Not only are roof vents needed but also soffit vents are needed. Air enters into the soffits and then exits out the high vents.
note; You can NEVER have too much ventilation in an attic area as long as there is sufficient(R value) insulation in the attic floor.
Also, if there is adequate ventilation via soffit and ridge vents ,there is no real need for any power venting at all. Save the cost of the fan and the electric power and just let natural air flow circulation do the job.