The next time you need to replace your roof be sure to consider your roof's ventilation at the eaves (intake) and ridge (exhaust). This key step to roof replacement is sometimes missed resulting in leaks from ice dams and shortened roof life. The recommended amount of cross sectional area all of venting holes or net free vent area (NFVA) is 1/300th of the square footage of climate controlled space from a birds-eyeview or plan view. If a climate controlled area has more than one story, only the area from a single story is counted. Below is a link to an article from Air Vent Inc. that was published by the Midwest Roofing Contractors Association (MRCA). This article explains some important concepts to keep in mind when designing a shingle roof that will vent properly.