Freezing and Thawing Damages
Use binoculars to see what's going on in the fall, before the freeze/thaw cycles; in the spring, see what the freeze/thaw did to your roof, checking for loose, cracked or missing slates.
Check for Leaks
It is also important to look in the attic for evidence of leaks, and to make sure gutters and downspouts are in good working order. If there is a leak, it is often the metal flashing at the seams, valleys and ridges of the roof is the real culprit. Slates usually outlive their flashing. Even copper flashing, the best in the business, typically lasts only 60 years (a spry and youthful age for most types of slate). Often, slate and wood shingle roofs are removed because of problems with flashings.
Sindelar, who coordinated a City-sponsored workshop on slate and shake shingles in 2001, cautions that before replacing a slate roof with something else, "You have to look at the economics of it." Because building codes prohibit putting new roofing material over a slate roof, the existing slates must be removed before their replacement may be laid down. That cost should be factored in.