Darren Tutt, Head of Technical Services, Calder Lead
During my time in this industry I have had the privilege and honour of both visiting and helping out with the leadwork design on some of the most important landmarks in the nation. The historical nature of some of these has meant that a great deal of time has had to be spent on getting things right, as these older buildings are unique in their construction with many intricacies to their outward appearance.
In times gone by, little importance was placed on the substrate design, other than ensuring it was structurally sound and of the highest quality available at the time. In those days, oak planks were commonly laid with gaps in-between. The advantage of this was that plenty of air could get directly to the underside of the lead, allowing lead’s natural protective patina to form. The airflow kept temperatures constant and condensation was a rare occurrence. Even if it did form, it dried out quite freely. Airflow and the ability to disburse moisture is very important in any traditional metal roof build-up.
In modern times, with the advent of improved heating and added insulation, the ability to control condensation is paramount. Cold and draughty historic buildings saw lead roofs last many hundreds of years in some cases, but by adding the ingredients that cause interstitial condensation this can suddenly change.
Interstitial condensation is a crude form of distilled or condensed water formed between the ‘layers’, or interstices, of a building, and it is this which can corrode lead sheet and other metals over time. Couple this with certain timbers that can leach aggressive tannins when wet and corrosion problems can occur, severely curtailing the longevity of a material that has seen the decades pass by.
This is why, today, all lead roofs should be laid on well-ventilated substrates. Doing this ensures that this amazing, traditional material lasts well beyond inferior alternatives, as it has done for centuries. As you can imagine, sometimes the structural changes needed to achieve this are impossible to create without affecting the look of a building, which is not always an option.
Over the years, solutions to this problem were sought, and up until now the only way of protecting the lead from corrosion in these circumstances was to apply a chalk slurry or a chalk-based paint to the reverse of the lead sheet. The idea here was that the chalk would provide the protective patina to the back of the lead, and so providing improved resistance to corrosion.
At Calder Lead we decided to tap in to our own 260 years of experience to see if we could find a traditional, yet effective answer to this problem. The result is our innovative product: Calder Heritage Lead. An easy to use, high-performance product that gives the ideal solution to this modern day dilemma.
Calder Heritage Lead uniquely combines Calder BSEN 12588 lead sheet with a layer of tin on the underside. The tin is plated to the lead sheet during the manufacturing process meaning that that there are no additional processes involved once it is on site, thus saving on both time and labour.
Tin has been used throughout history as an essential ingredient in resisting corrosion as it has an insoluble oxidised film on its surface. This natural oxidised layer protects the tin, and in turn anything it covers. Tin is also relatively unaffected by both water and oxygen at room temperatures and does not rust or corrode in any other way.
The combination of lead sheet with tin provides even more durability than any method currently employed. No works out of the ordinary would need to be undertaken, as a consistent level of protection is achieved over the entire sheet.
As the tin is integral to the lead, complete coverage is guaranteed with no weak spots. Traditional bossing methods remain unaffected and it is as easy to work as standard rolled lead sheet. Lead welding also provides no obstacle, with normal practices able to be used. As the tin coating is on the reverse, the outward appearance remains exactly the same.
History tells us that using tin as corrosive protection has been widespread. Perhaps the most common instance that springs to mind is the humble tin can. Interestingly, these weren’t actually made from pure tin – they were made from tin-coated steel, purely because of tin’s ability to protect the parent metal.
Combining tin with Calder Lead’s BSEN 12588 rolled lead sheet adds to the lead’s already impressive characteristics, making it even more durable in adverse situations. The dual layer is a significant advance in a world were innovative lead sheet products are rare. This material provides architects and specifiers with a robust solution when undertaking works in our amazingly rich and diverse heritage world. We need those buildings to be protected the way they have always been and in this modern age we show that you really can have the best of both worlds, with old meeting new and working in harmony.
This article was published in Roofing Today November 2018
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