Is Improper Ventilation Destroying Your Roof?

by luke on July 30, 2011

 Proper Attic Ventilation

Attic ventilation is an important but often overlooked aspect of your home. As a result, many homes are improperly vented, or not vented at all. This lack of circulation leads to many problems such as:

  • Ice Dams: Improper venting causes excessive heat buildup in your attic. This heat melts the ice and snow on the upper areas of the roof whre it flows to the edge of the roof. Once at the edge of the roof, without any warm attic space underneath, it refreezes and forms an “ice dam.” Ice dams cause water to backup on your roof and renders any roofing material virtually helpless.
  • Mold: When warm, moisture filled air is trapped in your attic, it often leads to mold and mildew on the rafters and roof sheathing. Depending on the type of mold, this can cause unpleasant odors and even pose a health hazard.
  • Wasted Energy: Excessive cooling costs during warmer months is often due to super-heated air being trapped in your attic. On a hot day, attic air temperatures can reach 150-160 degrees Fahrenheit!
  • Dry Rot: The adhesives used in roof sheathing, (usually 5/8″ plywood), break down under the high temperatures and moist air. I’ve encountered roof sheathing, so weakened by dry rot, that I easily stomped through the roof and into the attic space! The cost to re-sheath your roof and replace weakened rafters is extremely high compared to the relatively low cost of proper venting.

“Broken” Venting Systems

Many homes that appear to be equipped with attic ventilation systems are still inadequately vented.

  • Often, shed dormers, or similar additions are built onto the existing roof without connecting the new, unvented attic space with the main, vented attic space. This prevents the new attic space from properly venting and poses many costly, long-term problems.
  • Many roofing contractors install ridge vents, (warm air exits), without ensuring that there are proper soffit vents, (fresh air entrances). Ridge vents and soffit vents are dependent on each other. Lack of air-flow at either of these locations cripples the entire ventilation system.
  • “Proper Vents” are rigid pieces of styrofoam that are vital to keeping attic insulation from blocking your soffit vents. Once again, the cost of installing “proper vents” in your home is extremely low compared to the cost of damages caused by improper roof venting.

How Much Attic Ventilation Do You Need?

Example: Calculating the number of lineal feet of Cobra® Exhaust Vent needed for a 30’x 40’ attic:
Step 1Step 2Step 3Step 4Step 5




Calculate attic square footage
How: Multiply length of attic (in feet) times width of attic (in feet)
30’ x 40’ = 1,200 square feetCalculate NFA (Net Free Area) needed for this attic by using the “1 in 300” rule
How: Divide attic square footage by 300
1,200 sq. ft. ÷ 300 = 4 square feet of NFA neededConvert square feet of NFA to square inches
How: Multiply square feet of NFA by 144
4 sq. ft. x 144 = 576 square inches of NFA neededSplit the amount of NFA needed equally between the intake and the exhaust
How: Divide square inches of NFA needed by 2
576 sq. in. ÷ 2 = 288 square inches of NFA needed equally for exhaust & intakeCalculate # of lineal feet of Cobra® Exhaust Vent needed
How: Divide the square inches of NFA needed at the ridge by the NFA of the Cobra® Exhaust Vent
(Cobra® Exhaust Vent has 16.9 sq. inches of NFA per lineal foot)
288 sq. in. ÷ 16.9 = 17 lineal feet

This attic requires seventeen (17) lineal feet of Cobra® Exhaust Vent to meet FHA minimum requirements.

Intake ventilation MUST be at least equal to exhaust ventilation. To determine the number of intake vents needed, select your vent on page 12, and install enough vents so that their combined NFA is at least 288 square inches.Remember:
Always have a balanced ventilation
system. In no case should the amount of exhaust ventilation exceed the amount of intake


If you have any questions on how to correctly vent your attic…please feel free to ask!


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