Joints are positions in the deck which allow thermal contraction and expansion to occur. If joints are not maintained then runoff will fall through joints. Typically, snow melt runoff is concentrated on the low side of the structure. Snow melt is particularly brutal because it is laced with chemical agents concentrated in one area and flows for extended periods of time (length of meltdown). This combination of concentrated flow combined with corrosive agents subjected to extended exposure allows for corrosion damage to initiate and progress.