nail sickness

Nail sickness is the progressive corrosion of fasteners in roofing or marine structures, caused by exposure to moisture, salt air, or chemically aggressive environments. The nails gradually weaken, lose holding power, and eventually fail, allowing the roofing membrane or hull skin to shift, leak, or separate. The degradation happens from the inside out: rust forms at the nail shaft where moisture accumulates, then spreads outward until the fastener becomes brittle and loses tensile strength.

In residential roofing, nail sickness most often strikes in damp climates or where gutters clog and trap water against fasteners. Salt spray in coastal areas accelerates the problem dramatically. On wooden boats, it affects both the hull fastenings and cabin roof nails, particularly where freshwater mixes with salt and creates galvanic action. The nails most vulnerable are uncoated steel; galvanized, stainless steel, or hot-dipped fasteners resist the problem much longer, though they cost more at installation.

Early stages show surface rust streaking down from nail heads or small leaks appearing along fastener lines during heavy rain. As corrosion deepens, nails lose grip and the roofing membrane begins to lift or ripple in wind. Boat owners may notice deck seams opening or cabin crown molding pulling away from the hull. By late stages, nails crumble when struck or can be pulled by hand; structural members may shift enough to break adjacent fasteners or crack wood.

The name reflects the trade's old understanding of the problem: the nails were thought to be "sick" or defective, rather than recognizing that the fault lay in material selection, drainage design, or environment. Modern practice treats nail sickness as a design and specification issue. Proper roof pitch, gutter maintenance, ventilation, and choice of fastener material prevent most cases far more cheaply than repair after failure begins.