A recent investigation revealed an oversight in roof design. The construction under investigation had wide lead gutters behind masonry parapets. These gutter formed part of the roof and are in effect cold decks over thick thermal insulation. Other parts of the roof were provided with interstitial ventilation following current guidance. The gutters were not.
The natural durability of lead under normal conditions is due to the formation of protective insoluble salts which are formed on the surface. Moisture and carbon dioxide in the atmosphere combine to form weak carbonic acid and this reacts with bright metallic lead during periods of rain or overnight dews to form the normal grey patina typical of lead roofs. This thin continuous surface layer protects the metallic lead from further attack. On the underside of lead sheets these protective layers may not form.
The natural durability of lead under normal conditions is due to the formation of protective insoluble salts which are formed on the surface. Moisture and carbon dioxide in the atmosphere combine to form weak carbonic acid and this reacts with bright metallic lead during periods of rain or overnight dews to form the normal grey patina typical of lead roofs. This thin continuous surface layer protects the metallic lead from further attack. On the underside of lead sheets these protective layers may not form.
