Surprising facts about Lead Sheet Corrosion!

Do not underestimate the damage that interstitial condensation can cause to lead sheet

Lead is usually thought of as one of the most versatile and resilient building materials ever used. It was used extensively by the Romans more than 2,000 years ago in plumbing and construction of the great viaducts. In medieval cathedrals and churches lead was used in sheet form as roof coverings as well as for decorative rainwater gutters, downpipes and hoppers. In fact, lead more often than not, was and is used in connection with water, whether it be as a conduit for water, or as a waterproofing layer. If designed and installed right, lead can be highly durable and can sometimes last for hundreds of years in the most exposed locations. For example, the dome of St Paul’s Cathedral remains a sound example some three century’s after construction.

There are a few key facts that you need understand when constructing with lead.

The first is very well known. Lead has a very high ‘linear temperature expansion coefficient’, in other words it expands a lot when heated and more than most other building materials. For example, it expands almost three times as much as mild steel does under the same thermal conditions. This characteristic can be managed with regular expansion joints and ensuring a slip plane, or a separating membrane, between adjacent materials. Lead is normally fixed with clips or cleats that can accommodate movement.

The second is that there are plenty of other building materials that lead can chemically react with and should not be placed in contact with. For example; steel, aluminium and zinc will corrode lead. It is also best to be cautious when pairing lead with treated timbers, some specific species of wood such as Oak, or synthetic membranes.

The third is surprising; Distilled water is aggressively corrosive to lead.

What does this mean? It means rain water falling from above a lead sheet roof will not rot through the roof for tens if not hundreds of years, whilst; water vapour condensing on the underside of a lead sheet roof will rot through the sheet in fraction of the time. This can be even worse if the condensation is interstitial (forming between the lead and and another material layer) without ventilation to dry it out.

The chemistry behind this subtle difference is complicated. I have borrowed a quote from www.buildingconservation.com to provide a good explanation: “ conditions on the underside of the lead are entirely different, as carbon dioxide in rainwater and the air may be prevented from reaching the surface of the material in sufficient quantities to promote the development of a natural, protective patina. Where a film of moisture forms across the underside of the metal, the lower availability of carbon dioxide allows soluble lead ions to migrate from the surface through the formation of lead hydroxide, which is the first compound to form in the carbonation process. Any subsequent development of a carbonate layer will occur away from the surface of the metal, leaving moisture in contact with the lead. Further development of a protective carbonate layer is therefore prevented.” - https://www.buildingconservation.com/articles/leadsheetroofs/lead-sheet-roofs.htm

Ventilated voids and a vapour check membrane are key design features for lead roofs. The image below shows the extent to which lead sheet can corrode where there is neither a vapour check membrane nor a ventilated void. The adjacent roof that had a ventilated void beneath resisted corrosion much better.