Ice dam risk in Janesville, WI
Ice dams in Janesville, WI: how often
20.5 dam days a winter
High risk. On an ordinary house in Janesville, conditions for an ice dam come on about 20.5 days in a normal winter, mostly in January and February.
Janesville averages 37 inches of snow a year and about 60 days with snow on the ground. Most of it comes and goes within a few days. 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 Janesville
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 9.3 | Moderate |
| Typical, some insulation, not air-sealed | 20.5 | High |
| Older or poorly insulated, warm attic | 34.4 | High |
Sealing and insulating a typical attic takes out about 11.2 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 34.4 days.
Preventing ice dams on a Janesville 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.
Janesville winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 10.8 | 20.7 | 8.2 |
| February | 10.1 | 16.9 | 6.0 |
| March | 4.1 | 6.6 | 1.5 |
| April | 1.3 | 0.6 | 0.1 |
| October | 0.2 | 0.1 | 0.0 |
| November | 2.4 | 2.1 | 0.4 |
| December | 7.7 | 12.7 | 4.3 |
Janesville next to nearby towns
Janesville ranks 73 of 98 Wisconsin cities here for ice-dam days, behind Wausau at 41.6 days.
- Beloit, WI 11 mi, 20.5 days
- Whitewater, WI 18 mi, 21.9 days
- Roscoe, IL 18 mi, 20.5 days
- Fort Atkinson, WI 19 mi, 22.2 days
- Stoughton, WI 20 mi, 23.9 days
Where these numbers come from
NOAA 1991–2020 climate normals from Afton - Janesville Wwtp, 4.2 miles from Janesville. 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.
- Snow-on-the-ground counts come from a second, nearby station, because the closest one does not record them.