Ice dams on your roof: why they form, what to do now and how to stop them

Updated November 10, 2026 · KeystoneProGuide editorial team

What is an ice dam?

A ridge of ice along the edge of the roof that stops melting snow from draining off. The University of Minnesota Extension says the water that backs up behind the dam finds cracks and openings in the roof covering, runs into the attic, and from there can get into exterior walls or through the ceiling insulation and stain the ceiling below.

Why do ice dams form?

Because the roof is warm in the wrong place. The extension says an ice dam needs three things at once: snow on the roof, the higher parts of the roof surface above freezing while the lower parts are below it, and outside temperatures below freezing. Snow melts on the warm upper roof, the water runs down to the cold edge, and it freezes there. The dam then grows as more melt water arrives behind it.

The heat, it says, comes mainly from the house. Some passes through the ceiling and insulation, but in many homes the major route is air leaking from the living space into the attic, through gaps around light fixtures and other openings. Kitchen and bath exhaust fans that end just above the roof, chimneys, and leaky heating ducts in the attic add to it. ENERGY STAR describes the same thing from the other side: snow melting off a roof over an attic that is too warm and refreezing at the gutters.

Gutters are not needed for one to form. The National Roofing Contractors Association (NRCA), the trade's association, says ice damming can happen without gutters, and is more likely on lower-slope roofs, in valleys, and upslope of chimneys and other things that pass through the roof.

Do ice dams happen around Philadelphia?

The conditions for them do. At the National Weather Service's Philadelphia station, the normal low temperature is below freezing in December, January and February, and normal snowfall is 7.1 inches in January and 8.4 inches in February. The extension's advice is written for Minnesota, so take its specifics as a guide rather than a local rule.

What should you do while an ice dam is forming?

Here the sources differ, so it helps to know both.

• Clearing snow. The extension says removing snow from the roof takes away one of the ingredients, and that a roof rake or push broom can do it, though either may damage the roofing. NRCA also says to remove as much snow as possible, but to call a professional roofer if the roof is steep, the snow is deep or the ice is thick.
• Melting a channel. In an emergency where water is coming into the house, the extension says you can make channels through the dam by hosing it with tap water on a warm day, working up from the lower edge, and warns the channel will stop working within days. NRCA, by contrast, does not recommend hosing a roof with water, or using a hot air gun.
• Tools. NRCA does not recommend ice picks, shovels or any sharp-edged tool, because they can damage the roof covering and flashing. It says electric heat cables generally have limited effectiveness.
• Chemicals. NRCA says melting pellets generally do not shorten the life of asphalt shingles but can stain them until the residue washes off, and that calcium or magnesium chloride is less harsh than sodium chloride.

Whatever you do, the extension warns that anyone on the roof in winter, or working on it from below, risks injury and may damage the roof and house. What a homeowner can safely do covers where to draw the line.

What should you do about the water that got in?

Dry it out quickly. The extension warns that moisture from ice dams can lead to mold and mildew, and says to dry wet parts of the house immediately and deal with the water source. It also says not to repair interior damage until the ceilings and walls are dry, and to fix the heat-loss problem at the same time, or the damage will happen again.

How do you stop ice dams for good?

By keeping the roof cold. The extension's long-term advice comes in a set order: first make the ceiling airtight so warm, moist air cannot flow from the house into the attic, and only then consider adding insulation. ENERGY STAR agrees that attic air sealing combined with insulation can help alleviate ice dams, and lists ice dams among the symptoms that a house needs it.

Ventilation plays a supporting part. ENERGY STAR says natural airflow through the attic helps keep it cold in winter, and that the most common mistake when adding insulation is blocking that airflow at the eaves. The extension says that once heat loss is cut, only small amounts of roof ventilation are needed, and that mechanical attic ventilation is not a recommended fix for ice dams in Minnesota because it can cause other moisture problems. The extension suggests a weatherization contractor who can test the ceiling's airtightness with a blower door. Attic ventilation and insulation goes into this in more detail.

What does the building code require?

An ice barrier, where the local code calls for one. The 2021 International Residential Code requires an ice barrier under asphalt shingles, metal shingles, slate and wood shingles in areas with a history of ice forming at the eaves and backing up water, as designated locally. It must run from the lowest edge of the roof to at least 24 inches inside the exterior wall line, and further on very steep roofs. Ask your code office whether your municipality designates it, and ask the roofer whether the quote includes it. Fixr prices ice and water shield at about $0.50 a square foot.

The code deals with the water; the extension's point is that the dam itself is prevented by controlling the heat lost from the house. Roofers serving the area are listed on the roofers directory page.

Sources

1. Dealing with and preventing ice dams, University of Minnesota Extension
2. Resources (frequently asked questions), National Roofing Contractors Association (NRCA)
3. About Attic Ventilation, ENERGY STAR
4. Attic Air Sealing Project, ENERGY STAR
5. Chapter 9 Roof Assemblies, 2021 International Residential Code, International Code Council
6. How Much Does a Roof Replacement Cost? [2026 Data], Fixr
7. Climate Normals for KPHL, National Weather Service