A condition where a roof begins to fail not because the shingles or slates are worn out, but because the nails holding them have corroded, loosened, or completely degraded. This often happens in older slate roofs fixed with iron nails. Once the fasteners fail, tiles slip and the roof loses its weatherproofing.
Heard on the job
The slate is still good for another fifty years, but it's got nail sickness so we've to strip the whole thing.
Insider tip
Look for slipped slates or rust streaks on the underlayment as the first signs of nail sickness.