Ice dam risk in Missoula, MT
Ice dams in Missoula, MT: how often
20.5 dam days a winter
High risk. On an ordinary house in Missoula, conditions for an ice dam come on about 20.5 days in a normal winter, mostly in January and December.
Missoula averages 50 inches of snow a year and about 70 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 Missoula
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 12.8 | Moderate |
| Typical, some insulation, not air-sealed | 20.5 | High |
| Older or poorly insulated, warm attic | 30.1 | High |
Sealing and insulating a typical attic takes out about 7.7 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 30.1 days.
What to do about ice dams in Missoula
- 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.
- Rake the bottom three or four feet of roof after any storm that leaves more than about six inches, from the ground with a roof rake.
- 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.
Missoula winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 12.5 | 22.4 | 7.1 |
| February | 7.9 | 16.1 | 4.2 |
| March | 7.4 | 6.0 | 1.2 |
| April | 1.4 | 0.3 | 0.0 |
| May | 0.3 | 0.0 | 0.0 |
| June | 0.2 | 0.0 | 0.0 |
| October | 0.7 | 0.2 | 0.0 |
| November | 6.9 | 5.9 | 1.4 |
| December | 12.9 | 19.0 | 6.5 |
Missoula next to nearby towns
Missoula ranks 5 of 8 Montana cities here for ice-dam days, behind Bozeman at 33.6 days.
- Kalispell, MT 94 mi, 24.2 days
- Butte-Silver Bow, MT 94 mi, 31.4 days
- Helena, MT 97 mi, 17.2 days
- Great Falls, MT 135 mi, 15.8 days
- Moscow, ID 141 mi, 11.0 days
Where these numbers come from
NOAA 1991–2020 climate normals from Missoula 2 NE, 3.2 miles from Missoula. 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.