The answer to that question depends significantly on the type of roof rack as well as your vehicle. However, in most cases, if you can securely attach the piece of equipment to your roof, chances are you can use a roof rack to carry it.
Things to keep in mind is whether or not the item will cause your vehicle to have too much height, whether it will make your car too top-heavy, and whether or not it is too awkward in sizing to fasten securely.
For example, a small all-terrain vehicle or ATV would not be an excellent choice to attach to the top of your vehicle due to its overall weight and higher stance overall. It would provide too much of a chance of rollover for your car. However, a surfboard, although much longer and wider than an ATV would be a good contender since it provides a relatively low profile.