Freeze-thaw cycling
People assume a colder winter is harder on a roof. It is not — a steady deep freeze is comparatively gentle, because nothing moves. What damages a covering is water that melts and refreezes, over and over, and north Jersey sits almost exactly on the line where that happens most often.
What the cycling actually does
Water expands when it freezes. Any water that has found its way into a hairline crack, under a shingle edge, into the seam of a flashing or into the grain of an exposed board expands with it and makes the opening slightly larger. Then it thaws, more water gets in, and the next night it happens again. One cycle is nothing. Forty cycles in a season is a mechanism.
The visible results are familiar to anyone who has looked closely at an older roof here: shingle edges lifted and no longer sealed down, granules stripped in patches, sealant beads at flashings cracked open, and nail heads that have worked their way slightly proud of the surface. None of them is dramatic on its own, and together they are the reason a covering here rarely reaches the number printed on its wrapper.
What resists it
Three things, in order of how much they matter. A properly sealed covering going in: asphalt shingles bond with heat, so a roof installed in cold weather may not seal until the following spring, and one that never seals is exposed to every cycle that follows. Flashings replaced rather than re-sealed, because a bead of sealant is the part that cycling finds first. And ventilation, which keeps the roof deck closer to outside temperature and reduces the melting that starts the whole sequence.