Ice dam risk in Great Falls, MT
How often ice dams form in Great Falls
15.8 dam days a winter
High risk. On an ordinary house in Great Falls, conditions for an ice dam come on about 15.8 days in a normal winter, mostly in January and February.
Great Falls averages 66 inches of snow a year and about 60 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 Great Falls
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 9.9 | Moderate |
| Typical, some insulation, not air-sealed | 15.8 | High |
| Older or poorly insulated, warm attic | 23.3 | High |
Sealing and insulating a typical attic takes out about 6.0 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 23.3 days.
What to do about ice dams in Great Falls
- 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.
Great Falls winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 9.2 | 14.3 | 4.3 |
| February | 10.1 | 11.4 | 3.4 |
| March | 10.1 | 8.2 | 2.0 |
| April | 9.4 | 3.1 | 0.5 |
| May | 1.9 | 0.5 | 0.0 |
| June | 0.3 | 0.1 | 0.0 |
| August | 0.3 | 0.0 | 0.0 |
| September | 0.8 | 0.1 | 0.0 |
| October | 5.2 | 2.6 | 0.3 |
| November | 9.2 | 8.2 | 1.9 |
| December | 9.6 | 11.3 | 3.4 |
Great Falls next to nearby towns
Great Falls ranks 7 of 8 Montana cities here for ice-dam days, behind Bozeman at 33.6 days.
- Helena, MT 71 mi, 17.2 days
- Belgrade, MT 119 mi, 33.6 days
- Bozeman, MT 126 mi, 33.6 days
- Butte-Silver Bow, MT 128 mi, 31.4 days
- Missoula, MT 135 mi, 20.5 days
Where these numbers come from
NOAA 1991–2020 climate normals from Great Falls Airport, 4.4 miles from Great Falls. 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.