What Causes Ice Dams on a Roof?
"I'm battling this right now. It has created a leak in my roof into my family room."
That is how one commenter in r/HomeImprovement described an ice dam, and it is the worry behind this search.
An ice dam is a ridge of ice at the eave that stops melted snow from draining off the roof1. It forms when snow sits on a roof whose deck is above freezing2. The heat usually comes from your house, and the melt refreezes at the cooler eave3. So the cause is inside, not just at the edge.
Quick answer:
- Look from the ground for a thick ridge of ice along the eave, and from inside for stains on the ceiling.
- Stay off the roof; if snow has to come down, use a roof rake from the ground3.
- Water coming in: catch it, move belongings away and photograph the damage with the date.
- Ask any roofer to check the attic for air leaks and insulation, not only the ice at the edge.
On this page
Both checks are only a look from the ground and a look at your ceilings.
What causes ice dams on your roof?
Two things have to happen together. Snow sits on the roof, and the roof deck (the boards under the shingles) rises above freezing under that snow2. The snow melts and runs down until it hits the cooler parts near the eaves and freezes3. Ice builds up there, and more water pools behind it.
How heat from your house reaches the roof
Air leaking from the heated house into the attic is often the culprit2. That warms the deck from below, so snow melts on the heated part of the roof even when it is freezing outside. "Often" is not "always", so an attic check is how you find out about your own house.
Air sealing means closing the gaps where house air gets into the attic. Sealing those gaps, combined with attic insulation, can help reduce ice dams4.
Work at the edge alone leaves the heat source in place. Clearing leaves and debris from gutters does let melting snow and ice flow freely5, and it is worth doing, but it is housekeeping and not the cause.
Finding frost or damp insulation in the attic and unsure whether it is a leak? Condensation in the attic: leak or moisture from inside.
Will an ice dam go away on its own?
Not for good. One commenter in r/HomeMaintenance admitted, "I've never really even heard of ice dams till this year. I assumed they just go away." The ice may melt when the weather warms. But the air leaking into the attic2 is still there when the next snow arrives.
What an ice dam looks like, outside and inside
An ice dam is ice piled up at the eave, the lower edge of the roof. Some melt can overflow it and create icicles1, so icicles often hang below a dam. Icicles alone do not prove one, because the sun can make icicles too3.
- Outside, from the ground: a thick ridge of ice along the eave, often with icicles hanging below it.
- Inside: water stains on the ceiling3, and damp marks on the walls below.
A dam can leak because the ice buildup backs water up under the shingles, damaging them and letting water reach the ceilings and walls below6. Left in the house, that water can lead to structural damage and mold3. A stain is your cue to start a photo record and follow the steps below.
What to do this week (and what to skip)
- Stay on the ground. Removing snow from the roof can be dangerous3, so rake from below and keep the rake away from overhead power lines.
- Protect the inside. Put a bucket under any drip, move belongings away from the wet spot, and photograph the stain with the date.
- Prepare. Read your policy for how it treats ice dam damage. In Connecticut, for example, many policies do not pay to remove the dam itself7, so check yours and write down your questions.
- Call. If there is damage inside, call your insurance professional as soon as possible5 with your photos and questions ready.
Skip any quote that only deals with the ice at the edge and says nothing about the attic. If the dam has already created a leak, removal can still be worth pricing.
If the ice has to come off, how do you do it without harming the roof? How to get rid of ice dams.
Questions to ask a roofer, and what to have ready
Ask what goes along the eaves. In cold climates, building codes often require a self-adhered membrane1 there, a sticky waterproof layer on the roof deck that guards against leaks if a dam forms. It protects against leaks but does not stop dams from forming. Also ask whether the attic's air leaks and insulation will be checked, and whether the quote lists each item with its price.
Get that in writing. You can send this:
Please write in your quote what you will put along the eaves, whether you will check and seal air leaks in the attic, and the price of each item. If the attic work is not yours, please tell me who would do it. Please also send photos of anything you say is damaged.
If a roofer pushes you to sign today or won't list the work item by item, that is an answer too: weigh it against the quotes that do.
Before you call anyone, have this ready:
- Your photos of the ice and any stains.
- When the stain or drip first showed up, and whether it grows with a thaw.
- Your policy, and what it says about removing a dam.
- The roofer questions above.
Wondering what an inspection should cover in the attic? Roof inspection checklist: what to check and what to ask.
Quick questions
Does a metal roof prevent ice dams? Don't count on it. The melt starts with heat from the house, often air leaking into the attic2, so ask the roofer about the attic whatever the roof is made of.
Do I seal air leaks or add insulation first? Air sealing comes first: the insulation project is best done after air sealing4. Ask the contractor to confirm the order in the quote.
When is it worth paying to have a dam removed? As one commenter in r/HomeMaintenance asked, "how do you advise people when to decide it’s necessary to pay to have ice dam removed?" Weigh it against the leak risk: a dam with stains or drips inside is a stronger case than ice with a dry ceiling. Ask for the removal price in writing, and ask what is being done about the cause, so you aren't paying for the same job next winter.
Can a roof rake replace fixing the attic? No. Taking snow off the lower roof limits dams there3, but it does not remove the cause, so the attic check still matters.
Sources
3. North Dakota State University Extension, Reduce ice dam and icicle damage
4. ENERGY STAR (U.S. EPA), Attic Air Sealing Project
5. Insurance Information Institute (Triple-I), The homeowner's severe cold weather survival guide
6. Minnesota Department of Commerce, Division of Energy Resources, Home Energy Guide: Ice dams
7. Connecticut Insurance Department, Winter Damage: Tips for Making Insurance Claims
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