Proper roof venting is critical to good construction practices and maintenance. The purpose of roof venting is to reduce temperature build-up inside the roof cavity during the summer months and to reduce fungal wood rot and other moisture related problems associated with condensation in the roof cavity during the cooler months of the year.
A good venting system allows for a continuous flow of air which enters the roof cavity at a low spot in the roof (for example, under the eave) and exits the roof at a high spot (for example, at or near the ridge of the roof). As a general rule, there is no need to provide for any sort of mechanical assistance, fans, or other devices to the roof venting system. A good venting system works adequately by means of a natural convection current where warm air rises and pulls the cooler, drier air behind it. A good roof venting system must also be designed in such a way as to provide for proper air flow at all portions of the roof.
Improperly vented roofs often result in premature deterioration of the roofing material and fungal wood rot in the roof framing and sheathing. It is not uncommon to find an improperly vented fifteen-year-old roof with badly curled (and thus non-functional) roofing material and advanced fungal wood rot on the lower side of the roof sheathing. With the introduction of more insulation and weather sealing, as well as other building techniques which reduce venting, proper roof venting techniques are even more critical than in the past.
During the winter months, when the temperature outside is relatively cool and the temperature on the inside of the house is 20+ degrees warmer, the air on the inside of the house is capable of holding significantly more moisture than the air on the exterior of the home. When the home is occupied, one can expect that the warm interior air will be saturated with moisture, the result of normal living a