How to Prevent Ice Dams on an Oregon Roof (Gorge, Mt Hood, East Side)

Where in Oregon do ice dams actually happen?

An ice dam needs two things: a snow load that sits on the roof for days, and a roof deck that is warmer than the eave. West of the Cascades the first condition is mostly missing. Salem averages 11 inches of snow a year, Washington and Yamhill counties about the same, and Linn County 7. Valley snow is usually gone in two or three days, not long enough for a ridge to build at the gutter line. Corvallis and McMinnville homeowners worry about moss, not ice. The picture changes at the mountains. Hood River County, which takes in the Gorge and the north side of Mt Hood, averages 76 inches of snow and 55 freeze-thaw days a year. Wasco County east of the crest gets 22 inches of snow with 48 freeze-thaw days, and Lane County, which runs from Eugene up into the Cascade foothills, averages 31 inches, most of it above the valley floor. Multnomah and Clackamas counties average 19 inches, and the foothill towns toward Sandy and Estacada see more than Portland proper. Those repeated swings across freezing are what stack ice at the eave, and that is where the 13 roofs on record in Hood River and 12 in The Dalles since 1999 have to earn their keep. Portland is the valley exception. The city takes an ice storm every few years, when freezing rain glazes everything and a cold snap holds it for days. That is not a snow-fed ice dam, but it produces the same result: ice at the gutter, meltwater behind it, and water finding the joints in an asphalt roof on a Southeast Portland bungalow.

What causes ice dams on a roof in the Gorge or Mt Hood foothills?

The roof covering gets blamed, but the cause is under it. Snow on the upper roof melts because heat from the house is warming the deck from below. The water runs down under the snow blanket, reaches the eave, which hangs out past the heated wall and is as cold as the air, and refreezes. Each cycle adds a layer, the ridge grows, and the pool behind it backs up under the shingles. On a Hood River or Parkdale house with 55 freeze-thaw days that cycle can repeat most of the winter. The heat gets into the attic two ways. The first is air leakage: warm room air rising through recessed can lights, the attic hatch, bath fan housings, plumbing and wiring penetrations, chimney chases and duct joints. The second is conduction through thin insulation. Older Gorge and foothill homes, and a fair share of mid-century ranches around Sandy and Welches, were built with a few inches of batt on the ceiling and heating ducts run through the attic, which is about the worst arrangement for ice dams. Cathedral ceilings with no attic space at all are the other common offender in mountain builds. The third factor is ventilation. Even a well-insulated attic collects some stray heat, and if there is no path for cold air to move from the soffit to the ridge, that heat sits against the deck. Blocked soffit vents and ridge vents with no intake are common in east-side homes where someone tried to keep the wind out.

How do you prevent ice dams on an Oregon home?

Start in the attic, because that is where the fix lasts. Seal the leaks first: foam or caulk around every penetration, gasket the hatch, box over recessed lights with an airtight cover, and seal duct joints with mastic. Then bring the ceiling insulation up to what the Oregon Residential Specialty Code calls for in new construction, which is a great deal more than most pre-1990 mountain homes carry. Air sealing before insulating matters, since blown insulation over an unsealed ceiling still lets warm air through it. Then make sure cold air moves. Continuous soffit intake at the eaves, baffles to keep the insulation from choking it, and a ridge vent or equivalent exhaust at the top. The goal is a deck that reads close to outdoor temperature on a January morning in Hood River, so the snow on it does not melt any faster than the snow on the ground. Attic work is also the cheapest part of this discussion, and it lowers the heating bill in Wasco County, where winter nights are cold and dry. What does not work as a primary fix is heat cable. It melts channels through the ice so water can drain, but it does nothing about the warm deck, runs all winter on your electric bill, and fails after a few seasons. Cable is a stopgap while you plan the attic work, not a cure. Raking the lower few feet of roof after a storm helps on a single-story ranch in The Dalles; it is not realistic on a two-story chalet at Government Camp.

Does a metal roof stop ice dams in Hood River or Bend-side country?

A metal roof changes the odds without changing the cause. An interlocked aluminum roof, whether standing seam or the shingle and shake profiles, presents a smooth surface with no granules for snow to grip. Once the sun or a warm spell loosens the bond, the snow slides off in sheets instead of sitting for a week. Less snow on the roof means less meltwater, and a shorter window for a dam to form. Four-sided interlocking panels leave no exposed fastener heads or lifted tabs for backed-up water to enter, and aluminum does not rust where ice sits against it for weeks. The honest limit: if the attic is dumping heat against the deck, a metal roof will still melt snow from below and can still form a ridge at a cold eave, especially on a low-slope section or above an unheated porch. Metal is the second line of defense. The attic is the first. The two together are what keep a Mt Hood cabin dry. The snow shedding also has a downside you have to plan for. A roof-load of wet Cascade snow coming off a 6/12 pitch lands hard, and it lands on whatever is below: the front steps, a deck, a propane tank, a heat pump, or a person. Snow guards, set in rows above entries, walkways and mechanical equipment, hold the blanket in place so it melts off gradually instead of releasing at once. On a Parkdale or Rhododendron house we specify them over every door and above the driveway, and leave the back slope free to shed. Skipping them is a common mistake on mountain metal roofs put on by valley crews.

What should go under the roof at the eaves in Oregon snow country?

Whatever covering you choose, the eave is where the backup belongs. A self-adhering ice-and-water membrane, applied to the deck from the eave edge up to a point at least two feet inside the exterior wall line, seals to the deck and around every nail that goes through it. If a dam does form and water backs up under the roofing, it meets a waterproof layer instead of bare plywood. The same membrane goes in the valleys, around chimneys and skylights, and along the sidewalls where snow drifts pile up. In Hood River, Wasco and the Cascade foothills we treat that membrane as standard on every metal roof we put down, and on steeper mountain roofs with deep overhangs the run up from the eave is longer to clear the heated wall line. On a Salem or Albany roof, with 11 or 7 inches of snow a year, a narrower eave membrane still earns its place for the occasional ice storm. It cannot be added later without stripping the roof, so the decision belongs in the reroof conversation, not after the first leak. The complete picture for an Oregon home above the valley floor is a sealed, insulated, vented attic; a membrane at the eaves and valleys; and a roof surface that sheds snow and keeps water out for the 50-plus-year life of the aluminum. Get the attic right and the roof stays cold. Get the membrane right and a bad winter stays outside.

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