Proper Roof Ventilation

Well this is a fun topic.  And by fun topic I certainly mean that it seems like nobody cares about it.  But don’t worry.  That’s my job.  You may not care about venting your roof very well until you have some task in your attic that causes the need to be in your attic space for an extended period of time, at which point you may experience how hot your attic space is.  Roof ventilation is very important.  Let’s break down why and what proper roof ventilation looks like.

If you’ve spent just a few minutes in your attic space during the summer, you know it gets hot up there.  Who cares?  We do.  Here’s why.  As a roofer, I’m not concerned about the heat in your attic or what may be stored in your attic that may not withstand heat very well.  What I do care about is the extra heat on the underside of your shingles.  This extra heat causes materials within the shingle to breakdown faster and reduce the lifespan of your roof.  If you’re a smart one, you already knew that.

Here’s what you may not have known:

Just like a poorly ventilated roof makes for a very hot attic space, poor ventilation can also cause big problems in the winter.  If warm air from the home is reaching your attic space while it is cold outside and that warm air is trapped in your attic, this causes condensation to form on the underside of your roof.  The average family of 4 generates 2-4 gallons of water vapor every day in their home that can be transferred to their roof. Oriented Strand Board (most common roof decking material) is made to absorb water better than standard plywood, but it’s still very unhealthy for your roof decking to have condensation build on it.  This can cause decking to swell and bow or even rot and mildew.

With a perfectly ventilated roof, your attic space would be the same temperature as it is outside.  Of course it isn’t possible to do this, but it gives you some perspective  when you walk into your attic space and it is even hotter than it is outside.

Building code and reputable roof warranties require that you have adequate roof ventilation.  The point here is that your roofer may sell you a roof warranty that isn’t even valid if they don’t pay attention to your ventilation.

Mathematically proper ventilation loos like this:

Attic SQFT / 150 (300 w/o vapor barrier) = net free area

divide net free area by 2 (half intake vents and half exhaust vents)

A typical Lomanco 550 box vent provides 50 SQIN of net free area

So if we’re using box vents we can divide total NFA (in inches) by 50 to figure out how many box vents we need to install at a minimum.

There are a few other considerations but this should give you an idea.