Lag screw bolts are used in place of standard wood screws whenever the mounting surface must bear a heavy load. The hex head allows for lag screws to be tightened with more torque than can be used on phillips head screws, though a pilot hole is still required prior to installation. Note that lag screws do not accept nuts as they are technically thick heavy duty wood screws with threads designed to cut into wood. They are ideal for use on decks since installation can be accomplished without having access to both ends of the fastener, as opposed to standard hex bolts that require a nut on the opposite end.