Summary: Foundations for a building or garden wall: Marking out, excavating and laying strip footings.
Foundations are crucial for any building; the foundations of the building take the weight and distribute it to the soil below, preventing it from sinking down into looser uncompressed soil and increase the strength and durability of the building. Foundations by law are required to meet certain standards.
Before you can start laying bricks (see Building a brick wall), you will need to dig foundations for the wall. These consist of trenches filled with concrete and are called strip footings. The depth of foundations required depends upon the wall height and soil type. To build a wall higher than 1.2m (4ft) you must seek professional advice.
Marking out and excavating strip footings
- Walls up to 1m (3ft 3in) high will require a footing at least 150mm (6in) deep (for a single-skin wall) or 250mm (10in) deep for a double-skin wall.
- If the soil is clay, then a further 100mm (4in) should be added to the depth of the footing.
- The footing should finish 150mm (6in) below ground level and even further in extreme climates, where protection is needed from frost damage.
- The footing width should be 200mm (8in) wider than the wall -100mm (6in) either side.
- On loose, sandy soils, however, the footing should be 400mm (16in) wider than the wall.
- For walls only three or four courses high, shallower strip footings or concrete blocks may well be more suitable.
- Mark out the area of the wall on the ground with string attached to pegs or by using profile boards.
- Use a spade to cut into the soil directly underneath the string and dig out a trench to the required depth.
- If the subsoil is particularly loose, then dig down a little further and fill the space with compacted hardcore.
Laying strip footing
Once you have dug the trench you will need to lay the concrete footing.
- Start by knocking in a stake at each end of the trench down to the footing depth. Insert more stakes at 1m-2m (3ft-6ft) intervals between them. Ensure all stakes are driven into the ground vertically.
- Moving up the trench, position a spirit level across the top of the stakes to check they are level. Make any required adjustments.
- Mix the concrete to the ratio 1 cement: 5 ballast, adding water to produce a consistency that should pour easily without being too wet.
- Shovel the concrete into the trench up to the top of the stakes. Smooth the surface with a metal float.
- Leave the concrete to harden for 3 days before you start to build the wall.
How to build strip foundations video
Summary: DIY video showing you how to construct strip foundations.