There is a common misconception that the more exhaust you have in your roof, the better your ventilation system will be. In most cases more exhaust products will hurt the performance of your roof and end the life of your shingles prematurely. When it comes to ventilating your home, what is most important is that you have one form of intake and one form of exhaust. 

Replacing your roof is a home improvement project that most homeowners only want to do once in their life. With that said, wouldn’t you want to get the longest life out of your roof? Improper attic ventilation Is the number one reason why shingles will fail prematurely. This occurs so frequently due to a common misperception that more exhaust is better. When a roof is improperly ventilated, air will sit in the attic space and become stagnant, causing the shingles to cook from the underside. Once the shingles get too hot from the underside they will begin to crack, ending their life prematurely. 
The highest recommended ventilation system is a continuous soffit vent and a continuous ridge vent. Soffit vents are located underneath your overhang.  Correct soffit vents should have vents that enter the lowest part of your attic space. This is the intake point of your attic ventilation system and where you want to pull fresh air in from. The ridge vent is located at the ridge of your roof, the highest point of your attic space. This will be the exhaust point of your attic space where you want all the stagnant air to exit from. This system works when the wind travels over the ridge vent creating a vacuuming and pulling fresh air into the soffit vents. The fresh air coming in through the soffit will collect pushing the old stagnant air in the attic space through exhaust in the ridge which will keep the attic space cooler and ventilated.  
With ventilation, it is important to have a balanced system. You want to have an equal amount of intake to exhaust. In some cases, homes do not have an intake point so one will have to be created. Other times, homes might not have enough ridge length to match the intake so a power vent will be needed. A common improper practice is the use of a power fan next to a ridge vent. The problem with this system is when the power fan turns on, it pulls air from the closest hole, which is the ridge vent. As you know now, you want to pull fresh air from your soffits, so it collects all the stagnant air in the lowest portion of the attic space. When you have a power fan next to a ridge vent, you only circulate air from the top portion of the attic space leaving air sit in the lower parts of the attic space, to cook the shingles and shorten their life.
Not every home is going to have the same ventilation system because each home will have a different attic space. Attic ventilation is a conversation we recommend you bring up to your contractor when you’re getting estimates for your roof replacement. Cracked Shingles: