Ice dam risk in Bozeman, MT
Will your roof get ice dams in Bozeman, MT?
33.6 dam days a winter
High risk. On an ordinary house in Bozeman, conditions for an ice dam come on about 33.6 days in a normal winter, mostly in January and December.
Bozeman averages 91 inches of snow a year and about 118 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 Bozeman
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 21.6 | High |
| Typical, some insulation, not air-sealed | 33.6 | High |
| Older or poorly insulated, warm attic | 48.5 | Severe |
Sealing and insulating a typical attic takes out about 11.9 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 48.5 days.
Preventing ice dams on a Bozeman 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.
- 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.
- 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 215 kWh a winter in Bozeman, roughly $36 at 17 cents a kWh.
Which to spend on first: air sealing or heat cable.
Bozeman winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 12.7 | 29.6 | 9.4 |
| February | 13.0 | 23.8 | 7.0 |
| March | 13.1 | 16.4 | 3.7 |
| April | 12.9 | 4.1 | 0.6 |
| May | 3.6 | 0.4 | 0.0 |
| June | 0.8 | 0.0 | 0.0 |
| August | 0.1 | 0.0 | 0.0 |
| September | 0.4 | 0.0 | 0.0 |
| October | 6.0 | 2.7 | 0.4 |
| November | 12.5 | 14.8 | 3.7 |
| December | 16.2 | 26.4 | 8.8 |
Bozeman next to nearby towns
Bozeman ranks 1 of 8 Montana cities here for ice-dam days, the worst in the state.
- Belgrade, MT 9 mi, 33.6 days
- Helena, MT 78 mi, 17.2 days
- Butte-Silver Bow, MT 79 mi, 31.4 days
- Billings, MT 121 mi, 14.3 days
- Cody, WY 126 mi, 7.3 days
Where these numbers come from
NOAA 1991–2020 climate normals from Bozeman Montana State Univ, 1.5 miles from Bozeman. 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.