Ice dam risk in Johnstown, CO
Will your roof get ice dams in Johnstown, CO?
12.7 dam days a winter
Moderate risk. On an ordinary house in Johnstown, conditions for an ice dam come on about 12.7 days in a normal winter, mostly in January and December.
Johnstown averages 47 inches of snow a year and about 49 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 Johnstown
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 10.1 | Moderate |
| Typical, some insulation, not air-sealed | 12.7 | Moderate |
| Older or poorly insulated, warm attic | 16.0 | High |
Sealing and insulating a typical attic takes out about 2.6 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 16.0 days.
Preventing ice dams on a Johnstown 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.
Which to spend on first: air sealing or heat cable.
Johnstown winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 6.0 | 12.7 | 3.7 |
| February | 8.2 | 10.3 | 2.9 |
| March | 7.9 | 4.4 | 1.0 |
| April | 5.7 | 2.3 | 0.3 |
| May | 1.1 | 0.4 | 0.0 |
| October | 3.5 | 1.7 | 0.2 |
| November | 7.0 | 5.1 | 1.2 |
| December | 7.6 | 12.0 | 3.5 |
Johnstown next to nearby towns
Johnstown ranks 9 of 53 Colorado cities here for ice-dam days, behind Steamboat Springs at 51.0 days.
- Berthoud, CO 3 mi, 12.7 days
- Evans, CO 9 mi, 10.0 days
- Greeley, CO 10 mi, 10.0 days
- Firestone, CO 11 mi, 9.4 days
- Loveland, CO 11 mi, 12.7 days
Where these numbers come from
NOAA 1991–2020 climate normals from Loveland 2N, 12.7 miles from Johnstown. 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.