Ice dam risk in Oregon, WI
How often ice dams form in Oregon
23.9 dam days a winter
High risk. On an ordinary house in Oregon, conditions for an ice dam come on about 23.9 days in a normal winter, mostly in January and February.
Oregon averages 36 inches of snow a year and about 68 days with snow on the ground. Snow that lies that long is what turns a little attic heat into a ridge of ice at the eave. The conditions that make a dam need all three at once: snow, cold at the eave and heat above it.
What your attic does to that number in Oregon
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 10.7 | Moderate |
| Typical, some insulation, not air-sealed | 23.9 | High |
| Older or poorly insulated, warm attic | 40.4 | Severe |
Sealing and insulating a typical attic takes out about 13.2 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 40.4 days.
Preventing ice dams on a Oregon roof
- Air-seal the attic floor: the gaps around the hatch, light fixtures, plumbing stacks and the top plates of interior walls. This is the fix that removes the cause, and it usually costs less than a season of contractor ice removal.
- Rake the bottom three or four feet of roof after any storm that leaves more than about six inches, from the ground with a roof rake.
- At the next reroof, ask for ice-and-water membrane under the shingles from the eave to at least 24 inches inside the wall line. It does not stop a dam; it stops the leak when one forms.
Which to spend on first: air sealing or heat cable.
Oregon winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 10.4 | 22.2 | 9.0 |
| February | 10.6 | 19.9 | 7.3 |
| March | 4.5 | 9.0 | 2.2 |
| April | 0.7 | 0.5 | 0.0 |
| October | 0.4 | 0.1 | 0.0 |
| November | 1.3 | 2.0 | 0.4 |
| December | 7.6 | 13.9 | 4.9 |
Oregon next to nearby towns
Oregon ranks 59 of 98 Wisconsin cities here for ice-dam days, behind Wausau at 41.6 days.
- Fitchburg, WI 5 mi, 22.2 days
- Stoughton, WI 8 mi, 23.9 days
- Verona, WI 9 mi, 22.2 days
- Madison, WI 11 mi, 22.2 days
- Middleton, WI 14 mi, 22.2 days
Where these numbers come from
NOAA 1991–2020 climate normals from Stoughton Wwtp, 9.0 miles from Oregon. A dam day is a day with snow on the roof and freezing at the eave while heat or sun melts the snow above it. This describes a normal winter, not this one.