Functional Ventilation Louvers and Roof-Venting Trim
A roof has to breathe, which means that it has to allow air to pass through the attic and under the roof sheathing to keep the sheathing and the framing members from rotting. Without this air movement, condensation alone could cause severe damage. In the old days, house construction was much looser than it is today. As a result, more air could move through the structure and keep everything reasonably dry. As soon as houses started to tighten up, they began to have trouble. When fastidious use of caulk, vapor barriers, and insulation became the fashions of the day, people began to understand that tighter is not always better.
The only trim components that regularly involve ventilation are gable vents and soffit vents. Gable ventilation louvers come in round, triangular, and rectangular units that are made of high-density exterior grade polyurethane. These ventilation louvers are installed in the gable ends, just below the roof ridge. Air moving across one louver will pull air from the attic. The air that leaves is replaced with new air coming in through the ventilation louver on the opposite gable end. You can either paint louvers to blend in with the house's siding or make it stand out as a decorative element.
As mentioned earlier, in an old house, ventilation louvers usually provide enough ventilation. But in newer, tight houses, soffit ventilation is also required. Soffit vents are installed in the eave overhangs and allow air to wash under the sheathing. These vents have to be used in conjunction with other vents, such as gable, roof, or continuous ridge vents, or they won't work.