Ice dam risk in Northfield, MN
Will your roof get ice dams in Northfield, MN?
33.6 dam days a winter
High risk. On an ordinary house in Northfield, conditions for an ice dam come on about 33.6 days in a normal winter, mostly in January and February.
Northfield averages 43 inches of snow a year and about 90 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 Northfield
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 13.5 | Moderate |
| Typical, some insulation, not air-sealed | 33.6 | High |
| Older or poorly insulated, warm attic | 58.6 | Severe |
Sealing and insulating a typical attic takes out about 20.0 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 58.6 days.
Preventing ice dams on a Northfield 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.
Northfield winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 9.4 | 26.1 | 11.5 |
| February | 10.6 | 23.2 | 9.1 |
| March | 6.4 | 14.1 | 3.8 |
| April | 1.6 | 1.7 | 0.2 |
| May | 0.2 | 0.0 | 0.0 |
| October | 0.1 | 0.3 | 0.0 |
| November | 4.9 | 5.0 | 1.3 |
| December | 10.1 | 19.6 | 7.6 |
Northfield next to nearby towns
Northfield ranks 57 of 103 Minnesota cities here for ice-dam days, behind Hermantown at 52.9 days.
- Faribault, MN 12 mi, 33.6 days
- Farmington, MN 14 mi, 32.6 days
- Lakeville, MN 16 mi, 32.6 days
- Apple Valley, MN 20 mi, 32.6 days
- Rosemount, MN 21 mi, 32.6 days
Where these numbers come from
NOAA 1991–2020 climate normals from Faribault, 11.2 miles from Northfield. 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.