Midwinter thaws
St. Louis does not get the winter of the northern states, and in one specific way that is harder on a roof. A covering in a hard-freeze climate gets cold in December and stays cold; a covering here crosses the freezing line dozens of times a season — sixty degrees and sunny one week, an ice glaze the next. Every crossing moves every material on the roof.
Oscillation, not deep cold
Asphalt shingles, metal flashing, wood decking and the fasteners holding it all together expand and contract at different rates. In a climate that swings, they spend the whole winter sliding against each other in tiny increments: sealant strips that bonded one course to the next in October give up their grip by March; nails back out a fraction with each cycle until a head telegraphs through the covering; hairline gaps at joints open on the cold days and close on the warm ones, pumping moisture in and out as they go.
Water joins in wherever it can sit. A thaw sends meltwater into every seam and crevice on the roof; the refreeze that follows expands it. Repeated across a season, that freeze-expand rhythm opens the small openings a little wider each time — which is why a roof here can come out of a mild-looking winter with more new leaks than it earned in July.
What it looks like from the ground
The signature of thaw-cycle fatigue is scatter: a lifted course on one slope, a loose ridge cap, a stain that appears on a ceiling in late February and dries up by April. No single dramatic failure, just an accumulation of small ones on a covering that may still look presentable from the curb. On a roof past its fifteenth St. Louis winter, that scatter is usually the material itself reaching the end of its ability to keep flexing.
An inspection settles whether the fatigue is cosmetic or structural, and a covering that has stopped holding itself together is a replacement conversation, not a patching one — patches on fatigued material fail at their edges, where new meets old, within a few more cycles.