A single unit roof/rafters/insulation/ceiling will not benefit from ridge vents.
A "suspended ceiling" arrangement with the ceiling, then the insulation, then an empty space and then the roofing can use a ridge vent provided that the empty space goes all the way down to the eaves where there are soffit vents. This is rare unless you have a real suspended ceiling because a space of less than 18 inches between ceiling and roof will likely be completely filled with insulation.
Do you get coverage you can count on? From the bag of fertilizer? From your lawn sprinklers?