Slate Roofing: Patination Oil Prevents Leaching
New lead is commonly used on slate roofs for ridge capping and flashing around chimneys. Rain reacts with the lead to form an unattractive, lead carbonate stain that is white in colour. Over time, white leach marks gravitate down the slates and persist for many years.
These stains are unsightly and can be easily prevented by coating lead with patination oil that forms a semi permeable membrane on the lead. If lead is used on a new roof, it is essential to have it treated with patination oil prior to, or immediately after fixing.
A good reference on the use of patination oil on lead flashing can be downloaded from www.britishlead.co.uk/downloads/patination_data.pdf