Swelling is a sign that the siding has been failing for some time and its holding moisture. When moisture is absorbed through the siding the siding expands and begins to swell at the drip edge. Moisture will not stop at the drip edge and will begin to make its way up into the middle of the siding causing severe damage. Most swelling will occur at the bottom of the structure or where siding meets the roof. Snow sits on the ground and against the siding in these areas for a longer amount of time and therefore these areas are more susceptible to siding failure