Proper Roof Ventilation is Critical


Lucas Hamilton

I was in Pittsburgh recently and had the pleasure of visiting a home that was about to be reroofed again – it has been reroofed several times in the last few years.  The house was between 30 – 40 years old and originally had wood shakes which were replaced with asphalt shingles.  The roof was under ventilated to begin with but when they went to the new system it made the problem worse. Each contractor that came along tried to do different things to deal with the problem that the homeowner was encountering.

While putting away Christmas decorations a few years ago, the homeowners noticed ice forming in her attic.  Originally, they thought it was from a roof leak so they replaced the roof.  However, the next winter the ice returned. The roof leak wasn’t the cause. It was insufficient attic ventilation. The house had a great deal of moisture build up in the attic space which was causing ice to form in the winter on the underside of the roof.

The second contractor tried to add roof ventilation but did it in a way that didn’t help the situation.  He installed a power vent up high on the roof next to the ridge vent.  They put a humidistat on the power vent to activate the vent when the humidity rose in the attic.  The problem was that when the power vent kicked on, because of its position next to the ridge vent, it was pulling air in through the ridge vent and right out through the power vent which did not correct the humidity in the attic or solve the ventilation issue.

I met with them to discuss what was happening to the roof, make recommendations and work with the roofer to correct the ventilation issue. The roofer is going to optimize the power venting and eliminate the ridge vent. This was chosen because there is a concern that with the shape of the roof, you may never get sufficient soffit intake for the ridge vent alone to be sufficient.

As a result of the moisture in the attic, mold was developing on the roof decking.  While there are many ways to remediate mold, the homeowners wanted to take the most certain route which is to remove the contaminated wood. Of course, that adds cost to the project but is the best method of remediation.

Even though the knowledge base on ventilating residential roofs has expanded tremendously over the past 50 years,  professionals can sometimes have a difficult time properly ventilating a unique or challenging roof.  The homeowners were frustrated because they received different information from each contractor.  That can happen.

It is always a good idea to research the issues and ask questions.  In buildings where the attic ventilation requirements are not straight-forward the professional needs to look at the situation  from many angles to come to the right conclusion that solves the problem.

Need a roofer for your next residential or commercial project? We can help.

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Plus, stop ice dams before they form and why homeowners should pay attention to their roofs after a freeze and thaw winter.

Lucas Hamilton is Manager, Building Science Applications at CertainTeed Corporation


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  3. In order to create proper roof ventilation that adequately rotates there must be some air space between the power vent and the ridge in and out of the house it is important to get the level of intake and expulsion just right. This should be done by a Highly qualified professional who know how to figure the air flow to advise you of the best course of action.

  4. Thank you Lucas again for the excellent post.

    This is the number one reason why we have a comprehensive list of questions that we ask homeowners prior to starting a project. The clients sometimes get annoyed, but we assure them that it will help them in the long run. We don’t want to make mistakes in installing the wrong type of roof. Every roof has a reason – other than looking good.

    This is an interesting case study. I’m interested to see what the final decisions was for the homeowner.

  5. Hello,
    Excellent views described for ventilation. Conventional wisdom has established that an average-sized room requires at least one air change per hour when occupied. However this varies and is dependent on such factors as the number of occupants and the number and nature of the sources of pollution. Traditionally, ventilation was achieved with the use of air bricks and infiltration; however, as our energy conserving becomes more sophisticated, we need to develop a correspondingly more sophisticated ventilation strategy.


  6. There are various types of roof vents available, including Dormer and Turbine. The most important thing to consider, when choosing one, is the actual ventilation system.

  7. If your ventilation manifolds are damaged in a large storm, such as the hail storms that recently hit Denver, be sure to replace damaged manifolds. The damage couuld affect the proper air flow through the roof as well.

  8. Michael J Lovely on

    After almost 20 years of installation, I’ve had the most unfortunate pleasure of dealing with many ice, snow and water damming areas. I have found in more cases than expected, ventilation isn’t the actual problem we all face, it’s simply in the installation and especially around the ventilation, flange/ flashings, skylights, in valleys or even within pitch and transfer areas. If the roof is incorrectly vented into itself, but not venting out from the attack it will surely develop frost on the underside of the decking inside the attack which will in turn be susceptible to various molds, more common is black mold. To rectify this problem you may want to think about check if there are too many vents, if they are installed improperly or if they were installed at an improper location. 99% of failures occur at the point of installation. Be Safe! Be smart! (250) 540-0069

  9. In order to create proper roof ventilation that adequately rotates air in and out of the house it is important to get the level of intake and expulsion just right. This should be done by a qualified professional who can work out the figures required and advise you of the best course of action.

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