Masonry walls will quite literally have their history written in stone, from the original design intent to later alterations. What you do when you encounter an unexpected surprise is very important.

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In this instance we removed cement harl from a sandstone wall to discover a legacy of historic alterations. Windows had been filled in and new ones opened up.

There is an important observation to be made hear in terms of conservation philosophy – building are always undergone change and evolving. Presenting them as a pristine example that existed in one particular period is perhaps a deceit.

While it gave us a deeper insight into the history of the building and helped explain some unexpected issues elsewhere, these defects presented us with a technical challenge.

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The previous cement harl had been 30-40mm thick in places. To provide a weathering to the cement harl at the wall head, the contractors who made the alterations added a cement coating over the copings, to extend the width of the cope outward, beyond the depth of the harl. Over time this had weathered. It was covered with lichen and mossed so it was impossible to identify and diagnose the problem fully until we had a scaffold in placed and could physically touch the walls and copes. Overtime this cement coating had begun to crack and fail, allowing water into to wall head. This all had to come off. 

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With the harling off, and the legacy of historic alterations revealed, we could see that we were left with masonry that was neither structurally, nor weathertight. The bonding between stones is weak and we can see that there have been difficulties “toothing” in the masonry units. Effectively we are left with vertical cracks running through the wall where newer stone met older stone. These deep openings run right through the depth of the wall creating pathways for moisture ingress. As they go right through from front to back, it is difficult to fill them effectively, without grout pouring into the gap behind the lath and plaster internally.

Our solution was multifaceted, introducing crack stitching to improve the bonding of the masonry. To fill the voids, we developed a method for deep packing with long slate slips angles to through water to the outer face. Finally, a new lime harl coat will be applied over the face of the wall will give some added weather protection.

The key message is never to underestimate the potential surprises that will be encountered in a masonry repair. With a lot of experience, photographs and drone surveys may give you a clue as to what will be encountered, but until you get your hands on the stone, you just wont know.

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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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