Some roof leaks are harder to find than others. But leaks in the field of slate, away from flashings, or other obstacles, where there are no visible signs of a broken or missing slate are almost always caused by a failed or rusted bib.
The potential for the bib to fail is one of the two flaws in the “nail and bib” method of slate roof repair. The other significant flaw is that when nailing the slate in place, the two slates where the nail is driven is, are invariably damaged.
On the other hand, a slate repair using a slate hook will never fail. Of course, one can’t use a slate hook in every situation, but where it can be used, it should be used. I once heard of another contractor say he didn’t use slate hooks because they “stain” the roof, yet he had no problem install big, galvanized #100 snow guards that certainly left a much greater impact.
But I still don’t buy that theory. Slate hooks leave very little visual impact on the roof, especially galvanized hooks. Some like to use the stainless steel version, but I find them jarring on a sunny day.
The photo above is a zoomed in picture of a galvanized slate hook. Kind of hard to see, and given the location of a hook – in the keyway, any run off staining is virtually non existent — unless it is the zinc killing off roof mold, and then only slightly.
That leak where no slate are visibly broken is likely an old slate repair. If no slate repair can be found, there could be another cause — read about it here