Ice dam risk in O'Fallon, IL
How often ice dams form in O'Fallon
2.5 dam days a winter
Low risk. On an ordinary house in O'Fallon, conditions for an ice dam come on about 2.5 days in a normal winter, mostly in January and December.
O'Fallon averages 11 inches of snow a year and about 12 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 O'Fallon
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 1.8 | Low |
| Typical, some insulation, not air-sealed | 2.5 | Low |
| Older or poorly insulated, warm attic | 3.4 | Low |
The attic matters less in O'Fallon than in colder towns: there are only 0.7 days a winter between a typical attic and a sealed one.
What to do about ice dams in O'Fallon
- 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.
O'Fallon winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 3.4 | 5.7 | 1.4 |
| February | 3.3 | 2.2 | 0.4 |
| March | 1.0 | 0.4 | 0.0 |
| November | 0.6 | 0.5 | 0.0 |
| December | 2.9 | 3.0 | 0.6 |
O'Fallon next to nearby towns
O'Fallon ranks 218 of 225 Illinois cities here for ice-dam days, behind Arlington Heights at 21.5 days.
- Shiloh, IL 3 mi, 2.5 days
- Fairview Heights, IL 5 mi, 2.5 days
- Swansea, IL 5 mi, 2.5 days
- Belleville, IL 7 mi, 2.5 days
- Collinsville, IL 7 mi, 3.6 days
Where these numbers come from
NOAA 1991–2020 climate normals from Belleville Siu Rsch, 6.7 miles from O'Fallon. 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.