Do Metal Roofs Help With Ice Dams in Oregon's Snow Country?

Why do ice dams form on Oregon roofs in the first place?

An ice dam is not a roofing failure. It is a heating failure that shows up on the roof. Heat leaks out of the living space into the attic, warms the underside of the roof deck, and melts the bottom of the snowpack from below. That meltwater runs down the deck under the snow until it reaches the eave, which hangs past the heated wall and stays at outdoor temperature. There it refreezes. Each cycle adds a layer, the ridge of ice grows, and water backs up behind it until it finds a nail hole or a shingle lap and comes into the house. That cycle needs three things: snow on the roof, a warm deck, and a cold eave. West of the Cascades the first ingredient is usually missing. Marion County sees about 11 inches of snow a year and Polk about 9, and most of it is gone in two days. Multnomah and Clackamas average 19 inches with 36 freeze-thaw days, which is enough to build a small dam on a poorly insulated Portland bungalow once or twice a winter but rarely enough to do damage. East of the crest and up the Gorge it is a different climate. Hood River County averages 76 inches of snow and 55 freeze-thaw days. Wasco County gets 22 inches of snow with 48 freeze-thaw days and only 18 inches of rain, so snow lingers through long cold spells instead of being washed off. The Mt Hood foothills from Parkdale to Government Camp, and the Cascade foothills above Eugene in Lane County at 31 inches of snow, are where Oregon homeowners actually lose ceilings to ice dams.

Does a metal roof shed snow before it can dam in Hood River?

On the slopes where the sun reaches, yes, and that is the honest core of the answer. Asphalt has a rough granular surface that grips snow and holds it in place while the attic melts it from below. A factory-finished aluminum shingle or standing seam panel has a smooth, low-friction coating. Once the snow bonds break, usually after a few hours of sun or a mild afternoon, the pack slides off in sheets. If the snow is off the roof there is nothing to melt, nothing to refreeze at the eave, and no dam. We have 13 documented aluminum roofs in Hood River since 1999 and 12 in The Dalles, and the pattern on those homes is consistent. South and west slopes clear within a day or two of a storm. The eave stays bare. Homeowners who fought dams every February on the old composition roof stop seeing them on those exposures. What metal does not do is generate heat or change the physics. A low-slope roof, a 3:12 shed dormer, or a slope with no sun will hold snow on aluminum nearly as long as on asphalt. Metal reduces the fuel; it does not remove the cause. Anyone who tells you a metal roof guarantees no ice dams in the Gorge has not spent a winter there.

Where do ice dams still happen on metal roofs in the Mt Hood foothills?

North slopes are the first place. A north-facing roof in Parkdale or Rhododendron gets little or no direct sun from November through February, the snow never reaches the release point, and it sits there compacting into a dense pack that the attic slowly melts from underneath. Aluminum on that slope behaves like any other roof. If the attic below is leaking heat, you will get a dam at the eave, though it will usually be smaller because some of the pack still creeps off between storms. Valleys are second. Where two roof planes meet, sliding snow from both sides piles into the trough and bridges over. The valley becomes a snow reservoir that sits on the coldest part of the roof, right above the exterior wall, and the meltwater under it has nowhere to go but sideways under the roofing. The same is true of any spot where a chimney, a skylight, or a dormer wall stops the slide. The third trouble spot is a roof-to-wall intersection, common on Cascade foothill houses with a one-story addition off the kitchen. The lower roof collects what slides off the upper one, the wall above holds heat, and the addition's eave is where the dam forms. On any of these locations the roofing material is a minor variable. The construction details underneath it are the major one.

What actually stops ice dam leaks on an Oregon metal roof?

Two things, and the roofing surface is neither of them. The first is a self-adhering ice-and-water membrane on the deck along every eave, in every valley, and around every penetration before the aluminum goes down. Oregon Residential Specialty Code calls for ice barrier where there is a history of ice forming along the eaves, which describes every county on the east side of the crest and most of the foothill zone. The membrane seals to the deck and around fasteners, so when water does back up behind a dam it hits a waterproof layer instead of a seam. On Hood River and Wasco County installs we run it at least two feet inside the exterior wall line and often higher on low-pitch sections. The second is the attic. Air sealing the top plates, can lights, bath fan housings, and plumbing chases stops the warm air that melts the snow, and it does more for ice dams than any amount of added insulation. Insulation after sealing slows what is left. Continuous soffit intake with a ridge or high-point exhaust keeps the deck near outdoor temperature so the snowpack stays frozen. A metal roof installed over a leaky attic is a better-looking roof with the same ice dam. The interlocking aluminum system adds one honest advantage here. Every fastener is concealed under the next course, so there are no exposed screw heads on the field of the roof for backed-up water to find. Combined with membrane at the eave, a dam that does form is far less likely to reach the drywall than it was on the 25-year-old composition roof it replaced. Reduced snow, sealed attic, membrane under the metal, in that order of importance, is what keeps a foothill home dry.

Do I need snow guards on a metal roof in Hood River or Wasco County?

If a door, a walkway, a deck, a heat pump, or a gas meter sits under the eave, yes. The same slick surface that clears snow before it dams also releases it all at once, and a slab of wet snow off a 76-inch season is heavy enough to tear off a gutter, crush a shrub, or hurt someone on the porch. Snow guards spaced across the lower courses hold the pack so it melts off in place or slides in small pieces. Over entries they are not optional in the Gorge and on the mountain. On a Willamette Valley ranch with 11 inches of snow a year they are usually unnecessary. Understand the trade-off. A row of guards deliberately keeps snow on the roof above the eave, which is exactly the snow that a warm attic will turn into a dam. That makes the attic sealing and eave membrane more important on guarded sections, not less. We place guards where the snow has to be held for safety and let the rest of the roof shed freely. One more thing to be clear about. The Guardian Lifetime Limited Material Warranty on Interlock aluminum covers the material for the life of the original owner and is transferable once. It warrants against finish failure and covers wind to 120 mph and hail to 1.5-inch stones. It is not an ice-dam guarantee, and no roofing warranty is. Ice dam damage comes from heat and water behind the roofing, so the protection is the membrane and the attic work you pay for at the same time as the roof, not a document. That is the roof that holds up through a Hood River winter.

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