For effective roof ventilation you will need to have a cooler home in summer and a perpetual dry roofspace from moisture problems.
In order to achieve an effective roof ventilation system it is obvious that a single ventilator alone will not do the job.
How could you achieve effective roof ventilation? You would need more than one ventilator at least two of the same volume, these would be best to oppose each other to allow for effective air exchange. Then for each section of the roof it would be desirable to have a pair of ventilators, say on average 4, 6 or 8 ventilators depending on the size of the roof. These are the components for effective roof ventilation.