Roof mounted bike racks are a great way to transport bikes. They most commonly come in two styles, fork mount or upright. Fork mount carriers require you to remove the front wheel. This can allow for easier lifting as you lose a cumbersome front wheel and it's weight, but it does add a step to your process. Roof mounted front wheel carriers from brands like Thule or Yakima are available for your convenience, and are able to mount to the same crossbars as the bike carrier.  If removing the front wheel everytime you want to transport your bike sounds like a pain to you, then you may want to take a look at an upright carrier instead. Upright bike carriers much like fork-mount are offered in many styles and price ranges to suit your bike or your budget.