Ice Dam Basics

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Ice Dam Basics

How ice dams form, how to prevent ice dams, and what to do if you have an ice dam right now

Posted on Feb 27 2015 by Martin Holladay

What do you call the weeks between Valentine’s Day and Easter? It’s ice damA ridge of ice that forms along the lower edge of a roof, possibly leading to roof leaks. Ice dams are usually caused by heat leaking from the attic, which melts snow on the upper parts of the roof; the water then refreezes along the colder eaves working it's way back up the roof and under shingles. season, of course. Eastern Massachusetts is now the wet-ceiling capital of the world, but this winter, tens of thousands of homeowners from North Dakota to Maine are struggling with ice dams.

I’ve written about ice dams before. This article makes no attempt to be comprehensive; instead, it’s an introductory article to summarize basic information on ice dams. If you still have questions after reading the advice here, you should probably read “Prevent Ice Dams With Air Sealing and Insulation.”

How ice dams form

1. Ice dams form when a home’s escaping heat warms the roof sheathingMaterial, usually plywood or oriented strand board (OSB), but sometimes wooden boards, installed on the exterior of wall studs, rafters, or roof trusses; siding or roofing installed on the sheathing—sometimes over strapping to create a rainscreen. and melts the underside of the snow layer on the roof. Water trickles down the roof until it reaches the cold roofing over the eaves, where it freezes. After a while, the ice at the eaves gets thicker, forming an ice dam. If the water reservoir behind the ice dam is large enough, water can back up under the roof shingles and damage ceilings. (Image #2, below, depicts the complicated shape of a typical ice dam.)

2. The most common cause of ice dams, by far, is air leakage. Warm interior air leaks through through ceiling defects or recessed can lights, as well as through hidden passages in wall assemblies and roof assemblies, and this warm air contacts the underside of the roof sheathing.

3. The second-most common cause of ice dams is thin or missing insulation.

4. Bad roof design can make ice dams worse. Since ice can form in gutters, roofs with gutters (especially gutters that extend higher than the plane of the roof) are more susceptible to ice dams than roofs without any gutters. Roofs with valleys are more susceptible to ice dams than roofs without any valleys. Low-slope roofs are more susceptible to ice dams than steeply pitched roofs.

How to prevent ice dams

Scandalously, builders in snowy climates are still building new homes without testing the homes for airtightness. Many of these new homes have leaky ceilings and will soon develop ice dams.

If you are building a new house, the following steps should eliminate ice dams.

1. If possible, design a roof without any valleys. If you install gutters, make sure that the outer lip of your gutter is below the plane of the roofing.

2. If the house has a vented unconditioned attic, make sure that no ducts, water heaters, air handlers, furnaces, or other types of HVAC(Heating, ventilation, and air conditioning). Collectively, the mechanical systems that heat, ventilate, and cool a building. equipment are located in the attic.

3. Make sure that the ceiling above the uppermost story of the home is as airtight as possible. Verify airtightness with a blower door.

4. Install ceiling or roof insulation that meets or exceeds minimum code requirements. In areas of the country where ice dams are a problem, that usually means that you need a minimum of R-49 insulation. The insulation should extend to the outer edge of the top plates of the perimeter walls. Choose insulation with a reputation for good performance; avoid the use of fiberglass batts, which perform worse than any other type of insulation on the market. Make sure that the insulation is carefully installed and that the performance of the insulation is not undermined by thermal bridgingHeat flow that occurs across more conductive components in an otherwise well-insulated material, resulting in disproportionately significant heat loss. For example, steel studs in an insulated wall dramatically reduce the overall energy performance of the wall, because of thermal bridging through the steel. .

5. If you are designing an insulated sloped roof assembly (that is, a cathedral ceiling), the most robust design will include a generously sized ventilation channel under the roof sheathing. For more information on “vented over-roofs,” see Joe Lstiburek's article, “Mea Culpa Roofs.”

6. If you are worried about ceiling leaks due to ice dams, choose a type of roofing — for example, a membrane roofing like EPDM or standing-seam metal roofing — that does a better job of resisting leaks from ice dams than asphalt shingles.

What to do right now if you have an ice dam

Remember, many roofers and general contractors don’t have a good understanding of how ice dams form or how these problems should be fixed.

1. As long as you don’t have a great deal of water entering your home — so much water that you have a major disaster on your hands — it’s usually better to wait until spring before you begin implementing ice dam solutions.

2. Almost every expert warns homeowners that they shouldn’t climb up on the roof to try to remove ice. While this “safety first” advice is understandable, it is sometimes necessary to remove roof ice, especially if you have a major leak. Use common sense. You can either hire a contractor (one who has adequate insurance) to do this work, or you can take the risk yourself. The two biggest risks are falling off the roof — a risk which may be lessened by that enormous pile of fluffy snow that you will land in when you fall — and putting holes in your roofing or flashing because you stupidly decided to break up the ice with a hatchet. Again, common sense is very useful here; start with a wooden mallet or a baseball bat before you graduate to sharper tools. If you are unsure of yourself, don’t do it.

[Photo credit: Ryan McFarland]

3. When removing ice at the eaves is difficult or unwise, it may still make sense to remove as much snow as possible from the area above the ice dam to reduce future melting. This can be done with a shovel (if it's possible to get on the roof safely) or a long-handled roof rake.

4. Some people sprinkle road salt or calcium chloride tablets on their ice dams. You can fill old socks with rock salt, tie the open ends of the socks closed with a knot, and throw the socks onto the roof, aiming for the ice dams. This effort might help, especially if the weather ever warms up into the 20s, but the salt may not be very good for the vegetation below.

5. In the spring, ignore the contractors who talk about roof repair and ventilation improvements. Find a home performance contractor who knows how to seal air leaks, using such tools as a blower door, an infrared camera, or a theatrical fog machine. Remember: in most cases, adding insulation or changing the way a roof is vented will not solve an ice dam problem, especially if no attempt is made to seal air leaks first.

6. Fortunately, most HVAC contractors in cold climates know better than to install ducts or HVAC equipment in a vented unconditioned attic. If you are one of the unlucky homeowners with this problem — ducts or HVAC equipment in your unconditioned attic — the equipment and ducts should either be moved to a new location inside your home's thermal envelope, or your attic should be modified to turn it into an unvented conditioned attic. (For more information on this issue, see Creating a Conditioned Attic.) Trying to address this problem by leaving the ducts where they are, and merely improving the thickness of the duct insulation or sealing the duct seams, usually results in disappointment.

7. Once air leaks in your home's thermal envelope have been sealed, it may be useful to increase the R-valueMeasure of resistance to heat flow; the higher the R-value, the lower the heat loss. The inverse of U-factor. of the insulation between your ceiling and the underside of the roof sheathing. Aim for a minimum of R-49 insulation. An effective but expensive solution to ice dam problems is to install thick layers of rigid foam or nailbase above the existing roof sheathing, along with a “vented over-roof” above the rigid foam. Ideally, the vent channel above the rigid foam will be at least 1 1/2 inch high.

8. While changes to roof ventilation details rarely solve an ice dam problem, such changes may improve matters — but only if you have conscientiously finished your air sealing work and your R-value improvements.

9. Solving ice dam problems in an older building is often difficult. Air leaks can be tricky to track down; attic areas that need work are often impossible to access without opening up holes in a ceiling or roof; and undersized rafters often provide insufficient room for the required insulation. Most of these problems have solutions, but the solutions aren’t cheap.

Electricity to the rescue!

10. There are many more contractors pushing bad ice dam solutions — for example, electric-resistance heating cables — than there are contractors pushing good ice dam solutions.

Addressing these problems is worth it

Homeowners who bite the bullet and do a thorough job of addressing their ice dam problems — perhaps by installing a thick layer of rigid foam above their roof sheathing, followed by above-foam ventilation channels, a second layer of sheathing, and new roofing — can look forward to feelings of satisfaction next winter.

If all goes according to plan, your house will stay icicle-free — even when every other house on the block is loaded with 5-foot icicles. You'll also enjoy a useful side benefit: lower energy bills.

Martin Holladay’s previous blog: “Designing for the Future.”

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Image Credits:

  1. Image #1: Andrew Kuchling
  2. Image #2: CMHC
  3. Image #3: Tom Belknap
  4. Image #4: Security Construction
  5. Image #5: Nick LaGreca
  6. Image #6: Kirsten Durocher
  7. Image #7: Fleet Hill

Feb 27, 2015 10:38 AM ET

when venting isn't
by Hobbit _

Okay, I see one major issue with the whole vented thing, whether
it's an over-roof on top of a robust insulated structure or a
typical vented-attic setup. When the RIDGE VENT is covered with
snow, there's no more venting to speak of. There may be a bit
of thermal break across the vented space, but either configuration
is probably going to let a bit more heat than expected accumulate
in what is now basically a dead-end air space that's only open at
the eaves and wind up warming the roof deck anyway.

And having R-20+ of snow on top is going to put the deck somewhere
in the middle of the overall thermal gradient in almost all cases.
I noticed this when I went to shovel the weight off my shed dormer;
no ice dams, but still some evidence of melt-n-refreeze going on
under there.

Possibly one argument for gable vents, as outdated as they seem
to be these days... they won't get snowed on.


Feb 27, 2015 10:53 AM ET

Edited Feb 27, 2015 1:16 PM ET.

Response to Hobbit
by Martin Holladay

The R-value of snow varies depending on its density. Estimates of the R-value of snow vary. According to one source, the R-value of fresh dry snow is R-0.5 per inch. Other sources put the R-value of snow at R-1 per inch, or even at the (unlikely) value of R-2 per inch.

In most cases, an R-20 layer of snow would have to be at least 20 inches -- and more likely, 40 inches -- deep.

Snow-covered ridge vents still work, for the simple reason that snow is not an air barrier. Of course, air flow through snow-covered ridge vents may be reduced by snow -- but it won't be stopped.

Snow doesn't cover most roof ridges for long, because roof ridges are exposed to wind, and in some cases to sun. Snow will cover the ridge of a low-slope roof for longer than it will cover the ridge of a steeply pitched roof. My advice for a good (ice-dam-resistant) roof still holds: no valleys, no gutters, a steep pitch, and standing seam instead of asphalt shingles. These rules can be broken, of course, but breaking the rules moves your roof in the direction of an increasing risk of ice dams.

"Vented over-roofs" still work, as long as they are part of an overall approach that includes good roof design, attention to airtightness, and adequate R-value. The temperature of the air in the ventilation channels is going to be relatively low, even when the ridge vent has some snow on it, as long as there is an effective layer of R-49 insulation under the ventilation channels, and as long as there is at least a little bit of air flow through the ventilation channels. The air flow rate through these channels will vary, of course; the main driving force will be wind, so more air will move through the channels on windy days than on days when the wind is still.

The final proof is in the pudding: if you follow my design guidance, you won't have any ice dam problems. People who build roofs according to these guidelines know that these approaches work.

Feb 27, 2015 5:55 PM ET

ice dam formation
by Paul Eldrenkamp


Your description of an ice dam is a little dated. Check out pages 113-114 of Bill Rose's book "Water in Buildings." Bill refers to Canadian research that indicates that the real problem is not so much the dam itself but the lens of ice that is formed uphill of the dam. "It is this lens that allows the buildup of water, providing a water head sometimes several feet in height."

Bill's analysis of ice dams in general remains the best I've ever read (no surprise there). I wonder if you could get permission to reprint just that section here on GBA?

--Paul Eldrenkamp

Feb 28, 2015 6:44 AM ET

Edited Feb 28, 2015 7:53 AM ET.

Response to Paul Eldrenkamp
by Martin Holladay

Thanks for your comments. You wrote, "Your description of an ice dam is a little dated. ... I wonder if you could get permission to reprint that section [on ice dams] here on GBA?"

In fact, the shape of the ice that you call a lens is illustrated in an image (Image #4) posted in my first GBA article on ice dams. (You may have noticed that in this article, I referred readers there: I wrote that this is "an introductory article to summarize basic information on ice dams. If you still have questions after reading the advice here, you should probably read “Prevent Ice Dams With Air Sealing and Insulation.”")

The caption to that GBA image reads, "This drawing of an ice dam is more accurate than the other (more simplified) drawings. Based on careful observations by researchers from Canada, where residents know a thing or two about ice dams, the drawing comes from a CMHC publication, "Attic Venting, Attic Moisture and Ice Dams"."

Moreover, that earlier article on ice dams benefits from a posted comment by Bill Rose, who wrote, "Don Fugler and other Canadians have refined the picture [of the shape of the ice that forms on a roof]. It includes formation of an ice layer at the bottom of the snow extending up the roof, and melted water gets beneath this ice."

While the refined analysis by Don Fugler and Bill Rose, explaining the shape of the ice lens that forms on a typical roof, is helpful -- I always wondered how water could back up as far as it does to cause so many leaks -- a full explanation of the shape of the ice lens is (fortunately) unnecessary for those seeking to implement ice dam remedies.

I am reproducing Image #4 from my earlier article below. (I have also added this drawing to the article on this page, as Image #2, to clarify for readers what a typical ice dam looks like.)


Ice dam graphic 3.gif

Feb 28, 2015 11:28 AM ET

Ice Dams and HVAC
by Bruce Chyka

Attic duct work also needs checked for leaks. Its not just a blower door. Many supplies run to the edge of the exterior wall and are located close to windows. All connections sheet metal and flex duct need sealed.

Feb 28, 2015 12:28 PM ET

Edited Feb 28, 2015 12:31 PM ET.

Response to Bruce Chyka
by Martin Holladay

You're right, of course. Fortunately, most HVAC contractors in cold climates know better than to install ducts or HVAC equipment in a vented unconditioned attic. (That problem is much more common in Texas and Florida than in Vermont.)

If you are a cold-climate homeowner with an ice dam problem, and you are unlucky enough to have been bamboozled by an HVAC contractor who was stupid enough to put ducts in your attic, you have my sympathy.

The best solutions are either (a) to move the ducts and HVAC equipment to a new location inside the home's thermal envelope, or (b) to transform your vented unconditioned attic into an unvented conditioned attic. (For more information on this option, see Creating a Conditioned Attic.)

Trying to address the issue by leaving the ducts in place, and merely sealing the duct seams and improving the duct insulation, usually results in disappointment.

Thanks for your comments, Bruce. I have edited my article to reflect your points.

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