Rain and the northwest chimney

What the wet season does

Seven months of rain soaks mortar joints, washes out the lime, and leaves brick saturated. The occasional freeze-thaw then expands the cracks. By spring, joints that were tight in autumn have started to open.

The symptoms

Efflorescence (white salt staining on the brick face), spalling (the brick face shedding in flakes), water stains near the fireplace, and a sour smell from a damp flue are the tells. Any of them means the moisture cycle is already at work.

What keeps the water out

A good cap keeps the rain out of the flue, sound flashing seals the roof joint, and a vapor-permeable waterproofing coat on the brick stops absorption while letting trapped moisture escape. The repair happens at the entry point.