given to fixing nails that start to rust away. one indicator of a slate roof getting close to its use-by-date is when symptoms of ‘nail sickness’ start to appear, such as when the lower edges of slate appear twisted or don’t line up properly. although it is possible to replace several roof slate tiles suffering from nail sickness, bear in mind this is just a short-term measure. roofs suffering from nail sickness will eventually need to be completely replaced. 