Ice Dams Prevention & Ventilation of Low-Slope Roofs

Ice Dams can cause very serious and expensive to fix problems, and are usually caused by improper ventilation of your attic space and inadequate insulation of your roof. Ice Dams are a sign of heat loss in the attic, and this lost heat is money you throw out the window.

Ice dams may seem harmless at first, but they are known to cause thousands of dollars in roof leaks, structural  damages and repairs of your home. The can also create dangerous mold growth, which can cause or aggravate allergies, asthma and other respiratory diseases.  Fighting ice dams can be costly, and you also need to know how to approach the problem to make your efforts more efficient so that you can permanently eliminate them.

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This Ice Dams Prevention Guide will help you identify Ice Dams and eliminate either sources of the problem (preferred method, though not always feasible) or the effects, which are just as dangerous for you home.

What are Ice Dams?

Ice dams on a low slope roof

If you live in Northern US, Canada, or any other region with cold winters and lots of snow, you’ve seen and possibly experienced first hand Ice Dams – large ice formations along the eaves of the roofs. Although ice dams are common on most roofs in the northern regions of US and Canada, they cause most damages and leaks to Low Slope roofs, as it is much easier for water to “travel” up a low pitch roof vs. a steep roof.

Additionally, you may see many roofs in New Hampshire, northern Massachusetts, as well as other parts of the country with 2 x 2 feet aluminum pans locked together along the eaves of the roof. These metal sheets are installed so that ice formations would slide off, and ice dams would not build up. This may work for steep roofs, but on a low slope, the ice dams can form in the center of the roof as there is not enough pitch for water to run off.

Just a few of the most common ice dam related problems include roof leaks, rotted roof decking / exterior & interior walls / framing, mold and related respiratory illnesses (allergies, asthma, etc.), reduction in insulation effectiveness and associated heat-loss.

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What Causes Ice Dams?

Ice Dams on Low Slope (Shed Dormer) Roof

Massive Ice Dams on Low Slope (Shed Dormer) Roof

The main causes of ice dams are poor ventilation and / or inadequate insulation in the attic. When it snows outside, a thick layer of snow settles on the roof, and the warm air from the attic slowly melts the snow. Melted snow begins to run down the roof toward the eaves, which are usually colder than the rest of roof surface. There, water freezes, thus creating ice dams. As more and more water melts at the top of the roof and freezes at the bottom, ice dams grow to a thickness of over 4-5 inches.

As ice dams become larger, and more melted snow water runs down the roof, it hits the already large ice dams and can not go down any more. At this point it starts to refreeze under the shingles. At the same time, warm attic air melts the newly formed ice under the shingles, which starts to drip inside the attic and /or down the walls.

Ice Dams and Ventilation

Even if you have a very well insulated attic, which blocks most of the warm air from escaping your living space, still, some warm air will escape into the attic. This is where you need a working ventilation system, which will vent most of the warm air from the attic and will create temperature parity between outside and inside the attic. Only then, the snow will not melt artificially, and will not create ice dams.

Ice dam: diagram of proper ventilation and insulation.

Today, the most effective and commonly used attic ventilation system in residential construction and roofing is the Soffit / Ridge vent system. Although soffit/ridge system is the best-performing type of ventilation, it is not always possible to install it. While installing a good ventilation system on a new construction home is easy, some of the older homes were designed to have no ventilation at all, or optionally were build with two gable vents that are usually two 12 x 18 inches openings at the peak of the gable-side walls. These are not as effective as ridge and soffit vents but still work O.K.

In many cases however having a ventilation system is not an option due to many reasons, such as financial constraints or building limitations. For example, if a home was build with insulation stuffed between the roof rafters, but the the builder did not install baffles, which would create an air pocket to allow the flow of air. In this case no ventilation system will work, as there is no room for air to circulate. Of course, this situation can be fixed, but it would cost a lot of money and many homeowners are unwilling to spend that much.

Other types of roofs that are not designed to accommodate effective ventilation systems, such as ridge / soffit vents or gable vents, are Hip Roofs and low slope roofs. In the case of hip roofs, you could get away with static box vents or power vents with a thermostat. These are not as effective, but still provide some ventilation.

Hip roof ventilation

In the case of low slope roofs, builders and roofers not only need to solve the ventilation problems, but they also have to make sure that there are no leaks caused by the air vents. Unfortunately, too many builders and roofers do not include any ventilation on a low slope roof, as it is easier for them to avoid potential leaks. For example, a ridge vent on a low pitch roof will allow the wind driven water to enter the attic.

Since most low slope roofs are not vented, ice dams are much more likely to occur and cause extensive damages to the roof and the interior of the house, as it is a lot easier for water to travel “upward” on a low pitch roof. Most low slope and flat roofing systems are designed to have their seams put together with glues and adhesives, ranging from solvent based seam lap adhesives used with EPDM black rubber to Tar and roof cement used with asphalt and modified bitumen roofs. Du to the nature of adhesives, these roofs are prone to develop leaks, as adhesives break down with time and weather affecting their longevity. Ice Dams only speed up the process, as the ice melting and re-freezing process expands the gaps between the seams.

Additionally, asphalt shingles are often used on low slope roofs, which is blunt ignorance on the part of homeowners, who did not do the due diligence, and hired a roofing contractor to install such a roof, as well as homeowner abuse by shady roofing contractors, who do not let their client know that the roof they are about to install violates manufacturers installation guidelines and annuls the warranty on the roof. By definition, a low slope roof has a pitch pf less than 3, while ALL shingles manufacturers require at least a pitch of 3 to install their asphalt shingles product.

Roof heat cable fail to solve ice dam problems.

Heat Cables DO NOT Solve Ice Dam Problems

In many cases, homeowners with low sloped roofs, turn to heated electric cables to melt the ice dams. Despite common opinion that heat cables solve ice dam problems, it could not be further from truth.

First, just a reminder that ice dams are caused by hot air escaping your living space and lack of adequate ventilation. Heat cables use electricity to melt the ice formations. Therefore, you are trying to fight a heat / energy problem by throwing more energy into the cold atmosphere, so instead of fixing the problem you just add a band-aid, which does not even work. At the same time, you incur ever increasing energy expenses, while the problem not only remains there, but becomes worse.

Heat cables melt ice dams along the eaves, but there are still ice dams forming just above the effective range of heat cables, so your ice dam leaks now occur high up the roof slope, affecting more insulation and aggravating the problem even more.

Permanent Solution to Ice Dam Problems

The best thing you can do to eliminate ice dams is to improve your attic ventilation and add / improve insulation to minimize heat loss.  Insulation should be your first step, as poor attic insulation is the main cause of ice dams, and besides, you don’t want to keep wasting your money on heating up your unused attic. Seal the gaps between insulation, use expandable foam insulation for tight spots, etc. If you have the soffit vents/air intake slots, make sure that loose insulation does not block the air circulation.

Install proper ventilation. As mentioned before, soffit and ridge vent system is most effective, and does not cost a lot if you are having a new roof installed. You can also add it to an existing roof, but make sure there are appropriate conditions for adequate air circulation. Do NOT mix different ventilation systems – if you do so, there is a good chance that one system will minimize the effects of the other, therefore your ventilation will stop working.

What To Do If You Cannot Improve / Fix  Ventilation and Insulation Issues

Although it is best to eliminate the source of the problem – heat loss and improper ventilation, it is not always a viable option for many homeowners, and another solution is required. As I discussed earlier, heat cables do not work and only increase your energy expenses. The best solution for a sloped roof is to have a Metal Roof installed.

Standing seam metal roof eliminates ice dams.

Metal roofs are designed to eliminated all damages associated with ice dams, by preventing the water from traveling upward. Be it a standing seam metal roof or an interlocking shingles system, Ice formations may occur, but the design of the roof will not let the melting water rise and penetrate the roof surface.

In fact, due to their smooth surface, metal roofs shed ice and snow, which comes down like an avalanche. To prevent this snow from damaging things below and falling on someone’s head, special Snow Guards must be used, which you can see in the image above.

For low slope roofs, we install an IB roofing system, which features hot-air welded seams to prevent water from entering the roof, and is perfect for residential low sloped roofing, as it comes with an asphalt shingle pattern to give it an architectural look and design.

To properly ventilate a low slope roof, we’ve created a special assembly method, which allows the installation of ridge vent, and eliminates associated roof leaks. We raised a ridge vent by 2 inches from the roof surface. This clearance is more than sufficient to prevent any water from entering your house through the ridge vent.

Flat roofing ridge ventilation in Andover, CT

In conclusion, I will once again mention that it is best to go to the source of the problem and fix as much ventilation and insulation as you can. Combine a great, energy efficient attic insulation with a permanent, cool flat or metal roof, and you will get a long lasting, great looking roof that will not leak, will save you money on roof repairs and heating / cooling costs, and take away many headaches of being a proud homeowner, as well as allow you to enjoy your home without ever worrying about roof leaks.

If you live anywhere in Massachusetts, Rhode Island or Connecticut, contact us to get a free roofing price quote and schedule a roof inspection and estimate to install a lifetime flat IB roof or a beautiful metal roof. You can also use our online roofing price calculator to estimate your roof replacement costs and annual energy savings.

References and Resources:

  • Flat Roof repair guide, which includes cost estimates for basic and advanced repairs, as well as what you can expect and how to choose a roofing contractor to fix your roof. Useful information for DIY homeowners, property maintenance personal and business owners.

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  1. How can I get more info about the low slope roof ventilation technique you mentioned? “To properly ventilate a low slope roof, we’ve created a special assembly method, which allows the installation of ridge vent, and eliminates associated roof leaks. We raised a ridge vent by 2 inches from the roof surface. This clearance is more than sufficient to prevent any water from entering your house through the ridge vent.”

  2. thanks for all the information. we had a spinny roof vent installed on steel roof up at the peak which took some of the heat out, but we added to the house and it changed the roof peak, it would not cool the new piece. after adding gable vents to no avail, I put some soffit vents and that was the answer. thanks

  3. Thank you for this article. It opens up the debate on how to solve ice dam problems. The comment by Brian B offers one solution that initially looks promising. However, great care should be taken when using Icynene to create an unvented hot roof. This can create warranty problems with the shingle manufacturer and moisture problems that can lead to mold issues. Also, in the event of a roof leak, it is difficult to detect and more difficult to repair.

    The use of icynene on the attic floor to completely air seal is a good idea and should be the first step. Your article seams to point to insulation and venting being the most important. Building science disagrees. Air sealing first, insulation second. Venting is a minor player in ice dam prevention but is necessary for heat and moisture management. There is no science to suggest that a flat roof with conventional soffit and ridge venting moves much air. For passive venting to work, you need a difference in temperature combined with a difference in height. An example of this is a chimney that doesn’t draft well – height is added. Low sloped roofs do not have enough difference in height to effectively ventilate. You also show a power vent which seems to suggest you are recommending this as an alternative venting solution to ice damming. Power venting in the winter without completely air sealing the attic will only pull moist, conditioned air into the attic and can create moisture problems from condensation.

    Your criticism of heat cables is unwarranted as well. How do you propose to solve the issues of temperatures fluctuating above and below freezing and the effects of solar gain on the roof. These are real problems especially with today’s complex architecture. The proper use of self regulating heat cables is a solution to this problem.

    Perhaps a little more research and a little less selling is warranted in your blog posting.

  4. This is a great article for me to bring to customer’s to help explain the ice dam issue. A lot of people insist on using cables to solve the problem. I have been more recently insisting on using Icynene insulation to solve the problem. It is an unvented roof, contrary to what all of you are talking about. It will air seal the attic, add insulation to current standards and prevent ice dams very effectively. The roof stays cold and the snow melts from the top down, not the bottom up. I plan on insulating my house this year with this system. I have a gambrel roof and my ice dams are terrible. Does anyone have any issues with the unvented system I am talking about?

  5. So far in the article and comments all that I have seen is to “insulate the attic and provide better attic ventilation”. Sorry roofers but that is not up to current technology. I am a bit surprised that none has brought up the more obvious and energy efficient solutions to these problems. First: Stop air infiltration from reaching the attic from the conditioned space to begin with. Second: Make sure that there is a contagious and uncompromised vapor barrier between the conditioned space and the attic. Third: Provide for non-blocked soffit ventilation. Fourth and finally once all of these have been done then add to the existing insulation and provide for effective exhaust ventilation.

    Adding to the attic insulation in a home with minimum standard insulation is only marginally effective in keeping the attic cooler (e.g. not being heated by the conditioned space below) thus minimizing the melting of the snow and creation of ice dams. If the warm moist air from the conditioned space has been blocked and is not infiltrating into the attic by sealing off plumbing vent stack, electrical wiring, fixtures and other penetrations of the conditioned space envelope a third of the battle is over. If there is an effective vapor barrier on the warm side of the ceiling the need to ventilate the attic is further reduced because the condensation of that moisture in the attic another third has been largely eliminated. At this point adding additional insulation will have its desired impact.

    The methods of doing all of this is well documented elsewhere.

    • @ B J Bergen,

      Thanks for your input. I will take a note of it and update the article as needed.

      I do want to say though that there are numerous houses with non standard design, that no matter how much insulation you stuff them with, it will not solve the ice dams.

      For example: Salt box / gambrel roof houses. These, unless designed with soffit / ridge ventilation, will never get vented, because most of them have closed air space where roof changes slope from steep to low. Most slat-box homes will have ice dams, and that is where you need actual elimination of leaks. Yes, eliminating heat loss is great, but in most cases is not enough, or cannot be accomplished.

      In these cases a metal roof of a membrane roof on the low slope is almost a must. Often, even ice belts do not help. Example here.

      Another example, is the second image in this article, where you see ice belt at the bottom of the low section, but ice dams and leaks came in, where roof changes slope to steep. Idea is the same as with salt boxes (blocked ventilation at the pitch change, and actually complete lace of ventilation at eaves and the ridge. That roof only had gable vents, which did not do the job.

      The owner did as much insulation work as he could, but the 150 yrs. old house design did not allow for much. The only way we could solve those leaks was by installing a PVC membrane (yes, other types would also work – but we install PVC).

      Examples of low slope roofs, waterproofed with PVC membrane or Metal roofs here.


  6. I loved this article, & have been telling people this for years. I built a home in 1994, & used Continuous soffit & Ridge vents. I did put gable vents on the house, but they were just for looks.
    Last week a roofer called me about a house that he roofed, & it had Ice on under side of roof deck, & as it melted, it was leaking into the house. I told him, he does not have enough insulation first. Ventilation second. As warm air is hitting the cold surface of the roof deck, it’s condensing & freezing there.
    When I saw this article today in my e-mail, I forwarded it to the roofer, & will meet with the homeowner tomorrow.
    Chuck Peoples

  7. Ridge vents do not, by themselves, create the venting needed to curtail ice dams. Ridge vents work together with soffit vents, as part of a venting system, not with can vents or gable end vents. Here’s how the system works:

    Heat from the sun warms the air within the attic, which becomes less dense as it heats up, and, therefore, rises toward the peak of the attic. As the warm air rises, it creates negative pressure at the cooler bottom of the attic, which, through the soffit vents, introduces cooler air into the attic to make up the air volume. As the warmer air at the peak reaches the ridge vent, it dumps to the exterior, creating negative air pressure behind itself, which is filled by the rising, warming air coming in at the soffits.

    In other words, there is a convective air loop, with a steady stream of cooler air coming in at the soffits to replace the warmer air which is being exhausted at the ridge.

    Hear’s where this gets interesting: Wind blowing horizontally along the ground is blocked when it hits the side walls of the house, so the wind, being fluid, changes direction to go around the obstruction. That portion of the wind which goes up the walls and then flows up and over the roof acts just like the air flowing over the top of an airplane’s wing: it flows faster over the top than it does while going horizontally straight over the ground. This accelerated wind speed creates negative air pressure at the ridge, whoch increases the speed of the convective air loop between the cool air coming into the soffits, and the warmer air exhausting out of the ridge vent, thus accelerating the speed of that convective loop and promoting better venting within the attic.

  8. Low-slope roofs demand careful detailing and good workmanship. You can hire a roofing contractor to get rid of your ice dams like Roofers In Fife.

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  11. Frost, developed with nanotechnology to prevent the formation of frost zycosil I think a very good product, but only used mineral-based roof coatings of metal or Resistant Roof, roof, unfortunately, was placed on both the surface area of 6 mm deep into the snow to hold down both the surface hardener, offers a solution to the problem of frosting

  12. Ice dams can definitely cause external damage to the roof. Not only that the water can leak inside the house and cause water damage that could be weighted in thousands of dollars. Thank you for the videos also.

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  14. Pingback: Ice Dam Prevention – how to stop Ice Dams roof leaks.

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  19. I’m upstate NY where we get an average of 5′ of snow a year. I had a new asphalt shingle roof installed about 5 years ago with the rubber barrier and ridge vent. Last year we added a sun room of the back of the house (we have a ranch style house) where the roof meets from the house to the sun room (on both sides) I get ice dams. Stats in the gutters and works back, I have blown insulation in the attic. Any suggestions?

  20. If you live in the north you are going to get ice dams. The first thing to do when you have an ice dam is to open a channel in the ice to drain away the water and stop leaks into the house. A refillable snow melt sock or ice melt sock thrown on the roof and pulled into place will open a channel in minutes.

    Insulating properly goes along way to prevention of ice dams and there are many other expensive engineering solutions, but most of them kinda make you think about the story of the truck wedged under the bridge and how nobody could figure out how to pull it out until a little boy said, “Why dont you just let some air out of the tires and drive it out.”

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  24. Sam,

    From my experience and from what I’ve read in the reliable/credible building construction sources, a Ridge / Soffit vent system should not be combined with any other type of ventilation, and those other vents (passive box / can vents in your case) violate the flow /circulation of warm air in the attic and air in some cases either just stays there “spinning” inside attic or very little circulation occurs or parts of the roof are not being vented.

    Basically, if you have enough intake through the soffits, and a decent ridge vent, your roof should be ventilated just fine without the need to use any static or power vents. For ridge vent, I like the one that is corrugated plastic from Owens Corning and other brands as it does not obstruct the flow of air, where is a very “popular” Cobra from GAF allows less air to pass through.

    For more information on attic ventilation you should check out these sites: – General guide on Ice dam prevention and just a very good source of construction techniques and building product references. – a very good source for various ventilation issues and solutions. – Very good source for any roof-related questions. It is a forum with many knowledgeable and responsible roofing contractors, and I’m a frequent contributor there, under nic-name LAMetalRoof.

  25. In standard steep slope (4/12 or steeper) roofs I have ventilated the attic space in accordance with the IRC and have used soffit, ridge and roof can vents. Lately I’ve been advised that the best feature for the upper third of the roof is ridge venting. For roofs with large areas and where ridge venting can not supply the required free area I have used ridge vents and roof cans placed in the upper 1/3 of the roof. This method allows the soffit and upper 1/3 venting free area to be equal or balanced. However, I’ve been told that the use of ridge vent and roof cans is not recommended because the movement of air through the ridge vent will draw air through the roof cans and diminish the amount of air coming through the soffit venting. Please comment on the combined use of ridge and roof can vents.

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  27. Good day, I was researching ice daming for work and as someone who has had the problem over the winter I found the info to be very informative. I have ann older minihome with flat roof, 1970 roof! I have had 3 contractors come look at it and all have recommended rubber roofing. None have mentioned how to prevent ice daming so this article was very informative. Thank you!