Ice dam risk in Butte-Silver Bow, MT
How often ice dams form in Butte-Silver Bow
31.4 dam days a winter
High risk. On an ordinary house in Butte-Silver Bow, conditions for an ice dam come on about 31.4 days in a normal winter, mostly in December and January.
Butte-Silver Bow averages 15 inches of snow a year and about 98 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 Butte-Silver Bow
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 18.9 | High |
| Typical, some insulation, not air-sealed | 31.4 | High |
| Older or poorly insulated, warm attic | 47.0 | Severe |
Sealing and insulating a typical attic takes out about 12.5 days a winter here. It does not move the risk band, but it shortens every dam. An older house with a warm attic should expect closer to 47.0 days.
Preventing ice dams on a Butte-Silver Bow roof
- 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.
- Heat cable is worth pricing here even on a sealed house, along the eave and in the gutters and downspouts it drains into.
Running 60 feet of 5 W/ft cable whenever snow lies below freezing would use about 225 kWh a winter in Butte-Silver Bow, roughly $38 at 17 cents a kWh.
Which to spend on first: air sealing or heat cable.
Butte-Silver Bow winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 2.4 | 23.5 | 8.3 |
| February | 2.0 | 18.0 | 5.8 |
| March | 2.6 | 13.3 | 3.5 |
| April | 0.8 | 4.0 | 0.9 |
| May | 0.1 | 0.3 | 0.0 |
| September | 0.1 | 0.0 | 0.0 |
| October | 0.9 | 1.6 | 0.3 |
| November | 1.7 | 12.5 | 3.6 |
| December | 4.6 | 24.4 | 9.0 |
Butte-Silver Bow next to nearby towns
Butte-Silver Bow ranks 3 of 8 Montana cities here for ice-dam days, behind Bozeman at 33.6 days.
- Helena, MT 57 mi, 17.2 days
- Belgrade, MT 72 mi, 33.6 days
- Bozeman, MT 79 mi, 33.6 days
- Missoula, MT 94 mi, 20.5 days
- Great Falls, MT 128 mi, 15.8 days
Where these numbers come from
NOAA 1991–2020 climate normals from Divide, 11.0 miles from Butte-Silver Bow. 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.