External Walls And Foundations

Blockwork extension

Good foundations are absolutely critical to any build. There are a variety of different methods in common use today and which one you use is likely to depend on time, cost and the ground conditions. External walls are these days almost certain to be double-skinned, i.e. with a cavity in between them to provide better insulation than a single wall.

Types of Foundation

As long as the ground conditions are reasonably good, a modern foundation is likely to use either deep-strip or trenchfill construction. Both of these rely on a trench being dug to a depth of around 1m with straight, plumb sides and the bottom cleared of all loose material. To preserve the condition of the trench for laying concrete, it’s best if the trench digging and concrete pouring can be done on the same day. This allows less time for the condition of the trench to deteriorate.

Deep Strip

With deep strip, a layer of concrete to about 25cms is laid at the bottom of the trench. As soon as it is cured the inner and outer walls are started, usually with building blocks for the inner wall, which won’t be seen, and more decorative bricks or stone for the outer wall. Once the walls have reached a couple of courses above the ground level, the damp-proof course (DPC) is laid and the construction continues up to the required height.


Trenchfill is more expensive than deep strip but gets the build off to a quicker start, as the whole trench is filled with concrete. The building of the two walls starts off at ground level rather than below ground, but otherwise, wall construction is the same.


Two other methods of foundation construction are used where there are problems with the ground. Raft foundations are used where there may be a risk of ground movement, and the whole floor area of the build is excavated. This large trench is then filled with hardcore and then concrete, providing a ‘raft’ that will move as a whole with any ground movement.


If the ground is soft and harder ground is a long way down under the earth, piles can be used to reach the harder ground. Bridging pieces of concrete are then used to link the tops of the piles together to provide a solid platform for the walls. A specialist firm usually carries this out in conjunction with a structural engineer.

Brick Choices

Once the foundations are in place, wall building is relatively straightforward. The materials that you choose for the external skin of the double wall are largely a matter of choice, governed by cost and the required look.

Be aware that there may be some planning restrictions on your choice of brick if you are in an Area of Outstanding Natural Beauty, National Park or some other specially protected area. In the Cotswolds, for example (an ANOB), stone is demanded unless you are building in one the centre of one of the towns where there is already a good proportion of brick buildings.

Final Check

Of course, it’s better safe than sorry when it comes to planning and building, so you are strongly encouraged to check the latest regulations directly with your local planners, as they frequently change.

See Also
Bare walls
Internal Walls And Ceilings
Internal studwork
Re-arranging Interior Walls