Ice dam risk in Longmont, CO
Ice dams in Longmont, CO: how often
11.4 dam days a winter
Moderate risk. On an ordinary house in Longmont, conditions for an ice dam come on about 11.4 days in a normal winter, mostly in December and January.
Longmont averages 93 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 Longmont
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 9.6 | Moderate |
| Typical, some insulation, not air-sealed | 11.4 | Moderate |
| Older or poorly insulated, warm attic | 13.8 | Moderate |
The attic matters less in Longmont than in colder towns: there are only 1.8 days a winter between a typical attic and a sealed one.
Preventing ice dams on a Longmont roof
- 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.
More on when a roof rake is worth it.
Longmont winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 11.3 | 12.9 | 3.1 |
| February | 15.0 | 11.0 | 2.6 |
| March | 15.7 | 5.5 | 0.9 |
| April | 14.7 | 2.9 | 0.3 |
| May | 2.0 | 0.3 | 0.0 |
| September | 1.0 | 0.1 | 0.0 |
| October | 7.0 | 1.8 | 0.1 |
| November | 13.1 | 7.0 | 1.2 |
| December | 13.0 | 13.0 | 3.2 |
Longmont next to nearby towns
Longmont ranks 21 of 53 Colorado cities here for ice-dam days, behind Steamboat Springs at 51.0 days.
- Frederick, CO 8 mi, 9.4 days
- Firestone, CO 8 mi, 9.4 days
- Erie, CO 9 mi, 7.7 days
- Berthoud, CO 11 mi, 12.7 days
- Lafayette, CO 12 mi, 7.7 days
Where these numbers come from
NOAA 1991–2020 climate normals from Boulder, 15.0 miles from Longmont. 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.