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Your Roof: Snow Loads & Ice Dams, Oh My!

door-front-snow-loadPray for snow!  Yes, I mean it.  Why live in New England if you don’t enjoy the white stuff?  In December, snow gets us all cheerful and full of Christmas spirit!  Snow is also great fun for sledding, skiing, snowshoeing, snowmobiling, and it allows me to host an annual bonfire party in January. Let’s not forget that it’s also tremendously beneficial to our economy here in New Hampshire: snow inspires tourism!

At Team Engineering, we also love snow because it gives us the opportunity to answer your questions about safe snow loads on your house or building.  Further, there remains so much confusion in the construction industry about ice dams and what must be done to prevent/minimize them. Specifying “Ice and Water Shield” or similar bituminous membrane products as roofing underlayment may reduce leakage, but does not solve the underlying problems that cause ice dams.

For years, the primary focus was on ventilation, but we have learned that ice damming is a result of many additional factors, which are listed here in order of significance:

  1. Solar orientation and heating. The impact of the sun on roof performance and icing is tremendous.  You will rarely see a large ice dam on a south facing roof.  It is unfortunate how little consideration we New Englanders give to the solar orientation in our design.  A complicated, north facing roof is simply begging for ice damming trouble.
  2. Roofing color. The probability of ice damming problems with black shingles versus white or reflective is tremendous.  I know, we New Englanders love our black roofs, but they are not ideal from an ice-dam and energy design standpoint.  Light color or reflective type shingles will not only reduce the tendency for ice damming but also reduce cooling demand in the summer months.
  3. home-ice-dam-nhDraft Stopping.  Far too many homes and buildings have plenty of insulation, but have poor vapor separation between the living space and the attic.  One of the classic examples of this is office buildings with suspended acoustical ceilings and fiberglass batt insulation installed on top of the ceiling.  The heat loss through direct exfiltration (drafts through the ceiling assembly) is incredible.  Assuming a sloped roof above, ice dams are guaranteed in this situation.  Most homes also suffer from similar conditions – lots of direct exfiltration to the attic. This is a great example of ice dams indicating tremendous heat loss and inefficiency.
  4. Heating System Location.  Too many of our modern homes and buildings are constructed with the heating system in uninsulated attics. The Energy Codes and industry are finally beginning to address this practice. While it may save on construction cost with ease of installation, you will pay dearly for energy costs, and ice dam problems are a virtual guarantee.
  5. Insulation. Most of our clients are familiar with insulation R-values. The more the better, and since 1986, most homes have been required to have at least R-30 installed in the attic.  Sounds good, right?  Unfortunately, 10” of blown fiberglass or a 10” thick fiberglass batt – both theoretically R-30 – are relatively useless when installed in an open, well-ventilated attic.  My favorite analogy is that if you wear three wool sweaters on a blustery winter day, you’ll still be cold – as the wind blows right through those layers of wool. Properly insulated envelopes require a combination of insulation and vapor control.  No different than how you’ll appreciate your Gore-Tex shell when you ride the Cannon Mountain chair!
  6. Ventilation. Traditional, insulated, ventilated attics still work great.  Unfortunately, industry momentum has managed to convince most that ridge vents are an adequate alternative to end vents.  The trouble with the ridge vent theory is that four letter word: snow!  In most cases, ridge vents do not provide sufficient ventilation to counteract the solar heating and heat loss described above. There is growing consensus toward the viability of insulated (non-vented) roof decks.

If you are concerned about how much snow your roof can support, if you are frustrated with ice damming problems with your home or building, or if you are designing an addition or new building, please contact us and let us help you enjoy the snow!

About the Author

John TurnerJohn Turner is a Multi-Discipline (Architectural and Electrical) Licensed Professional Engineer in New Hampshire and Massachusetts, specializing in building technology and construction. He is also a Board Certified Building Inspection Engineer (BIE). As president of Team Engineering, John has diverse experience in all facets of construction. His specialties include residential and commercial inspection, design, energy-efficiency, renovation consulting, condominium/association properties and forensic engineering.View all posts by John Turner →

  1. Mike Hippert
    Mike Hippert01-17-2013

    In the blog post “Snow loads and Ice Dams Oh My” in step 5 “insulation” you mention the need for vapor control to stop wind from blowing right threw the insulation, it almost seems that you are indicating that I would need the vapor barrier to be on the roof side of the insulation rather then on the house side in order to protect the insulation from having air blown threw it. My understanding on insulation and vapor barriers were that the vapor barrier went between the house and the insulation to keep moisture from inside the home from getting into the insulation. So is there some other product that i should add over (between the roof and the insulation) to prevent cold air from blowing through my insulation? I would like to add another R-30s worth of insulation to the existing R-30 in my attic but if I need to add a vapor barrier of some type i want to be sure to do it correctly to stop air from moving through the insulation. Thanks!!!

    • Maureen Heaney
      Maureen Heaney01-25-2013

      This is an excellent question. The best analogy I like to use is that if I stepped outside last night with 0 degree temps and 30 mph gusts in my turtleneck and 3 layers of rag wool sweaters, I would still be cold – the wind would simply wash through those nice layers of insulation and still make me cold. This is exactly why Gore-Tex and “Windstopper” fleece have become so popular in clothing. The point is, you need an air barrier on the exterior of the insulation for it to be effective. This wind barrier naturally occurs with a dense pack blown cellulose, but does not exist with fiberglass batt type insulation. As a result, if you wish to add that R-30, we would generally advocate blown cellulose as the preferred method. Also, remember that before you add insulation, you should always have a qualified engineer evaluate the roof framing to verify that the roof will be able to handle the additional snow load. Draft stopping at the ceiling level is another important step. You should also make sure that all electrical and hvac systems are modern and safe – as once you’ve got a sea of cellulose up there, modifications to the electrical and hvac systems will be difficult.

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