With a Slate roof we can always tell when your roof is getting to the final stages of its life though, and this will be evidenced by previous attempts at securing your Slates back in place using lead tags. But the one tell tale sign that you are fighting a losing battle and throwing good money after bad at the problem will be the sagging of your horizontal slate lines. With a new or good slate roof, the bottoms of your slates should be in a nice straight line as you look down the roof from one side to the far end. When these lines start to deviate and show sagging and Slate slippage it is down to the steel nails holding your Slates in place having eroded and failed. This problem doesn’t get any better and is it false economy to keep throwing your money at it?