Tips about Roofing and Attic Ventilation

The most common problem with roofs installed today is poor ventilation.

Ventilation is the single biggest indicator of how long your new roof is going to last.  I’ve seen 50-year shingles fail after only 8 years, and conversely I’ve seen 25-year shingles last well past their warranty period.The key to achieving longevity with your new roof is to create a “Balanced Ventilation System”.  A balanced ventilation system means that the amount of exhaust ventilation is equal to the amount of intake ventilation.  Most roofers today understand that you need to have exhaust ventilation to allow hot, moist air that accumulates in the attic to be expelled.  However, most roofers today do not fully understand that it takes equal or greater intake to truly flush out the hot air.

 

A well-ventilated attic not only prolongs the life of your roof, but also helps reduce utility costs and make the home more comfortable.  The ideal ventilation system will provide a constant flow of cool, dry air along the entire underside of the roof decking.  Think about the old Hawaiian Punch cans that you used to have to punch two holes in for the juice to pour out.  What happened if you only punched one hole in the can?  You would have a huge mess, because the juice would not pour out smoothly.  The second hole was necessary to allow air to enter the can at the same rate as the juice poured out.  The same principle applies to roof ventilation.  You can have plenty of exhaust ventilation (ridge vents or can/box vents), but if they are not matched with an equal or greater amount of intake, they are of little use.  Today’s building codes require a minimum of 1 square foot of N.F.A. (Net Free Area) per every 300 square feet of attic floor space.  It is O.K. to exceed minimum code requirements, as long as you can balance the exhaust with equal or greater intake.

For Example: a 1200 square foot attic would require a minimum of 4 square feet of N.F.A.  To achieve balance, you would want to have 2 square feet of exhaust and 2 square feet of intake.  This is now an exact science, so when choosing a company to replace your roof, make sure they have a good understanding of roof ventilation and the math involved.

There are several factors that can “short-circuit” your ventilation system.  You must take into account any gable vents on the home , as they may actually disrupt the flow of air from your intake vents to your exhaust vents.  Some homes do not have overhangs, thereby making intake ventilation more of a challenge.  However, experienced roofers can create adequate intake even on homes without any overhangs at all.

Keep the following basic rules, and common mistakes in mind when interviewing a contractor for your roof replacement:

  • How long has the company been in business? (96% of roofing companies fail or change their name within the first 5 years)
  • Is the company licensed and insured?
  • Are they providing any additional coverage other than the manufacturer’s limited warranty?
  • How do they plan to achieve a balanced ventilation system?
  • You should never combine 2 different types of exhaust ventilation (ridge and can vents) on the same home.
  • When using a ridge vent for exhaust , all gable vents should be blocked off from the inside to prevent a short-circuit.
  • The best performing ridge vents will have an external baffle to deflect wind driven rain and snow, and create better airflow.

The key thing to remember is that while exhaust ventilation, such as ridge or can vents, are important, just as important, but more often overlooked, is intake ventilation.  Exhaust ventilation is of no use if it is not matched by an equal or greater amount of intake.

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