Cross-Joint (Perpend) Width and Vertical Deviation
The terms ‘perpend’, or ‘perp’, is derived from the fact that vertical joints are perpendicular to the bed joints. Cross-joints or perpends should be of a consistent width throughout the wall and a standard cross-joint is 10mm wide. In practice, however, they sometimes have to vary in size to allow for the slight variation in the sizes of individual bricks (or sometimes at the bricklayer’s discretion to accommodate wall lengths that do not quite ‘work bricks’). An ‘industrial tolerance’ of + or -3 mm is considered adequate in dealing with the former, meaning that cross-joints can be as narrow as 7mm or as wide as 13mm and still be deemed acceptable. However, wide cross-joints can look particularly unsightly even when staying within the acceptable tolerance of +3mm, and applying a joint finish tends to make joints look wider still. On this basis, if cross-joints need to be adjusted, it is probably more acceptable to narrow them.
The vertical alignment of cross-joints must not ‘wander’ – a deviation from the vertical of only 5mm in the height of the wall is deemed acceptable. The process of adjusting the width of cross-joints away from the usual 10mm tends to lead to a lack of consistent width and ‘wandering’ cross-joints will inevitably appear, particularly on long walls. Vertically eyeing in the cross-joints as the bricks are laid will undoubtedly help, but it is also good practice to plumb cross-joints at regular intervals along the face of the wall. If the cross-joints are allowed to wander away from a true vertical, the appearance of the finished brickwork is detrimentally affected (see Fig 122). It must be acknowledged, however, that significant dimensional
deviations in the bricks can make it very difficult to obtain a consistent 10mm cross-joint and perfect vertical alignment! Sometimes, the only way forward is to dry bond the proposed wall carefully, and make every effort to maintain the best compromise between aesthetics and staying within the accepted tolerances. It is a question of striking a balance between what is right and what looks right, given the limitations of the materials being used.