Adjustments to Gauge Below Ground
In theory, if both ends of a wall are erected to the same gauge from a horizontal, base level then every course of the wall will be level throughout its full height. In practice, however, a bricklayer cannot rely on a foundation being perfectly flat and/or level so a reference point must be established in the form of a datum level.
In terms of site practice for long walls, a datum level peg is fixed, typically, at the height of the horizontal DPC (on a building no less than 150mm above finished ground level) and this datum level is transferred to pegs at both ends or all corners of the wall or building being constructed.
For small walls or piers, where the length of the wall is within the limits of the length of a spirit level (or, at worst, a straight-edge), only one gauging point is required at one end. Level can be easily transferred from this, so only one datum peg needs to be positioned adjacent to the wall being constructed. Where a small wall or pier is completely isolated and remote from any other brickwork, the brickwork can simply be gauged straight off the foundation concrete as there is no practical or aesthetic requirement to marry up the gauge or level of the new work to anything nearby.
For details of the establishment of datum, its importance and methods of transferring levels, see Chapter 6.
The idea is that the brickwork, as it is built upwards from the foundation, finishes level at a point that coincides with the datum level. Any adjustments in terms of increasing (‘picking up’) or reducing (‘grinding down’) the thickness of bed joints to achieve this objective must take place below ground, where they will not be seen!
Once a datum peg has been transferred to each corner or end of the wall, a spirit level and gauge lath can be used to determine the extent to which gauge needs to be adjusted below ground (see Fig 125). Where the brickwork falls short of datum by less than half a course (37.5mm), it is easier to ‘pick up’ the thickness of the bed joints to make up the difference. This adjustment should be gradually spread over a number of courses rather than being done with a couple of huge bed joints, since joints that are overly thick could squeeze out or sink with the weight of any brickwork built immediately on top. For this reason, where the shortfall exceeds half a course – for example, where it is around 50mm – it is better to add in another course of bricks and ‘grind down’ the joint thickness to accommodate the extra course, rather than ‘picking up’.
A simple strip foundation would require a minimum thickness of 150mm and assume a distance between the top of the concrete to finished ground level of 1000mm. Assuming that datum has been established at DPC level of 150mm above finished ground level, the overall distance from datum to the top of the foundation concrete is 1150mm below datum. This figure (1150mm) divided by a brickwork gauge of 75mm works out to 15.33 courses of brickwork from the top of the foundation to datum level, which clearly does not work to gauge – fifteen courses would finish short and sixteen would finish too high. Accordingly, the bricklayer will have to make adjustments to gauge by either ‘picking up’ the bed joints in fifteen courses to gain 0.33 of a course (approximately 25mm) as the brickwork is built out of the ground, or by laying sixteen courses instead of fifteen and ‘grinding down’ the joints by 50mm to lose 0.67 of a course.
The adjustments to gauge/bed joint thickness must be made below ground level where they are not seen, so no adjustment is possible in the two courses between finished ground level and DPC/datum level. In practice, the bricklayer’s choice is either to ‘pick up’ 25mm in thirteen courses or to ‘grind down’ 50mm in fourteen courses. The former will require every bed joint below ground level to increase in average thickness by a fraction under 2mm. This should not prove too difficult to achieve consistently and it is very unlikely that the slightly thicker bed joints will start to squeeze out as the brickwork is built higher. The latter option, on the other hand, requires each bed joint to be ground down/reduced in average thickness by 3-4mm. This would make for very thin bed joints and would be more awkward to achieve successfully. ‘Picking up’ the thickness of the bed joints slightly is clearly the easier and more practical option in this case.
Whichever method is adopted, the gauge should be regularly checked and re-checked as the brickwork is built out of the ground, to ensure that the bed joints can revert to the correct thickness of 10mm by the time finished ground level is reached.
A third alternative, albeit more involved, is to excavate the foundation trench to a precise, calculated level where the brickwork will work to gauge from the top of the foundation up to datum level without the need for adjusting the thickness of bed joints. For more on this, see Chapter 6.
Once the brickwork has reached the datum level – in this case DPC level – then a second peg can be inserted into the ground upon which the gauge lath can be positioned in order to gauge every course above DPC (see Fig 126). The ability to position the second peg assumes, of course, that the foundation trench has been back-filled. If this is not the case, a masonry nail can be inserted into the bottom of the bed joint at DPC level, and the gauge lath can be rested on this. This has a couple of disadvantages: the hole will need to be made good later, and the nail will cause the gauge lath to sit too high by a distance equal to its thickness. While this is a very small amount, some bricklayers prefer to make a small notch in the bottom of the gauge lath to accommodate the nail and eliminate any error.
Fig. 126 Gauging above datum level.