Manufacturers often specify ventilation requirements as part of the warranty covering the shingles. Improper ventilation may reduce the length or void the warranty if damage occurs because of the ventilation issues.
Ventilation helps control the temperature and moisture levels in the attic. Excessive heat can cause damage from underneath the shingles, while moisture can rot the roof below the shingles. A manufacturer's warranty specifies the amount of ventilation necessary with a ratio of vent area to attic area. New construction building codes often specify a ratio of 1 to 150, or 1 square foot of vent area for every 150 square feet of attic space. Reviewing the terms of the warranty help determine if the existing ventilation meets or exceeds the requirements.