Repointing is a construction technique used in the maintenance of masonry structures. The ‘pointing’ is the external, i.e. visible, edge of mortar joints between masonry units which are typically 10 mm-wide.
When masonry structures are first constructed, mortar is applied as a thick paste which sets hard as it cures, creating a tight seal between bricks and blocks to prevent air and moisture entering into the construction. It can compensate for variations in brick or bock size to produce an aesthetically-pleasing and structurally-sound construction. However, generally, mortar is structurally weaker than the blocks or bricks it bonds, creating a sacrificial layer that is more easily repaired than defects would be in the bricks or blocks themselves.
Pointing on older buildings was often made of lime mortar which, compared to harder cement pointing typically used today, may feel soft or powdery, but this does not mean that it needs to be repointed. Only where joints are badly eroded such that voids have formed which are at risk of rainwater and ice making them progressively worse, will they need repointing. In this case it is important that a similar mortar is used, to prevent erosion of the masonry itself (see the image at the top of the page and see: Lime mortar for more information).
Masonry structures in exposed locations tend to be at more risk of weathering than in other locations. Due to variances in exposure, it may only be small patches of masonry that require repointing as opposed to the whole structure. Regardless of the extent of the repointing, it is important that it should resemble the original (both in type, texture, surface profile and colour) as closely as possible, as otherwise the newer pointing can clash with the old.
The process of repointing begins with preparation of the masonry by raking out the defective mortar to a depth of at least 20 mm. The joint should then be dampened with a fine water spray before fresh mortar is applied to the back of the joint using a pointing tool. The type of mortar to be used should be slightly weaker than that of the surrounding masonry; usually (but not always) a 4:1 sand/cement mix. For softer masonry (such as some sandstones), a 'weaker' lime mortar can be used.
For more information see: Mortar.
Mortar can applied slightly protruding from flush face of the masonry until it begins to harden and then any excess can be scraped off. The profile of the mortar joint (pointing) can be varied depending on exposure or to create a specific visual effect. The most common profiles are; flush (rag joint), bucket handle, weather struck, weather struck and cut, and recessed.