Ice dam risk in Helena, MT
How often ice dams form in Helena
17.2 dam days a winter
High risk. On an ordinary house in Helena, conditions for an ice dam come on about 17.2 days in a normal winter, mostly in January and December.
Helena averages 37 inches of snow a year and about 57 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 Helena
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 10.0 | Moderate |
| Typical, some insulation, not air-sealed | 17.2 | High |
| Older or poorly insulated, warm attic | 26.1 | High |
Sealing and insulating a typical attic takes out about 7.2 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 26.1 days.
Preventing ice dams on a Helena 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.
Which to spend on first: air sealing or heat cable.
Helena winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 6.6 | 17.8 | 6.1 |
| February | 6.6 | 11.0 | 3.3 |
| March | 4.6 | 5.3 | 1.2 |
| April | 2.9 | 1.1 | 0.1 |
| May | 0.1 | 0.0 | 0.0 |
| August | 0.3 | 0.0 | 0.0 |
| September | 0.2 | 0.0 | 0.0 |
| October | 2.8 | 1.6 | 0.2 |
| November | 5.4 | 6.0 | 1.5 |
| December | 7.7 | 13.7 | 4.7 |
Helena next to nearby towns
Helena ranks 6 of 8 Montana cities here for ice-dam days, behind Bozeman at 33.6 days.
- Butte-Silver Bow, MT 57 mi, 31.4 days
- Belgrade, MT 69 mi, 33.6 days
- Great Falls, MT 71 mi, 15.8 days
- Bozeman, MT 78 mi, 33.6 days
- Missoula, MT 97 mi, 20.5 days
Where these numbers come from
NOAA 1991–2020 climate normals from Helena Airport, 1.5 miles from Helena. 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.