Ice dam risk in Billings, MT
How often ice dams form in Billings
14.3 dam days a winter
Moderate risk. On an ordinary house in Billings, conditions for an ice dam come on about 14.3 days in a normal winter, mostly in January and December.
Billings averages 57 inches of snow a year and about 55 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 Billings
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 8.7 | Moderate |
| Typical, some insulation, not air-sealed | 14.3 | Moderate |
| Older or poorly insulated, warm attic | 21.4 | High |
Sealing and insulating a typical attic takes out about 5.6 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 21.4 days.
What to do about ice dams in Billings
- 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.
Which to spend on first: air sealing or heat cable.
Billings winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 10.6 | 15.8 | 4.8 |
| February | 9.1 | 9.4 | 2.7 |
| March | 8.2 | 6.6 | 1.3 |
| April | 7.5 | 2.3 | 0.2 |
| May | 0.9 | 0.2 | 0.0 |
| September | 0.3 | 0.0 | 0.0 |
| October | 4.5 | 2.0 | 0.2 |
| November | 6.5 | 6.5 | 1.4 |
| December | 9.8 | 12.4 | 3.8 |
Billings next to nearby towns
Billings ranks 8 of 8 Montana cities here for ice-dam days, behind Bozeman at 33.6 days.
- Cody, WY 91 mi, 7.3 days
- Sheridan, WY 103 mi, 22.9 days
- Bozeman, MT 121 mi, 33.6 days
- Belgrade, MT 127 mi, 33.6 days
- Helena, MT 175 mi, 17.2 days
Where these numbers come from
NOAA 1991–2020 climate normals from Billings International Airport, 1.3 miles from Billings. 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.