All masonry buildings have one element in common: mortar, the material that separates the individual masonry units, whether they be brick, stone or some other material, such as concrete bricks or even glass bricks. The mortar’s function is not, as most people suppose, ‘to stick the bricks together’, but to hold them apart — and to fill the irregularities on the bedding faces. Gravity is the element that holds masonry together.
At some stage in a building’s life this mortar will – hopefully – show signs of age and wear, and will gradually become recessed. The mortar ought to be softer and more porous than the masonry units and will consequently take the brunt of the weathering process; this is more desirable than the alternative situation where the masonry units are softer than the mortar. In these circumstances the mortar will remain sound whilst the bricks erode, subsequently requiring rebuilding rather than the relatively simple task of renewing the outer portion of the mortar joint, the process known as repointing.
It is usually necessary to repoint when the depth of the open joint is approaching the thickness of the mortar bed. The work is generally straightforward but labour intensive, and though materials are cheap, the ultimate cost of employing a builder may be considerable, so wholesale cosmetic repointing may not only be unnecessary but also expensive.
One advantage of repointing when necessary is that the preparatory works of raking and cleaning out the joint are minimised. At this point a few words of caution are in order regarding the use of grinders to clean out the joints. Even with great care exercised in their use, slips and over-runs are inevitable and will leave permanent unsightly scars in the masonry. The only time a grinder should be employed is in removing modern over-strong cement-based pointing where the conventional chopping out with a plugging chisel will cause more damage. Here the correct technique is to put a single thin cut in the middle of the joint to the depth of the cement mortar, then, with a sharp 2 1/4” bolster, gently tap the remaining mortar towards the cut groove. This will minimise damage to the adjoining brick faces.
When undertaking repointing it is customary to begin the work at the top, gradually working down the wall. This ensures that all the dust and damping down is below any completed work. Preparation, as mentioned, requires cleaning the joints out to a minimum depth of the mortar thickness. On brickwork this will be 3/4” or more, and most importantly the cleaned-out face needs to be a square face, not concave. It takes a little more care and time but does ensure sufficient hold for the new mortar. Clean the joints out with a hand brush and then give a good soaking of water with a spray or hose — the objective being not just to wet the surface, but to create a reservoir of dampness within the wall to facilitate even curing and an even drying out of the new mortar.
On dry and porous brickwork this damping process may need repeating. Once the surface has dried, the wall is fully prepared and ready for repointing.
Choosing an appropriate mortar mix can be a fraught process and needs to be considered on an individual job basis. Generally it is aesthetically pleasing and sensible to replicate the original mortar in colour, texture and strength, and this may mean mixing different sands together for the desired result. For ensuring consistency of mixes, use a gauging measure, which can be any appropriately sized container, and don’t forget to make a note of the mix proportions!
Most Victorian or earlier buildings will be built with lime mortar and, whatever some modern builders say, the use of cement in repointing mixes is generally injurious, causing accelerated decay and damp problems. One characteristic of mortars containing cement is their vulnerability to salt and even sulphate attack, and whilst this sounds esoteric, the destructive effects of salt attack are all too visible in roadside buildings that are splashed by salty water in winter by salt intended for the highway. Mortars containing only lime, either putty or hydraulic, and sand aren’t susceptible to this damage.
The mortar used for pointing needs to be firm but workable and pushed firmly into the prepared joints without leaving any voids, and then allowed to firm up before the chosen finish is worked. With all mortars, whether lime or cement, it is preferable to have a slightly open texture finish rather than a dense, smooth ‘ironed’ finish. This might be one of the reasons why the simple ‘rubbed flush’ joint, where the filled joint is gently rubbed with a stick or rubber before a final light brushing removes any surplus mortar fragments, is considered so attractive.
Finally, care is required in protecting the repointed areas from rapidly drying out. Occasional spraying with water is most effective. The unwanted effects of heat, cold and rain can generally be avoided by tarpaulins or hessian hung in front of the wall.
Successfully completed repointing should last 50 or 60 years.