‘A Stitch in Time – Guidance Notes in Maintaining your Place of Worship’
Guidance Note 4 – External Masonry
The intention in this guidance note is to give a quick overview of the main issues to consider when reviewing the condition of external masonry, in particular stonework. I propose to go into more detail in future guidance notes on specific issues such as managing salt-damp and repointing walls. In the meantime please take this note as an introduction to the general subject of the care for traditional masonry walls.
Our fore bearers used natural materials to build traditional structures that worked in harmony with their environment. Because stone and early brick buildings used lime mortars in their construction they were able to breath to control moisture and move to manage expansion and contraction of structures.
Massive masonry walls were needed to construct large buildings such as churches. As well as being structural their thick stone and brick walls help to regulate internal temperature.
Repairs to historic masonry walls in recent decades have largely been carried out using cement which is hard, inflexible and unbreathing. This exacerbates salt damp and accelerates the decay of stone and brick, especially when masonry materials are naturally soft. Current practice is now to remove cement pointing and repairs from traditional masonry walls and replace it with lime mortar.
Salt damp deterioration at the base of a church wall in North Adelaide.
Historically lime pointing was only meant to be sacrificial. It was accepted that it would be raked out and replaced every 25 or 30 years. By using lime, which is softer, more flexible and more breathable than stone or brick, the more expensive masonry would be protected from deterioration. This situation reverses, though, when cement pointing is used in place of lime. Walls are then forced to dry out through the masonry which suffers accelerated deterioration as soluble salts are deposited within the surface layers of the stone or brick instead of within the sacrificial lime pointing.
The hard, unbreathing cement pointing here has caused slat damp deterioration to the adjacent blue stone.
Metal fixings embedded within stonework to secure decorative features can rust resulting in ‘oxide jacking’ – the process of expansion that iron undergoes as it rusts, putting surrounding stonework under enormous pressure leading to cracking and disintegration of masonry.
It is very common to have high level decorative stonework supported by embedded ironwork. Where pointing is not maintained and/or where movement cracking occurs and moisture is allowed to penetrate to rust the ironwork heavy stone decoration can fall with potentially dangerous consequences.
Embedded iron rusting and cracking a surrounding stone moulding that is now in danger of falling from height.
Decorative stonework on the spire of an Adelaide church. Note the exposed and rusted iron fixing with a surrounding cement repair. This is where stone features have formally been located and have either been removed for safety reasons or have fallen off.
It is therefore important that stonework, particularly at high level, be inspected at least annually, and at least using high powered binoculars from the ground, to check for evidence of cracks and potentially unstable masonry. While there are hundreds of cases annually of people injured in the UK by falling masonry unlike South Australia Great Britain is not an area prone to earthquakes.
Headlines from 2000 when an Australian waitress was killed by falling masonry in Edinburgh.
Our soils in South Australia often react to the heavy rainfall by absorbing moisture and expanding, whilst long dry spells cause the ground to dry out and contract. Footings beneath the walls of traditional buildings are often small, and sometimes non-existent, and expansion and contraction of the adjacent ground usually results in cracks in external walls that, if not repaired promptly, can lead to water ingress. This situation is often exacerbated by the use of cement to point up masonry walls instead of lime.
As a result of movement cracking has taken place between the hard cement pointing & the stonework. The lack of flexibility of pointing has also led to the cracking of the ‘kneeler’ stone.
The cement’s inability to accommodate movement causes it to crack along the joints and sometimes forces the stonework to split as it is weaker than the cement.
Hard cement pointing causes accelerated erosion of the adjacent stonework, as seen above in a South Australian place of worship where the stone has eroded away leaving the cement joints more exposed.
Another issue with cement pointing is that because it is generally harder than surrounding masonry the brick or stone erodes more quickly as water is thrown off the cement and onto the masonry. In a traditional lime-pointed wall the sacrificial joints are softer and erode first.
There has been a long legacy over the decades since WWII of cement used in South Australia for repairs to masonry walls.
While this guidance note points out the drawbacks in using non-traditional cement it is unrealistic to expect that all owners of historic masonry churches repaired with cement will rush to remove it in favour of lime pointing. In many cases where cement is used to point up hard bluestone its detrimental impacts may be limited.
It is important, though, to understand the tell-tale early signs of masonry deterioration where circumstances may lead to expensive future remedial repairs. If you are in any doubt contact a well-respected heritage architect who has experience in dealing with church buildings to advise on a suitable course of action.