Ashlar stonework was designed to provide a smooth polished surface to walls. The mortar joints joists were built to be very thin, almost invisible, only a couple of millimetres wide. It required great skill and craftmanship to achieve such a high degree of accuracy.
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Maintaining and repairing ashlar stonework can present a challenges for property owners. As the mortar deteriorates, water will enter the walls, but the tight joints make it difficult to repair loose mortar. A thin saw bade can be worked into the joint, but it will inevitably end up widening the joint. In practice this is a necessity in order to get fresh mortar packed tight into the joint. Even the mortar must be specified to have very fine grained sand, as the grains of regular sand will be too large to get into the joint.
An advantage of this approach is that there can be greater control over the durability of the mortar and its colour, as the sand content has a significant influence on colour. Getting a good colour match with the stone can ensure that visual impact of the joints remains very low and any difference in joint width is almost imperceptible.
An alternative approach is to try and pour grout into the joints using clay cups. This requires a very fluid and lime rich mortar mix. This will do the least damage to the stone, but there is less confidence that it will flow into all the voids. The high lime content may impact on durability and the colour is likely to be bright white, making the joints very noticeable. This was not the visual effect that the original architects were trying to create.
The difficulties of treating thin joints effectively can lead to a strategy that avoids carrying out repair work unless absolutely necessary.
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To complicate things further, ashlar sandstone was often face bedded to create a very smooth polished surface. This places the grain vertically and makes it extremely vulnerable to splitting, if the stone gets saturated and then suffers frost or salt attack. This can remove the do-nothing approach from consideration. In this situation, it becomes even more important that the joints are closed to prevent rain penetration. The damage arising from the repair will be far less than the damage caused by the weathering and erosion of the stone if left untouched.
So we have three different strategies, each with advantages and disadvantages. As so often in conversation, there is no obvious solution, what is important is to understand the heritage of the building that you are working on, understand the range of options available and their limitations and then to make a reasoned argument for the decision that you make.
In this instance it is important to get professional advice, any repairs that may result in changes in the appearance of a listed building will require listed building consent and will come under scrutiny.
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MAAC studio are accredited conservation architects, PAS 2035 Retrofit Co-ordinators and Principal Designers working with traditional buildings throughout Scotland.
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