timber frame or masonry will share precisely the same foundations, and no distinction will be made between the two. In a few cases, where loads are critical, lightweight timber frames or other prefabricated structures may be able to have lesser foundations. This is particularly applicable with a piled and ring beam foundation, where it might sometimes be possible to reduce the number of piles.

All foundations should be accurate to the plan, but in the heat of battle that is not always possible. However, if you are building with a timber frame, the foundations should be level to within 20mm and square to within 12mm.

Still not sure on the right option for you? For a specialist build, it’s always best to speak to an expert, like us at Mayflower Mortgage. Book in for a FREE, no obligation call today, and we will be happy to chat through your plans and get you on the right path.