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What’s the Purpose of Roof Vents?

If you stand in your back yard and look up at your roof, you may notice a series of vents along the ridge line.  They looks like mushrooms — with a wide top and narrow, hidden stem.  If the contractor who originally installed them went cheap, the vents are probably plastic.  Otherwise, they’re sturdier and longer-lasting metal.  The main cause of failure for plastic roof vents is cracking.

Or if you don’t see vents along the ridge line, check the shingles at the top of the ridge.  If they have a heightened profile, it’s likely due to a continuous-style vent that travels along the entire length of the ridge.

Framers (or roofers) cut a thin line into the plywood on both sides of the ridge line, install a thick mesh over top, then cover it with the ridge cap shingles.  If your home is new, this is probably the style of roof ventilation you have.  Unless your roof is very hipped, because then the amount of ridge line it has may not be enough to ventilate properly.  In which case, you should find the mushroom style vents right next to the ridge instead.

If your roof has neither mushroom vents nor ridge vent, then you’ve probably noticed some real problems by now.

What does roof ventilation do?


Most homes have pitched roofs and attics.  The living space of the home is designed as an envelope that seals and manages the flow air, moisture, and temperature.  Yet, what’s above your living space — your attic and roof —  are not in the same envelope.  The reason is to manage condensation and pressure between inside and outside.

There is gaseous water all around us all the time.  We can’t see it and usually call it water vapor.  If you conducted an experiment and lowered the temperature, at some point of continually lowering the temperature, you would finally see the water.  It would condense on surfaces, like glass.

The temperature at which this happens — where water in gas form condenses to water in liquid form — is the “dew point”.

Well, because the temperatures (and moisture amounts) inside your home and outside your home are usually different, there is typically a dew point at some place in the structure between inside and outside.

And if moisture reaches that point, it will condense.  This water over time can cause rot, damage, and can even appear to be a leak at times.

Roof and attic ventilation solves this problem by producing enough airflow that any water vapor in the attic gets blown out of the roof instead of traveling up to the plywood — where it can condense.

How to check if your roof is ventilated enough


The easiest method is to get in your attic.  Is it the same temperature as the outside?  If yes, then your roof almost certainly ventilates as well as it could.

Sometimes people use their attic, or they think it should have similar temperature to the inside of the house (because it looks like it’s inside the house).  But really the idea is to treat the attic like it’s an outdoor open-air structure.  The insulation along the bottom of the attic — along with the vapor barrier along plywood between the living space and attic — is the true barrier between the envelope of the inside to the outside.

This helps put into perspective that the roof exists to stop the elements and solar damage.  The attic exists as a byproduct of that effect.  You’re not meant to use it, because then you can create moisture and air travel that can damage wood components of the house (like the roof deck substrate).

What happens when you get too much condensation?


A new roof sometimes costs more than you’d expect.  Part of the reason is that if there’s been enough condensation damage, not only are you buying a new roof, but you’re buying new roof deck substrate (plywood).

If your attic isn’t properly ventilated, then water vapor is probably traveling up to the bottom of the roof plywood.  Then as it migrates through the plywood, it can hit the misplaced dew point and condense on the wood.

The properly placed dew point will not be on wood.  It’ll be inside insulation, and there should be enough insulation and vapor barriers that the air/temperature travel between the two is minimal.

Roof shingles deforming or the roof deck substrate losing integrity are often a sign that ventilation is inadequate.

Shingles can deform like getting buckled up.  This can be from long time periods of inside air pushing up through the roof.

Or a sagging roof deck can signal that water has condensed on the plywood long enough that it’s no longer sturdy because it’s been rotting away.

What’s the cost to poor ventilation?


It can cause a 50-year roof to last 10 or so years.

Bad ventilation doesn’t immediately ruin a roof, but enough of it shortens the lifespan significantly.  Often, homeowners will find leaks on their ceilings and think the water is coming from the outside.  But it might not be.  An improperly ventilated roof can do that as easily as a hole in the roof during rain.

Don’t forget intake ventilation


The vents along the ridge either always exhaust or mostly exhaust (depending on the type).  As you know, any properly ventilated system also needs an air intake source.

Many homes do not have enough intake ventilation.  Stay tuned and we’ll cover what you need to know about intake ventilation next week.


Are your roof shingles buckling or blowing off?  Is your attic too warm?  Is your roof sagging in spots?  Give us a call now and we’ll have an expert estimator examine your roofing system for free.  Together, we can make sure your roof lasts along as it’s supposed, and you won’t have to worry about buying a whole new roof early because of inadequate ventilation.

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