Ice dam risk in Blackfoot, ID
Will your roof get ice dams in Blackfoot, ID?
20.1 dam days a winter
High risk. On an ordinary house in Blackfoot, conditions for an ice dam come on about 20.1 days in a normal winter, mostly in January and December.
Blackfoot averages 15 inches of snow a year and about 64 days with snow on the ground. Snow that lies that long is what turns a little attic heat into a ridge of ice at the eave. 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 Blackfoot
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 11.0 | Moderate |
| Typical, some insulation, not air-sealed | 20.1 | High |
| Older or poorly insulated, warm attic | 31.5 | High |
Sealing and insulating a typical attic takes out about 9.1 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 31.5 days.
What to do about ice dams in Blackfoot
- 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.
- At the next reroof, ask for ice-and-water membrane under the shingles from the eave to at least 24 inches inside the wall line. It does not stop a dam; it stops the leak when one forms.
Which to spend on first: air sealing or heat cable.
Blackfoot winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 6.0 | 19.6 | 7.0 |
| February | 2.2 | 14.8 | 4.3 |
| March | 0.3 | 4.2 | 0.8 |
| April | 0.7 | 0.8 | 0.1 |
| October | 0.8 | 0.8 | 0.1 |
| November | 1.4 | 4.5 | 1.0 |
| December | 3.3 | 19.3 | 6.8 |
Blackfoot next to nearby towns
Blackfoot ranks 5 of 26 Idaho cities here for ice-dam days, behind Rexburg at 42.8 days.
- Chubbuck, ID 19 mi, 20.8 days
- Pocatello, ID 23 mi, 9.4 days
- Idaho Falls, ID 26 mi, 36.4 days
- Ammon, ID 27 mi, 36.4 days
- Rexburg, ID 52 mi, 42.8 days
Where these numbers come from
NOAA 1991–2020 climate normals from Blackfoot Fire Dept, 0.2 miles from Blackfoot. 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.