To get the most effective attic ventilation, it's best to choose different types of roof vents that work well together, such as the soffit vent, the ridge line vent and the gable vent. Add an attic floor or gable fan and the combination is even more effective.
The idea is to channel cool air into the bottom of the attic and force the hot air through the roof. Soffit vents are placed around the perimeter of the roof, under the eaves. These vents pull in cooler outer air through a number of vents, pushing the hot air up.
Gables are the peaked triangles found on either end of a peaked roof. The gable vents are installed at the tip of the triangle. While gable vents on their own don't provide much circulation, combined with the soffit vents they help bring air into the roof.
The hot air is discharged through the roof either through exhaust vents or ridge line vents. Exhaust vents come in square of round shapes and are placed near the peak of the roof. They also work well on flat roofs.
Ridge line vents are installed along the top of the ridge line, which is where both halves of a roof meet. The vents angle slightly downward from the middle of the roof to the edges to prevent water leaks. Ridge line vents can be covered with shingles or installed as a roof cap with a folded space that contains the vent grill.