Ice dam risk in Herrin, IL

How often ice dams form in Herrin

3.1 dam days a winter

Low risk. On an ordinary house in Herrin, conditions for an ice dam come on about 3.1 days in a normal winter, mostly in January and February.

Herrin averages 11 inches of snow a year and about 14 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 Herrin

Attic	Dam days	Risk
Air-sealed and well insulated	2.2	Low
Typical, some insulation, not air-sealed	3.1	Low
Older or poorly insulated, warm attic	4.2	Low

The attic matters less in Herrin than in colder towns: there are only 0.9 days a winter between a typical attic and a sealed one.

What to do about ice dams in Herrin

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.

Herrin winter, month by month

Month	Snow (in)	Days on ground	Dam days
January	3.2	5.6	1.4
February	4.0	3.7	0.8
March	1.3	0.4	0.0
October	0.2	0.1	0.0
December	2.7	3.6	0.7

Herrin next to nearby towns

Herrin ranks 215 of 225 Illinois cities here for ice-dam days, behind Arlington Heights at 21.5 days.

Marion, IL 7 mi, 3.1 days
Carbondale, IL 12 mi, 2.3 days
Mount Vernon, IL 36 mi, 2.8 days
Cape Girardeau, MO 44 mi, 2.1 days
Jackson, MO 45 mi, 2.1 days

Where these numbers come from

NOAA 1991–2020 climate normals from Carbondale Sewage Plt, 8.7 miles from Herrin. 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.