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Q&A Spotlight

Heading Off Ice Dams Before They Form

As a roof replacement approaches, a Boston area homeowner looks for precautions against winter ice dams

A new roof is in store for this Boston area home. Will the choice of roof cladding have any bearing on whether ice dams form in winter?

Long icicles hanging from the eaves make for great ski country advertising, but ice dams that form along the lower edges of a roof may lead to damage as melting water backs up and has no place to go but inside the house.

Writing from the Boston, Massachusetts, area, Kevin says in a Q&A post that he is aware of the danger and is looking for ways to eliminate that threat when he puts a new roof on his house.

Although the roof has been shaded until recently, the removal of some nearby trees means the two-story house will be in full sunlight during the day. The front of the house faces west.

“I’m looking to re-roof,” Kevin says, “and curious how this might impact my ice damming risk and what precautions or answers I should be looking to get from contractors, i.e. amount of ice+water shield, if needed.”

Before the trees were removed, Kevin would occasionally see ice in the front gutter, although the ice did not creep very far up the roof. He also noticed a little ice accumulation in the shadows around the roof dormers.

“Again, never water in the home,” he adds, “but occurred under more shady conditions with a lot of snow as well. I probably should have roof raked that area in hindsight.”

Does the choice of roof cladding have anything to do with ice dams? That’s the question for this Q&A Spotlight.

Solar gain is not the problem

Air leaks and a lack of insulation are the principal causes of ice dams, not the type of roof cladding, writes GBA editor Brian Pontolilo. Nor is the amount of sunlight falling on the roof a major contributor to the problem.

“Solar gain could definitely cause some ice dams as it happens when snow melts to water and then the water freezes,” Pontolilo says. “But I have never heard of solar gain as a significant cause of ice dams. The most common causes are heat loss through a leaky and/or under-insulated attic or roof. Some roof designs exacerbate the situation, but are not the cause.”

There is an exception to that rule of thumb on houses in the Boston area, says Dana Dorsett, although it’s rare and usually insignificant.

Here’s the scenario: On houses with deep roof overhangs and a south-facing wall with no soffits, a column of rising hot air on the exterior side of the wall may create a pocket of warm air a few inches out from the wall when the sun strikes it.

That possibility, however, looks unlikely given the photo shown at the top of this page that Kevin has provided to GBA.

Waterproof the lower reaches of the roof

If Kevin has experienced ice dams here in the past, it’s worth adding an extra layer of protection when the new roof goes on, Zephyr7 recommends. A common choice is Grace Ice & Water Shield, a peel-and-stick membrane that many builders in New England use.

“When the Ice and Water Shield goes up, you want at least two roll widths worth at the edge of the roof,” Zephyr7 writes. “One roll width isn’t really enough. Make sure the the Ice and Water Shield is installed on top of the drip edge, too. I’ve seen that done wrong lots of times.”

And even if the roof doesn’t experience ice dams, Dorsett adds, it’s a good idea to make sure roof valleys get an extra level of waterproofing—30-pound felt is inadequate. Valleys can become major snow-traps in big storms, with snow depth reaching two or three times what it is on the ground.

Open-cell foam in the roof

Open-cell spray polyurethane foam has apparently been sprayed into the rafter bays of Kevin’s house. Should that, Kevin asks, affect any decisions on applying a waterproof membrane to the entire roof?

If there is no ventilation channel between the roof deck and cavity insulation in the roof, the membrane can safely extend up and over the ridge, Dorsett says. But open-cell foam allows the passage of too much water vapor to be completely safe when the roof is unvented.

“A smart vapor retarder or vapor barrier latex applied to gypsum board on the conditioned space side of the [open-cell spray polyurethane foam] can substantially mitigate against rot risk on the roof deck,” Dorsett says. “The riskiest section of roof is anything north-facing or otherwise substantially shaded.”

Dorsett refers Kevin to a Building American report on moisture-safe unvented wood roof systems.

Other problem areas in the roof

Kevin adds some details on a few other areas of concern. They include the area above the porch with the dormer, where photos he provides show open-cell foam in the knee wall.

Another problem spot is the area between that dormer and the roof bump-out. That’s where a small laundry room is located, with the dryer exhaust running to the porch ceiling.

“Not sure on insulation in the knee wall but the room is so small it’s hard to move the appliances around to get back there,” he says. “The washer drain has thickened up once in sub-zero temps as it is unheated. We throw a space heater in there on occasion.”

If the area above the porch is a completely trapped, dead-air space, Dorsett suggests that filling it with cellulose would be fairly low risk because it’s on the sunny side of the house. Kevin could do that job himself with a rented blower, but if working from the interior is too complicated or messy, he could drill holes in the porch ceiling to blow in the cellulose.

As to the laundry room, Dorsett says a dryer vent passing through an uninsulated space will definitely contribute to ice dams.

“It’s important to keep both the dryer vent and drain on the conditioned space side of any new insulation, which may preclude just packing the whole shebang full of cellulose,” Dorsett says.

Our expert’s opinion

Peter Yost, GBA’s technical director, added this:

There are some interesting issues packed into this one! Let’s take them one at a time.

The cause of ice dams: Bill Rose has a great section on ice dams in Chapter 4 of his book, Water in Buildings: An Architect’s Guide to Moisture and Mold. Ice dams occur at overhangs in northern climates due to both conductive and convective heat loss, where ducts or chimneys run through attic spaces particularly near eaves and also particularly in roof valleys. It’s the melting above as the conditioned space loses heat and the refreezing at the cold overhang that forms a lens that can back water up the roof. Water can extend sometimes several feet up the roof. Continuous insulation and air sealing at eaves and roof-attic penetrations manages ice dams. If this were my project, I would do a blower door to assess the continuity of the air control layer, particularly at the eaves and at knee walls.

Deciduous trees shade even in winter: It still surprises me how much shade a leafless tree can still provide—leafless branches block as much as 40% of available sunlight. See, for example, this research: “Effects of Individual Trees on the Solar Radiation Climate of Small Buildings.”

Unvented roof assemblies and continuous peel-and-stick membranes: Many roofers like to cover entire roofs with peel-and-stick membranes to eliminate the chance of backed-up water from ice dams resulting in a leak to the interior. But the building science price of this is essentially no drying potential to the exterior with unvented roof assemblies. I would stick with what the code requires; here is what the IRC 2015 states in section 1.2:

“In areas where there has been a history of ice forming along the eaves causing a backup of water as designated in Table R301.2(1), an ice barrier shall be installed for asphalt shingles, metal roof shingles, mineral-surfaced roll roofing, slate and slate-type shingles, wood shingles and wood shakes. The ice barrier shall consist of not fewer than two layers of underlayment cemented together, or a self-adhering polymer-modified bitumen sheet shall be used in place of normal underlayment and extend from the lowest edges of all roof surfaces to a point not less than 24 inches (610 mm) inside the exterior wall line of the building. On roofs with slope equal to or greater than 8 eight units vertical in 12 units horizontal, the ice barrier shall also be applied not less than 36 inches (914 mm) measured along the roof slope from the eave edge of the building.”

Sequencing drip edge and ice and water shield: It’s not hard to find strongly held different opinions on how to lap the drip edge and underlayment. Here is what the 2015 IRC states, and this “over/under” method makes sense to me from a water management perspective:

R905.2.8.5 Drip edge.

“A drip edge shall be provided at eaves and rake edges of shingle roofs.

Underlayment shall be installed over the drip edge along eaves and under the drip edge along rake edges [emphasis added].”

-Scott Gibson is a contributing writer at Green Building Advisor and Fine Homebuilding magazine.


  1. johns3km | | #1

    A pleasant surprise when I visited the homepage. Thanks for featuring my home and I’ll be sure to update if I discover any issues with the sheathing from the open cell when we settle on a roofer. Takeaways for me are 2 strips of I+W including valleys (I’ll look in to the math above the porch), getting a blower door test, blowing in insulation over the porch, and repainting finished attic with vapor barrier paint. Hoping to get that uninsulated foundation up to R-15 soon...

    Thanks again,
    Kevin (Brookline)

  2. Peter L | | #2

    I've seen ice dams form with the high-desert southwest sun hitting the roof during the day, melting the snow and then the melted snow refreezes at night. No leakage happens as the panel is watertight with no exposed entry points as it's a standing seam metal roof with no exposed fasteners. Diurnal swings of 60F during the day to 20F at night is common for high-desert southwest areas during winter. You get intense solar gain (elevation of 5,000+) during the day with 60F air temps and then nighttime temps drop into the 20's and even into the teens.

    With that being said. In the above scenario, would not the geography of such areas result in ice dams being more common?

    Also, would not the above mentioned metal roof be more resistant to ice dams?

  3. User avater
    Peter Engle | | #3


    Ice dams require, first and foremost, snow laying on the roof for some period of time. In conditions like you describe, I expect the snow melts relatively quickly. And in the high desert, while there is occasional heavy snow fall, it is probably somewhat rare. Of course, this varies by location with some high-altitude locations receiving hundreds of inches of snow per year. Those are the ones most likely to see ice dams.

    In my experience, temperature swings like you describe rarely cause ice dams. Since the outdoor temperatures don't drop below freezing until well after dark, the ice dam potential is reduced. It is the bright, cold days that seem to cause the most ice dams from solar exposure. If the sun is warm enough to melt snow on the roof (especially when combined with attic heating) while the temperatures are consistently below freezing, this provides the conditions that continue to feed water to the cold eaves as fast as it freezes. You need both a volume of water flowing down the roof and cold enough eaves to form the dams.

  4. User avater
    Jon R | | #4

    I find it interesting that most ice dam recommendations revolve around reducing the amount of heat that reaches the snow. No matter how much of this you do, the upper roof is still warmer than the eaves - so ice dams are reduced (perhaps to the point of being inconsequential) but not eliminated.

    The only 100% cure (eave surface is warmer than the roof above) that I've seen is electric heat. I have seen recommendations to install a completely internal attic fan to provide better attic air mixing. Said to be quite effective, even if it never gets the eaves quite as warm as the attic peak.

    Strictly speaking, ice dams aren't about heat loss (from the interior) or heat retention (in a vented attic or vented cathedral ceiling). They are about uneven heating. Part of this is caused by the natural (but not inevitable) tendency for warm air to rise.

  5. User avater
    Paul Kuenn | | #5

    Hey Jon, the "tendency for warm air to rise" is due to stack affect with a lack of insulation and no air sealing. All the homes I've sealed (from 1870-2010) stopped having ice dams. Three of them I pass during the week and the last few years we've had up to 3 feet of snow sitting on them for weeks. Proof is in the pudding. The last thing I'd want is wasted electricity under constant snow accumulation.

  6. Ken Field | | #6

    Lots of good information in the article. Raised heel rafter framing and proper insulation eliminates most ice dam problems but that needs to start at construction time. The very narrow gap where the rafter tail sits on the top plate makes insulation a challenge even if you can get to that area to work. Air leakage from within the wall exacerbates the problem if there is just fiberglass stuffed there. A good rafter vent and a plug of 2 lb. closed cell foam beneath it seems to do the trick but most homeowners won't go for something that expensive until after they have had a round or two of expensive ice damming damage.

  7. user-6987585 | | #7

    Our house looks exactly like this one and we had ice dams for years. We insulated in the knee walls as you mention but the key to solving the issue I believe was a metal roof. We haven't had any problems since.

    1. johns3km | | #8

      I’d love to see a picture as I have yet to come across one like ours. It’s 1940 outside of Boston, a couple of Sears kit homes were similar but not exact.

      I think a metal roof would looks really sharp but quite the cost. Where did you get ice dams?

  8. codeslack | | #9

    Stupid question: would painting rain gutters black help them thaw and generate enough heat to melt the ice dam above?

    I have a south facing sloped roof on an addition, and it has terrible ice dams. Part may be due to uneven insulation, but part is also due to proximity to the boiler exhaust stack off to one side—just enough warm air blowing around to melt snow at the top of the roof. The roof is metal and there hasn’t been moisture incursion.

  9. Malcolm Taylor | | #10


    I don't know. One way to test it might be to watch whether the snow on black cars melted appreciably faster than their lighter neighbours parked nearby.

  10. But Why? | | #11

    Al Gore promised us no more snow by 2016...why bother with any of this...problem solved.

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