WARNING!!! It is better to put the wood on top. Some people even say NOT to use wood underneath to get extra height. The jack must be able to roll so if you put wood underneath it must be a large enough board for all the jack's wheels to roll on even while the jack moves as the jacking arm arcs upward. Either that or keep the parking brake off and let the car's rear wheels roll... In either case be very careful if you do it this way.