A lap is when the fiberglass wraps around the rail to the other side of the board, providing more strength. When glassing the bottom of the board, the fiberglass '˜laps' onto the deck. When fiberglassing the deck, the fiberglass laps around the rail onto the bottom. The distance of the lap (measured from rail to stringer) has a effect of the board's overall breaking strength and flex. A longer lap will make a more snap-resistant, stiffer board because. A shorter lap will be a weaker and flexier board. A standard lap distance is about 1" in towards the stringer from the apex of the rail.