What Are Ice Dams, and Where in Oregon Do They Form?

What is an ice dam on a roof?

An ice dam is a band of solid ice that builds along the lower edge of a roof, over the eave and gutter, during a stretch of snow and freezing weather. Snow higher on the roof melts, the water runs downslope, and when it reaches the overhang it freezes again into a ridge. Each day of melt adds a layer, and after a week the ridge can be several inches thick and run the full length of the eave. The ice itself is not the damage. The damage comes from the meltwater that arrives behind the ridge, finds nowhere to drain, and pools on the roof above it. In Oregon the picture most people have of this is a cabin at Government Camp or a house on the Hood River valley floor after a January storm, with icicles as long as a fence post. A dam that matters can be a two-inch lip of ice on a Cascade foothills ranch house outside Sandy or Estacada, invisible from the driveway, holding an inch of water against the roof for a week. What counts is whether standing water sits above the eave line with a route under the covering.

How does an ice dam actually form?

Three things have to be true at once: snow on the roof, a roof surface above the living space that is warmer than freezing, and an eave that is colder than freezing. The warm surface comes from the house. Heat leaks through thin insulation, open chases around chimneys and plumbing, recessed lights and attic hatches, and warms the underside of the roof deck. The snow on top melts from the bottom. The overhang gets no heat because there is no house under it, so it sits at outside air temperature and the water refreezes the moment it arrives. Once the ridge exists, the next melt backs up behind it. Water climbs the slope, and on a 4/12 or 6/12 pitch it does not take much depth before it is above the top edge of the first course of shingles. Asphalt shingles and cedar shakes shed water running downhill; they are not built to hold water pushed uphill under them. The water passes the lap, reaches the deck, and follows the sheathing down to the top plate of the exterior wall. This is why weather alone does not explain ice dams. Wasco County around The Dalles gets only 22 inches of snow a year but 48 freeze-thaw days, and a 1960s ranch there with four inches of loose attic insulation will build a dam most winters. A well-insulated, well-vented house next door with the same snow stays cold all the way to the ridge, and no dam forms. The attic is the cause. The eave is where you see it.

Where in Oregon do ice dams form?

West of the Cascade crest and below about a thousand feet, ice dams are rare. Marion County averages 11 inches of snow and 39 freeze-thaw days, and most of those freeze-thaw days happen with no snow on the roof at all. Salem, Albany, Corvallis and McMinnville homeowners can go a decade without a proper dam. Multnomah and Clackamas counties see 19 inches of snow and 36 freeze-thaw days, so the Gresham and Oregon City areas get a short-lived dam every few winters, gone within days when the rain returns. The real ice-dam country in Oregon is the Columbia Gorge and the Mt Hood corridor. Hood River County averages 76 inches of snow and 55 freeze-thaw days a year, which is both the snow load and the repeated melt-and-freeze cycling that grows a dam. Hood River, Parkdale, Odell and the Highway 35 side of the mountain all qualify. Government Camp, Rhododendron, Welches and Brightwood along Highway 26 sit under Mt Hood's snow, and the seasonal cabins there often have the worst attic sealing in the state, because they were built for summer use and heated later. East of the crest, Wasco County gets less snow but stays cold long enough that 48 freeze-thaw days build ice at the eave on any house with a warm attic. The Cascade foothills on the west side are the transition zone. Lane County averages 31 inches of snow, and homes up the McKenzie and Middle Fork valleys toward Blue River and Oakridge see dams in a normal winter while Eugene on the valley floor does not. Benton County's 18 inches and Columbia County's 20 inches land mostly in the hills, so a house on the ridge above Scappoose or behind Philomath can dam while the town below stays clear.

What are the warning signs of an ice dam?

The first sign is outside. Long, thick icicles hanging from the eave and gutter after a snowfall mean water is melting above and freezing at the edge, which is the dam mechanism in progress. Icicles on a roof with no snow left on it are just gutter drainage refreezing and are not a concern. A band of ice sitting on the roof above the gutter, with snow still above it, is the dam itself. On a Hood River house you can often spot it from the street by late January as a grey-white bulge along the eave line. The second sign is inside, and it is the one that matters. Water stains or bubbling paint at the top of an exterior wall, where the wall meets the ceiling, mean water has already passed under the roofing and run down the deck to the top plate. Damp insulation in the attic near the eaves, a musty smell in a north-side bedroom, or drips from a window head on the eave wall all point to the same thing. Older Craftsman bungalows in Hood River and mid-century ranches in The Dalles with knee walls and cathedral ceilings are especially prone, because the roof deck is close to the heated space and the eave is the coldest spot on the house. A stain that reappears every February in Parkdale or Government Camp is a roof and attic that will keep doing this until the cause is addressed.

Do Portland ice storms cause ice dams?

Portland's signature winter event is not snow but freezing rain, the silver thaw that comes when Gorge east wind pours cold air into the metro area under a warm Pacific front. The ice that coats trees and power lines also coats roofs, and afterward the eaves of Portland houses are lined with ice and the gutters are frozen solid. It looks like an ice dam and is often called one. Mechanically it is not the same thing. Freezing rain coats the whole roof from the outside, with no snow above melting from attic heat to back up behind it. Where the two overlap is the day or two after the storm, when Portland warms to the high thirties and the glaze on the upper roof melts while the shaded eave, still in the east wind, stays frozen. For a short window a true dam can form on Craftsman bungalows and foursquares in the older east-side neighborhoods, and that window is when the occasional Portland ceiling stain happens. With Multnomah County at 36 freeze-thaw days a year it rarely lasts long enough to do serious harm. A roof with no lap for uphill water to pass through changes the arithmetic wherever you are. Interlocked aluminum shingles fold into one another on all four sides, so water pushed back up the slope meets a locked seam rather than an open edge, which is why the 13 roofs on record in Hood River since 1999 and 12 in The Dalles have not had the eave leaks their asphalt neighbors get. It does not stop the dam forming, because the dam is an attic problem. Preventing a dam and getting rid of one already on the roof are their own subjects and have their own articles.

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