Ice dam risk in Alton, IL
How often ice dams form in Alton
3.7 dam days a winter
Low risk. On an ordinary house in Alton, conditions for an ice dam come on about 3.7 days in a normal winter, mostly in January and February.
Alton averages 17 inches of snow a year and about 17 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 Alton
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 2.4 | Low |
| Typical, some insulation, not air-sealed | 3.7 | Low |
| Older or poorly insulated, warm attic | 5.4 | Moderate |
The attic matters less in Alton than in colder towns: there are only 1.3 days a winter between a typical attic and a sealed one.
What to do about ice dams in Alton
- Check the gutters before the first snow and clear them, so meltwater has somewhere to go. If you see icicles thicker than a finger along the eave after a storm, that is the early sign to look at the attic.
Snowy places are not always the worst: why snowfall misleads.
Alton winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 5.7 | 7.2 | 1.9 |
| February | 4.3 | 4.3 | 0.9 |
| March | 2.3 | 1.4 | 0.1 |
| April | 0.2 | 0.0 | 0.0 |
| November | 0.9 | 0.7 | 0.1 |
| December | 3.2 | 3.8 | 0.8 |
Alton next to nearby towns
Alton ranks 205 of 225 Illinois cities here for ice-dam days, behind Arlington Heights at 21.5 days.
- Wood River, IL 5 mi, 3.7 days
- Godfrey, IL 5 mi, 4.8 days
- Bellefontaine Neighbors, MO 11 mi, 3.7 days
- Edwardsville, IL 12 mi, 3.6 days
- Florissant, MO 12 mi, 3.7 days
Where these numbers come from
NOAA 1991–2020 climate normals from St Louis Lambert International Airport, 15.9 miles from Alton. 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.