A wall is only as strong as its weakest point, and the lower this point is in the wall the greater the potential for collapse. Poor foundations are the source of most serious collapses, and no amount of careful building higher up can correct problems at the base. With between one and two tonnes of stone in every metre length of wall, the foundation must be wide, solid and firm, with an even distribution of stone sizes to resist unequal settling.
Lining out
As the following information relates to rebuilding, there will be an existing foundation trench left by the removal of the old foundation stones. It’s best not to disturb this too much, as it will be very well compacted by the weight of the old wall.
- First remove any loose soil or stones from the old trench, piling them neatly to each side. Then using pegs and lines, set the lines about 1″ (25mm) above ground level.
- ‘Nick out’ along each line. Face the line, holding the spade vertically with the back of the blade about 2″ (50mm) from the field side of the line, and parallel to it. Push the blade down to the existing trench level using the foot if necessary. Do this along the entire line, and then work along from inside the line removing the nicked out section level with the base of the foundation trench, leaving a right angled rather than a rounded profile to the base.
Digging and profiling the trench slightly wider than the proposed foundation ensures you have space to maintain the correct width, and makes it easier to set the stones in line.
Placing the footings
- Set the lines about 1″ (25mm) higher than the proposed top height of the foundations. If the foundation stones are fairly even-sized and fit within the trench, you can use pegs and line. Where the foundation stones are more blocky or irregular, you will need to set the batter frames up at this point in order to get the line high enough.
- Use the biggest stones in the foundation, except those which would make good throughs, or which you want to reserve for wall heads or other features. Ideally the stones should come up to but not above ground level, but any stones larger than this should be used here, rather than higher in the wall.
- Be sure that each stone sits on a good bed or base. Check for steadiness by standing on them. Avoid using wedges as much as possible, since wedged stones are likely to tilt or slip as the wall settles. Minor wedging at the centre of the foundations is permitted, but major irregularities in the base of the stone should be fitted by ‘sculpting’ the trench.
- Lay the foundations in two parallel lines along the foundation trench with their long edges into, not along, the side of the wall. Very large stones may have to be placed along the wall, to avoid having to use stone which is far too small on the other side of the foundation.
- Make sure that the inside edges of adjacent stones are touching for as much of their length as possible, selecting complementary shapes wherever appropriate.
- Alternate longer stones on opposite sides of the trench, rather than grouping them on one side only.
- Try to match adjacent stones for height to make building on them easier. Small differences can be accommodated by digging in the larger stone. Larger differences are accommodated during the subsequent building.
- Most professional wallers leave a gap of at least 1″ (25mm) between the inside edges of the foundation stones, to promote drainage within the wall. Others maintain that water will drain through anyway, and that it makes a stronger foundation to butt the stones up as close as possible, effectively making a solid foundation across the width of the wall. In practice, irregularities in the stone will create spaces, but try and make them no more than 1-2″ (25-50mm).
- Pack between the two rows of footings with angular, free-draining fillings.
- Fill any gaps between the outside faces of the foundation stones and the edge of the trench to neaten the base and prevent the chance of twisted ankles.
