How Ice Dams Cause Leaks 1
Ice dams form when portions of a snow-covered roof warm up enough to melt snow, but the water refreezes at the colder roof edge. This creates a ridge of ice that traps water behind it, forcing moisture under your shingles and into your home’s structure.
What Causes Ice Dams to Form on Your Roof?
An ice dam forms due to a temperature difference between the main part of a roof and its edge. For an ice dam to develop, the upper sections of the roof must be above freezing (warmed to >32°F) while the lower edge stays below 32°F.
The section of the roof directly above the heated interior of the home is a warm zone. Here, heat from the interior escapes and warms the underside of the roof deck, which in turn melts the snow on top of it.
This meltwater flows down the roof until it reaches the eave or overhang, which is a cold zone. Heat from the home’s interior cannot reach this area, so the roof surface is cold. As the meltwater from above flows onto this cold section, it refreezes, forming a ridge of ice known as an ice dam.
The process follows four distinct steps:
- Attic heat warms the roof – Warm air leaking or radiating from your house heats the higher portions of the roof, raising the surface temperature above 32°F
- Snow melts and drains down – Meltwater flows down the roof under the snow, staying liquid as long as it’s over the heated part of the roof
- Refreezing at cold eaves – When water reaches the cold, unheated roof edge above the exterior wall, it refreezes upon contact
- Ice ridge builds up – Repeated melting and refreezing cycles cause ice to accumulate into a ridge along the eaves
According to the University of Minnesota Extension, this cycle continues each day as more snowmelt feeds the growing ice dam at the edge.
How an Ice Dam Causes Damage
As the ice dam grows, it traps melting snow behind it, creating a pool of water on the roof. This standing water can seep underneath the roofing shingles and through the roof deck, leading to leaks inside the home.
When the leaking water reaches the building’s frame, it hits the top plate of the wall. From there, it goes behind the drywall or plaster and drips down through the interior of the wall cavity. This can cause damage to structural components and interior finishes, including the joists, ceiling, top plate of the wall, drywall/plaster, window trim, window frame, and window sash.
Where Water Goes
Backed-up water works its way beneath shingles and through nail holes or seams in the roof deck. This moisture path affects multiple building components:
| Building Component | Type of Damage |
| Roof deck & rafters | Rot, structural weakening |
| Attic insulation | Saturation, loss of R-value |
| Ceiling materials | Staining, sagging, collapse |
| Interior walls | Moisture intrusion, mold growth |
| Window frames | Rot, staining around sills |
| Gutters | Loosening, tearing off |
| Fascia & soffit | Water damage, warping |
| Exterior siding | Paint damage, deterioration |
The exterior components of the roof and wall structure affected by this process include the snow, roofing, roof deck, fascia, gutter, soffit, soffit vent, and siding. The visible signs are the ice dam itself, the water pooled behind it, and the ice and icicles that form on the eaves.
Any ice seen between the fascia and the exterior wall is bad news.
Water forced out by the ice dam can also spill into the soffit or behind fascia boards. In severe cases, you might see water or ice on the exterior siding, indicating that water has penetrated the wall and is escaping outside. Such hidden moisture can warp siding, damage paint, and deteriorate wooden sheathing or trim.
Warning Signs You Have an Ice Dam
The most visible sign of an ice dam is the ice dam itself—often seen as a ridge of ice along the eaves with long icicles draping down from the gutters or roof edge. But there are other warning signs to watch for:
| Warning Sign | What It Means |
| Visible ice at eaves | Ice dam is present or forming at the roof edge |
| Large icicles | Indicates an ongoing melt-refreeze cycle feeding the dam |
| Icicles behind the gutter or soffit | Water is leaking behind the roofing materials |
| Ice between the fascia and the wall | Meltwater has breached the roof edge and frozen on the vertical plane |
| Interior water stains or drips | Indicates water leakage through the roof into the living space |
| Ice on exterior siding or soffits | Water has entered the walls and frozen on exterior surfaces |
According to Peak Roofing Contractors, even a relatively thin ice accumulation (an inch or two) at the roof edge can be enough to cause problems. A dam is often obvious as a mound of ice with water behind it, especially in valleys or along lower roof sections.
Any ice seen between the fascia and the exterior wall is particularly serious—this means water is escaping where it shouldn’t and has likely already caused hidden damage to your roof structure.
The Skinny
Warm air escapes the heated interior, warms the roof deck, melting snow above which sends meltwater down the roof to the unheated eave, where it refreezes to form an ice dam.
As discussed in Case Study #128, the origin zone of ice dams on eaves is typically directly above the top of the exterior wall.
The 6″ Rule: How High Should Heat Cable Go?
The 6″ Rule is simply this: The height of the heat cable tracing pattern you install—how far up the roof the cable is installed—is determined by the ‘Origin Zone’ of the ice dam.
This is exactly why most heat cable manufacturers stipulate the tracing height go at least 6″ higher than the plane of the exterior wall below. When energized, the heat cable will create melted pathways through the ice at the Origin Zone, allowing meltwater from the roof above to run harmlessly off the roof.
Why 6 Inches Above the Wall Line?
The origin zone is the section of the roof above the exterior wall where ice dams typically start—right where the warm interior meets the cold overhang. To ensure cables melt through this critical area, the zig-zag heat cable pattern must go upslope past the exterior wall line.
According to Kuhl’s Contracting, running the cables about 6″ beyond where the outside wall meets the roof creates melted pathways through the exact spot where ice dams form.
Example: If your roof has a 12-inch eave overhang, you would run the heat cable roughly 18 inches up from the roof edge (12″ for the overhang + 6″ extra). By extending slightly above the heated portion of the house, the cables cross the typical dam origin point, ensuring meltwater can drain off instead of refreezing at the roof edge.
How Much Cable Do You Need? Click here to use our Heat Cable Calculator
Need help with installation? Find a Heat Tape PRO installer in your area or call us at 1-877-387-4218. Our team has been solving ice dam problems for over 35 years—we’re here to help.
Radiant Solutions Company
1515 5th Street South, Hopkins Minnesota, 55343
Phone: 1-877-387-4218
www.radiantsolutionscompany.com