What is a natural slate rosetta?
Rosettas are naturally occurring formations of Calcium or Magnesium Carbonate on the slate surface, often circular or oval in shape. These are formed during the development of the slate layers and are then exposed when the slates are split. Rosettas can sometimes be mistaken for bird droppings.
Rosettas are will not affect the performance or life span of your natural slate roof. Over time they will dissolve away when weathered by the rain, which is naturally slightly acidic. This process will also not affect the performance of your roof.