Ice dams are the winter problem. Heat escaping from a poorly insulated or poorly ventilated attic warms the roof deck and melts the snow sitting on it. That meltwater runs down to the eaves, which stay at outside temperature because there is no heated space beneath them, and it refreezes there. The ice builds into a ridge, and every subsequent melt pools behind it. Water then gets pushed backward up under the shingles, which is the one direction a shingled roof cannot resist. This is why so many Chicago roof leaks appear during a January thaw rather than during a rainstorm, and why the fix is frequently in the attic rather than on the roof.

Freeze thaw works on everything else. Water enters a small gap in flashing, a nail hole or a seam in siding, freezes, expands and opens it slightly. Chicago runs through that cycle dozens of times a winter. Small defects do not stay small here.

Summer brings hail and severe storms. Chicago sits in an active severe weather corridor, and hail bruises shingles by fracturing the mat beneath the surface without necessarily breaking through. That damage frequently does not leak for another year or two, which is why post storm inspections matter even when the roof looks intact from the ground.

Wind off the lake never stops. Sustained wind lifts and works at shingle edges, siding seams, flashing and fascia. North and west elevations take the worst of it and almost always fail first, which is why one side of a Chicago house often looks a decade older than the other.

The temperature range is enormous. Chicago roofs go from well below zero in January to genuinely hot surface temperatures in July. That is a large annual expansion and contraction cycle for every material and every fastener on the building, and it is what eventually opens seams that were sound when installed.