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Icicles along the eaves look like the picture of winter — right up until they aren’t. Most are harmless. But once they thicken past two or three inches in diameter, they stop being decoration and become the visible edge of an ice dam: a ridge of ice locked along the roof’s edge that can tear down gutters, force water up under the shingles, and stain the ceilings in the rooms below.
The cause is rarely the weather alone. Heat escaping from the living space into the attic warms the underside of the roof and melts the snow lying on it. That meltwater runs down to the cold overhang, refreezes there, and builds a dam that traps everything draining behind it. In the short term the answer is steam and a colder attic. The lasting answer is a roof and gutter deicing cable system along the eaves, gutters and downspouts, which holds a drainage path open all winter so ice never gets the chance to dam.
Harmless icicles vs. destructive ice dams
Not every icicle is a warning. Before you call anyone, work out which of these two things you are looking at.
| What you see | Harmless icicles | Destructive ice dam |
|---|---|---|
| Icicle size | Thin, short, pencil- to finger-width | 2–3 inches or more in diameter, long and heavy |
| Where the ice sits | Hanging from the gutter lip only | A ridge of solid ice built up along the roof edge and inside the gutter |
| Snow on the roof | Even snow cover across the whole roof | Bare or melted patches up high, thick snow and ice at the cold eaves |
| Cause | Sun melting surface snow on a mild day, refreezing at night | Warm air leaking from the living space into the attic, melting snow from underneath |
| Timing | Follows a brief thaw-freeze cycle | Follows heavy snowfall plus several days of sustained freezing temperatures |
| Inside the home | No interior signs | Water stains or sagging ceilings, peeling paint, damp attic insulation, mold or mildew smell |
| Risk | Falling ice near walkways and entries | Torn-off gutters, loosened shingles, water backing up under the roof and into the house |
That two-to-three-inch threshold comes from Rob Robillard, a general contractor and carpenter who edits AConcordCarpenter.com and writes on home repair for the Boston Globe. His point is that the icicle itself matters far less than what is going on above it.
How ice dams form
“Ice dams form as a result of warm air inside your home leaking into the attic and warming the underside of the roof, causing snow and ice to melt,” Robillard writes. “The melted water will drain along the roof, under the snow, until it reaches the cold overhang. The overhang tends to be at the same temperature (below 32 degrees) as the outdoors, and the melted water will refreeze and form icicles and possibly an ice dam.”
As more melted water reaches the dam, it backs up under the roof shingles and escapes through cracks and openings in the roofing, leaking into your home, he explains. Ice dam problems typically appear after heavy snowfall followed by several days of freezing temperatures.
What an ice dam does to your home
According to an article in This Old House Magazine, dams have the ability to “tear off gutters, loosen shingles, and cause water to back up and pour into your house.” In turn, this results in “peeling paint, warped floors, stained and sagging ceilings, [and] soggy insulation in the attic, which loses R-value and becomes a magnet for mold and mildew.”
What to do if you already have an ice dam
Never chip, hammer or salt an ice dam off your roof
Hammers, chisels, axes and ice picks crack shingles, puncture the roof deck and tear off gutters — the repair often costs more than the leak you were trying to stop. Tossing calcium chloride or rock-salt pucks into the gutter is just as risky: the salt corrodes aluminum gutters, flashing and fasteners, damages the plantings below, and only melts a narrow channel that refreezes within days. Working from a ladder on ice is also how most ice-dam injuries happen.
For an active leak, This Old House Magazine suggests aiming a box fan at the underside of the roof where water is getting in, to freeze that water and buy yourself time. To remove the dam itself, Robillard recommends steam delivered through a hose and wand to cut the ice apart from the outside and lift it off the roof — a job best left to a professional. Keeping the attic cold and well ventilated stops the dam from rebuilding a week later.
How to prevent ice dams for good
Steam and box fans are triage. The only way to stop meeting ice dams every February is to keep them from forming at all — which means giving meltwater a route off the roof before it reaches the freezing overhang and stalls there.
Deicing cable is not trying to melt the snow off your roof, and it does not need to. It opens a narrow channel straight through the ice — along the eaves, into the gutter and down the downspout — so meltwater always has a route to the ground instead of pooling behind a dam and backing up under the shingles. The WarmlyYours Roof and Gutter Deicing Cable System is run through exactly the places ice reliably starts: gutters, downspouts, overhangs, valleys, and the pinch points between roof features such as dormers or adjacent rooflines. For most homes the simplest version is a pre-assembled plug-in kit, which arrives ready to fasten along the roof edge and plugs into a standard outlet.
See a roof and gutter deicing system in action
This two-minute walkthrough shows how a pre-assembled, plug-in roof and gutter deicing kit is laid out along the roof edge, gutters and downspouts to keep a drainage path open all winter.
Plan your roof and gutter deicing system
Not sure which cable to specify? A pre-assembled plug-in kit covers most straightforward gutter and downspout runs, while a self-regulating cut-to-length heating cable is cut on site to fit valleys, dormers and complex rooflines and varies its output with the temperature, drawing less power as conditions warm.
Two free resources make it easier to spec the right cable and get a price:
- PRO Pocket Guide: Roof & Gutter — Self-Regulating vs. Constant Wattage Cable — a quick-reference guide for trade professionals, contractors and installers on when to quote self-regulating versus constant wattage heating cable.
- Roof & Gutter Deicing Quotation Form — send us your roof, gutter and downspout measurements and we will put together a custom quote and installation plan.
And while you are preparing the rest of your property for winter, see how outdoor radiant heating makes life easier in winter by keeping driveways, walkways and steps clear of snow and ice.
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