Ice dam risk in Mattoon, IL
How often ice dams form in Mattoon
5.4 dam days a winter
Moderate risk. On an ordinary house in Mattoon, conditions for an ice dam come on about 5.4 days in a normal winter, mostly in January and February.
Mattoon averages 15 inches of snow a year and about 20 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 Mattoon
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 2.9 | Low |
| Typical, some insulation, not air-sealed | 5.4 | Moderate |
| Older or poorly insulated, warm attic | 8.5 | Moderate |
Sealing and insulating a typical attic takes out about 2.4 days a winter here. It drops the risk from moderate to low. An older house with a warm attic should expect closer to 8.5 days.
Preventing ice dams on a Mattoon 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.
Which to spend on first: air sealing or heat cable.
Mattoon winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 5.2 | 8.1 | 2.6 |
| February | 3.6 | 6.6 | 1.6 |
| March | 1.9 | 1.2 | 0.2 |
| April | 0.1 | 0.1 | 0.0 |
| November | 0.6 | 0.4 | 0.1 |
| December | 3.1 | 3.6 | 1.0 |
Mattoon next to nearby towns
Mattoon ranks 201 of 225 Illinois cities here for ice-dam days, behind Arlington Heights at 21.5 days.
- Charleston, IL 10 mi, 6.8 days
- Effingham, IL 27 mi, 5.2 days
- Decatur, IL 40 mi, 6.9 days
- Champaign, IL 44 mi, 8.8 days
- Urbana, IL 45 mi, 8.8 days
Where these numbers come from
NOAA 1991–2020 climate normals from Mattoon, 0.5 miles from Mattoon. 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.