Ice dam risk in Rochester, MN
How often ice dams form in Rochester
33.3 dam days a winter
High risk. On an ordinary house in Rochester, conditions for an ice dam come on about 33.3 days in a normal winter, mostly in January and February.
Rochester averages 53 inches of snow a year and about 89 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 Rochester
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 12.5 | Moderate |
| Typical, some insulation, not air-sealed | 33.3 | High |
| Older or poorly insulated, warm attic | 59.3 | Severe |
Sealing and insulating a typical attic takes out about 20.8 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 59.3 days.
Preventing ice dams on a Rochester 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.
Rochester winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 12.2 | 25.4 | 11.2 |
| February | 10.7 | 23.3 | 9.4 |
| March | 8.6 | 13.6 | 3.5 |
| April | 3.3 | 1.9 | 0.2 |
| May | 0.5 | 0.1 | 0.0 |
| October | 0.9 | 0.3 | 0.0 |
| November | 4.5 | 4.5 | 1.1 |
| December | 12.4 | 19.8 | 7.8 |
Rochester next to nearby towns
Rochester ranks 59 of 103 Minnesota cities here for ice-dam days, behind Hermantown at 52.9 days.
- Austin, MN 34 mi, 33.8 days
- Owatonna, MN 38 mi, 33.5 days
- Red Wing, MN 40 mi, 36.2 days
- Winona, MN 40 mi, 30.7 days
- Faribault, MN 44 mi, 33.6 days
Where these numbers come from
NOAA 1991–2020 climate normals from Rochester International Airport, 7.7 miles from Rochester. 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.