Ice dams form when heat from the living space and air leaks melts snow on the warm part of the roof, and the water refreezes at the cold eaves. The lasting fix is in the attic: air sealing, insulation and ventilation. Massachusetts code also calls for an ice barrier at eaves where ice has a history.
Key takeaways
- An ice dam forms when heat escaping from the house melts snow high on the roof and the meltwater refreezes at the colder eaves, where it pools behind a ridge of ice and can work under the shingles.
- The usual causes are poor insulation, air leaks into the attic around recessed lights, ducts and attic hatches, and poor ventilation. The roof covering is rarely the original cause.
- Boston averages about 43.8 inches of snow a year, and the winter of 2014-15 reached 108.6 inches, with 34.3 in January and 64.8 in February. Heavy winters make the problem visible, but an attic that leaks heat can build dams in an ordinary one.
- Under 780 CMR R905.1.2, where a history of ice along the eaves is designated, asphalt shingle, slate and wood roofs need an ice barrier running from the eave to at least 24 inches inside the exterior wall line.
- Mass Save offers a home energy assessment, and search results describe air sealing and insulation rebates through the program. Those offers can change, so confirm the current terms with Mass Save.
- Do not climb onto an icy roof or chip at ice with a hammer or an axe. Shingles and gutters break easily, and a fall is the larger danger. Call a roofer, or a contractor who deals with attic insulation, instead.
What is an ice dam, and why does it form on a Newton roof?
An ice dam is a ridge of ice at the roof edge. Heat from the living space and air leaks melts snow on the warm part of the roof, and the water runs down and refreezes at the cold eaves. The ridge then holds back more meltwater, which can back up under shingles.
Think of the roof as having two temperature zones. The upper part sits over the heated house and the attic, and it is warmer. The eaves overhang the outside wall, with nothing heated beneath them, and they are colder. Snow melts in the first zone and refreezes in the second.
The ice at the eave grows into a dam. Meltwater coming down behind it has nowhere to go, so it pools and creeps up under the shingles, where shingle laps are meant to shed water running downhill, not water standing still. From there it can reach the sheathing, the insulation and the ceiling below.
That is the whole mechanism, and it is why the cause is usually inside the house. Newton's housing is old, with a median construction year of 1942 and about 49 percent of homes built before 1940, and older houses often have thin insulation and many gaps where pipes, chimneys and wiring pass into the attic. The conditions are in place long before the first snowfall.
What actually causes ice dams?
The sources reviewed name three causes: poor insulation, air leaks from the living space into the attic, and poor ventilation. Common leak points are recessed lights, ductwork and attic hatches. All three let heat reach the underside of the roof deck, which is what melts the snow.
A state ice dam leaflet and Mass Save material list the same short set of causes, and they all come back to heat. Insulation slows heat moving up through the ceiling. Air sealing stops warm indoor air from rising through gaps. Ventilation moves cold outside air along the underside of the roof so the deck stays near the outdoor temperature.
Air leaks matter more than people expect. A recessed light in a ceiling is a hole into the attic, a duct run through the attic can leak warm air, and an attic hatch that is not sealed lets heat straight up. Insulation laid over these gaps does not stop warm air from moving through them.
Snow depth makes the problem visible but does not create it. A heavy winter, such as the 108.6 inches Boston recorded in 2014-15, gives a leaky house more snow to melt, and so more ice. A lighter winter with a few freeze-and-thaw cycles can still build dams on a roof that loses heat.
- Thin or compressed insulation over the ceiling.
- Air leaks at recessed lights, duct runs and attic hatches.
- Gaps around chimneys and plumbing stacks where they enter the attic.
- Soffit and ridge ventilation that is blocked or missing.
- Warm air from bathroom or kitchen fans vented into the attic instead of outdoors.
How much snow does the Boston area get, and does it matter?
Boston averages about 43.8 inches of snow a year, according to National Weather Service figures. The snowiest winter, 2014-15, totaled 108.6 inches, including 34.3 in January and 64.8 in February. Heavier snow means more meltwater, but ice dams follow heat loss, not snow total alone.
The numbers help you judge how often your roof is tested. In an average winter Boston-area roofs see a modest snowpack. In an extreme one, such as 2014-15, they carry months of it, and the January and February totals show how fast it arrived.
Wind and storms add their own pressure. The January 2018 bomb cyclone brought at least 17 inches of snow to Boston and a record 15.16 foot storm tide, and a storm like that can leave drifted snow against walls, dormers and chimneys where meltwater collects. For the damage side of storms, see storm damage roof repair.
No design ground snow load is stated for Newton here. Design snow loads in the state code range from 45 to 65 pounds per square foot depending on the community, and the figure that applies to your house is the one in the code for your town.
| Measure | Figure | Source type |
|---|---|---|
| Average annual snowfall, Boston | About 43.8 inches | National Weather Service |
| Winter 2014-15 total | 108.6 inches, Boston's snowiest | Published weather records |
| January 2015 | 34.3 inches | Published weather records |
| February 2015 | 64.8 inches | Published weather records |
| January 2018 bomb cyclone | At least 17 inches in Boston; record 15.16 ft storm tide | Published weather records |
What fixes ice dams for good?
The lasting fix is to stop the heat reaching the roof deck: seal the air leaks into the attic, add insulation, and make sure the roof ventilation works. Cutting away ice treats the symptom. Where a roof leaks, repair the flashing or covering that failed, then address the attic.
Think of the fixes in order. First, air sealing: close the gaps around lights, ducts, hatches and penetrations. Second, insulation over the sealed ceiling. Third, ventilation, so cold air can move under the roof deck. The order matters because insulation laid over open air leaks gives less than insulation over a sealed ceiling.
A home energy assessment is a sensible starting point, since it looks at the whole house and not at the roof alone. Mass Save, the state's energy efficiency program, offers one, and results describe air sealing and insulation rebates connected to it. Terms change, so ask Mass Save for what applies to your house today.
Work on the roof is sometimes part of the answer. If water has already got under the shingles, a roofer can find and repair the damage, and where gutters have been pulled off by ice, they can be reattached or replaced. Our pages on ice dam prevention and repair and attic ventilation and roof insulation describe how those jobs go.
No fix is a guarantee. Weather varies, and a house with several leaks may need work in more than one place. A roofer or insulation contractor who looks at your attic can tell you what the house needs.
What does Massachusetts code say about an ice barrier?
Under 780 CMR R905.1.2, where there is a history of ice forming along eaves, asphalt shingle, slate, wood shingle and shake roofs need an ice barrier. That means two cemented layers of underlayment or a self-adhering polymer-modified bitumen sheet, from the eave to at least 24 inches inside the exterior wall line.
The ice barrier is a second line of defense. It does not stop ice from forming, but if meltwater backs up under the shingles, the barrier is the layer that keeps it from reaching the deck. It goes on during a re-roof, which is why the requirement shows up on quotes for replacements.
The code applies where a history of ice along the eaves is designated in Table R301.2(1), and the designation is made by community. Whether your house sits in a designated area is a question for the Newton building department. Do not assume the barrier is already there on an older roof.
A full roof replacement in Newton needs a building permit, so the barrier should be on the permit plan for a re-roof. For what the permit involves, read whether you need a permit to replace a roof in Newton, and for how the barrier affects price, the cost guide explains the lines to look for on a quote.
What should you do when an ice dam is already leaking?
Do not climb onto the roof or hack at the ice. Move belongings out of the wet area, place a container under the drip, and call a roofer. Take photographs of the staining. A roofer can trace the source, while an attic specialist can address the heat loss behind it.
Ice on a roof is slippery and heavy, and shingles and gutters are easily damaged by a hammer, an axe or a shovel. A fall from a roof is far worse than a stain on a ceiling. If you can see ice at the eaves and water inside, the right step is a phone call, and calling is the fastest fix.
Inside the house, deal with what you can reach. Keep furniture out from under the leak, catch the water in a container, and note where the stain started and when. A photograph of the ceiling and one of the roof edge from the ground help a roofer understand what happened.
A leak that starts during a thaw is not always an ice dam. Chimneys, valleys and wall-to-roof joints leak when snow sits against them, and our guide pages on roof leak repair and roof flashing repair cover the other usual sources. A roofer tracing the water to its entry point is more useful than a patch over the visible stain.
When you are ready, call (833) 751-2102, describe what you see from the ground, and you will be put through to an independent roofer who serves your town. Ask what they can do about the roof and who would handle the attic side.
Are gutters part of the ice dam problem?
Gutters can be affected, since ice and snow load them and a dam can pull them from the fascia. Gutters are rarely the root cause, though. Heat loss drives the dam, and a gutter that is clean and well fixed drains meltwater better than one that is full or sagging.
A gutter at the eave sits right where the ice forms, so it takes the load. Ice inside a gutter adds weight, and a dam that grows over the edge can pull the gutter away from the house. Leaves left in the gutter from fall make the load worse, a point that matters on wooded lots in Weston, Lincoln and Wellesley.
Removing the gutters is not an ice dam cure, because the ice forms from heat loss whether there is a gutter or not. But a gutter that drains well, is firmly fixed and is cleared in the fall gives meltwater an easier path. The page on gutter installation and repair describes the work.
Frequently asked questions
Why do my Newton neighbor's roofs have icicles and mine does not?
Icicles show where meltwater is running off the eaves, and they depend on how much heat each attic loses. A roof with no icicles may simply lose less heat, or the snow may not have melted at the time. The attic's insulation, air leaks and ventilation decide the pattern.
Will a new roof stop ice dams?
Not by itself. Ice dams come from heat reaching the roof deck, so a new covering over a leaky attic can still build them. A re-roof can add the ice barrier the code calls for in designated areas, which is protection if water backs up, but air sealing and insulation address the cause.
Does Mass Save pay for air sealing?
Search results describe a home energy assessment and air sealing with insulation rebates through Mass Save. Terms and eligibility change, so this page cannot confirm what applies to your house. Ask Mass Save directly, and book the assessment before you hire anyone to insulate.
Is the ice barrier required on every Newton roof?
The code requires it where a history of ice along the eaves is designated in Table R301.2(1) of 780 CMR. Whether that designation covers your house is a question for the Newton building department, so ask before you assume the barrier is or is not required.
Can I knock the ice off myself?
It is a bad idea. Shingles and gutters are easily damaged, ice breaks unpredictably, and a ladder on frozen ground is a fall risk. Call a roofer, tell them what you can see from the ground, and let them decide how to deal with the ice and the leak.
Do ice dams only happen after big storms?
No. They need snow on the roof, heat coming through it and cold at the eaves. Boston averages about 43.8 inches a year, and a leaky attic can build dams in a normal winter. A record winter such as 2014-15 makes them worse but is not required.
Sources
- National Weather Service Boston/Norton – climate and snowfall records
- Mass Save – home energy assessments and insulation incentives
- Massachusetts Board of Building Regulations and Standards – 780 CMR, Massachusetts State Building Code
This guide is general information for Middlesex County homeowners, not a substitute for an on-site inspection. Prices are estimates. See our disclaimer.