Ice dam risk in Ontario, OR

Will your roof get ice dams in Ontario, OR?

4.4 dam days a winter

Low risk. On an ordinary house in Ontario, conditions for an ice dam come on about 4.4 days in a normal winter, mostly in January and December.

Ontario averages 15 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 Ontario

Attic	Dam days	Risk
Air-sealed and well insulated	3.1	Low
Typical, some insulation, not air-sealed	4.4	Low
Older or poorly insulated, warm attic	6.0	Moderate

The attic matters less in Ontario than in colder towns: there are only 1.3 days a winter between a typical attic and a sealed one.

Preventing ice dams on a Ontario roof

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.

Ontario winter, month by month

Month	Snow (in)	Days on ground	Dam days
January	6.1	6.5	1.9
February	2.3	3.2	0.7
March	0.5	0.3	0.0
November	1.1	0.7	0.1
December	5.4	5.8	1.6

Ontario next to nearby towns

Ontario ranks 4 of 57 Oregon cities here for ice-dam days, behind Klamath Falls at 9.8 days.

Middleton, ID 28 mi, 2.9 days
Caldwell, ID 31 mi, 2.9 days
Star, ID 33 mi, 2.9 days
Eagle, ID 36 mi, 2.9 days
Nampa, ID 37 mi, 2.9 days

Where these numbers come from

NOAA 1991–2020 climate normals from Malheur Branch Exp Station, 4.0 miles from Ontario. 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.