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My roof is being reshingled due to hail damage. One contractor has advised me to change to a ridge vent, but another advised to keep the individual roof vents already in place. Which is more effective in terms of heating, cooling and longevity of the roof?
The major consideration is the available length of ridge for a ridge vent. The general rule is to use 1/150 of the total floor area for roof venting. A typical ridge vent system uses a slot cut at the ridge about two to three inches wide. So, for every foot of ridge ventilation at two inches wide, you are getting about 1/6 of a square foot of ventilation. Or, one square foot of ventilation two inches wide is six feet long. A typical 2,000 square foot home, then, would require about 13 linear feet of ridge venting two inches
wide using the 1/150 rule. In homes with hip roof systems, ridge length is at its minimum; a typical gable roof has the greatest ridge length.
Box vents are also useful means for ventilation. The advantage of ridge vent systems is that they are located at the very highest point in a roof system and therefore vent most effectively. Box vents cannot be located at the ridge and therefore because of a lower location on the roof than the ridge, do not vent as fully. Some would argue that the difference is marginal at best. I would argue, that if the conditions are equal, a ridge vent is preferable. Sufficient "marginal" improvements in design typically contribute to significant improvements overall.
Ronn Phillips
State Housing and Environmental State Extension Specialist
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Updated 8/15/08