Lead Roofs and Statuary: Understanding, Monitoring and Conservation
Treating corrosion and staining in historic lead work
Lead is certainly the best and lightest covering, and being of our own growth and manufacture, and lasting, if properly laid, for many hundred years, is without question the most preferable. (Sir Christopher Wren)
Sir Christopher Wren’s acknowledgement of the longevity of lead ignores the fact that it can be subject to attack which can either drastically shorten its life or radically change its appearance. Historic England, currently and in its previous guise as English Heritage, has been researching two problems; underside corrosion and topside staining.
In the latter decades of the twentieth century, a significant number of historic lead roofs were suffering corrosion on their underside. Not only was this shortening the life of the lead, it was encouraging specifiers and clients to turn to replacement materials, some of which were not in keeping with the character of the historic buildings. This was a major problem for conservation bodies because in many cases lead had been the only covering used for centuries, and on many complex roofs it was the ideal material. So in the late 1980s, the Building Conservation and Research Team at English Heritage commissioned a major research project to understand the causes and mechanisms of underside corrosion and find a solution.
Lead has one important weakness. It can be attacked by pure water, such as condensation.
When lead sheets are first laid on a roof, white staining of corrosion will appear on the top side, but gradually the lead combines with carbon dioxide in rainwater to build up a protective layer of lead carbonate. On the underside, where no carbon dioxide is present, any condensation will begin the corrosion process. This gets worse if there are acids present, coming from woods such as oak and from glues commonly used in hardboards and plywood.
Industry’s solution was to change the design of the roof to accommodate a ventilated structure by lifting the plane of the slopes by 200 millimetres, causing all manner of detailing problems on many historic roofs.
What was needed therefore was a coating applied to the underside that could resist the onset of corrosion in order to maintain the existing design of the roof.
The mix was improved with the addition of a chalk-based paint which produced ‘chalk emulsion’, providing a far more adhesive and effective coating.
Both systems have been monitored over the last twenty years on sites where the corrosion problem is particularly bad and so far they have proved effective at resisting corrosion. We will publish guidance later this year on the Historic England website.
Solutions to the blotchy appearance of many cathedral and church roofs will remain elusive until we understand how and why it is happening.
Over the last twenty years the distinctive light-grey colour of lead roofs and statues has started to turn purple/brown, either in small patches and stripes or over the whole surface. The change is most prevalent on aspects exposed to direct sunlight, particularly in rural and coastal areas, and it affects new as well as centuries-old lead. At present it does not appear to reduce the longevity of lead; the main problem is the blotchy appearance that now afflicts many prominent cathedral and church roofs as well as valued artworks. Solutions will remain elusive until we understand how and why it is happening.
Many theories have been suggested, ranging from the greater use of fertilisers to aeroplanes discharging excess fuel. Testing to date has shown that the composition of lead was not a factor (old lead has more impurities), nor was surface topography responsible, but rainwater running over a surface appears to stimulate staining.
Historic England commissioned laboratory testing at the University of Aberdeen, which confirmed that a typical protective layer on the lead surface comprises a complex mixture of different lead compounds including plattnerite (beta lead dioxide). It is this mixture which produces the purple/brown stain. On the samples provided from various historic buildings, it was clear that this was not a corrosion product but a conversion of one existing compound to another.
Why it is happening is not clear. A mild acid will remove it, so possibly today’s cleaner air has reduced the amount of dilute sulphuric acid which might have removed the staining. So far it has not been possible to recreate starting conditions in the laboratory to simulate the creation of the stain, but that will have to be the next stage in unlocking this mystery.
About the author
Chris Wood, BSc, MSc(Oxf), GradDip(BldgCons), AA, MRICS, MRTPI, IHBC, FSA
Senior Architectural Conservation Advisor, Historic England
Chris was the Head of the Building Conservation & Research Team at English Heritage, and is now a part-time Senior Architectural Conservation Advisor with Historic England. He is retiring later this year after 26 years with English Heritage and Historic England. He has led dozens of research projects prompted by casework priorities or other urgent issues affecting historic and traditional buildings. He has also written extensively on many materials, including lead, the research on which he managed. He has worked in private practice architecture and was a conservation officer for over a decade.
English Heritage 2013 Practical Building Conservation: Roofing. Farnham, Ashgate Publishing, 417-443
Teutonico, J M 1998 ‘Metals’
English Heritage Research Transactions 1, 21-72