Chuck wrote:
I live in Columbia SC area, single story, no dormers, moderate pitch
(you can walk on it being careful.. I don't know the actual pitch)
with asphalt shingle roof in place. I have no visible leaks yet, but
noticed vertical cracks in shingles that run 3 or 4 ft long.

Vertical cracks? Do you mean that there are 4' long cracks running up
the slope of the roof? If so, you're looking at the edges of the
plywood telegraphing through the shingles. You may have a bigger
problem than the dying shingles.

Since you have access in the attic you should get up there and see if
you can see where there are gaps between the rafters and roof
sheathing. To locate the areas in question count the number of tabs
that the worst crack is from the edge of the roof. One tab equals one
foot, then go inside and measure - make sure to allow for roof
overhangs.

R