Ice dam risk in Decatur, IL

How often ice dams form in Decatur

6.9 dam days a winter

Moderate risk. On an ordinary house in Decatur, conditions for an ice dam come on about 6.9 days in a normal winter, mostly in January and February.

Decatur averages 14 inches of snow a year and about 25 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 Decatur

Attic	Dam days	Risk
Air-sealed and well insulated	4.0	Low
Typical, some insulation, not air-sealed	6.9	Moderate
Older or poorly insulated, warm attic	10.5	Moderate

Sealing and insulating a typical attic takes out about 2.9 days a winter here. It drops the risk from moderate to low. An older house with a warm attic should expect closer to 10.5 days.

What to do about ice dams in Decatur

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.

Decatur winter, month by month

Month	Snow (in)	Days on ground	Dam days
January	5.9	11.0	3.5
February	2.5	6.5	1.7
March	0.9	1.5	0.2
April	0.4	0.1	0.0
November	0.4	0.6	0.1
December	3.5	5.7	1.4

Decatur next to nearby towns

Decatur ranks 196 of 225 Illinois cities here for ice-dam days, behind Arlington Heights at 21.5 days.

Taylorville, IL 28 mi, 6.3 days
Lincoln, IL 31 mi, 7.4 days
Mahomet, IL 37 mi, 8.8 days
Springfield, IL 38 mi, 7.1 days
Champaign, IL 39 mi, 8.8 days

Where these numbers come from

NOAA 1991–2020 climate normals from Decatur Wtp, 2.1 miles from Decatur. 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.