Ice dam risk in Rapid City, SD
Ice dams in Rapid City, SD: how often
11.8 dam days a winter
Moderate risk. On an ordinary house in Rapid City, conditions for an ice dam come on about 11.8 days in a normal winter, mostly in December and February.
Rapid City averages 54 inches of snow a year and about 46 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 Rapid City
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 7.0 | Moderate |
| Typical, some insulation, not air-sealed | 11.8 | Moderate |
| Older or poorly insulated, warm attic | 17.8 | High |
Sealing and insulating a typical attic takes out about 4.8 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 17.8 days.
What to do about ice dams in Rapid City
- 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.
Rapid City winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 5.3 | 9.2 | 2.8 |
| February | 9.1 | 9.6 | 2.9 |
| March | 10.4 | 6.8 | 1.6 |
| April | 10.0 | 3.6 | 0.4 |
| May | 2.2 | 0.4 | 0.0 |
| June | 0.1 | 0.0 | 0.0 |
| September | 0.1 | 0.0 | 0.0 |
| October | 3.4 | 1.8 | 0.1 |
| November | 6.5 | 5.2 | 1.1 |
| December | 6.6 | 9.7 | 2.9 |
Rapid City next to nearby towns
Rapid City ranks 14 of 14 South Dakota cities here for ice-dam days, behind Watertown at 35.4 days.
- Box Elder, SD 7 mi, 13.9 days
- Spearfish, SD 41 mi, 17.4 days
- Gillette, WY 114 mi, 18.7 days
- Pierre, SD 145 mi, 21.9 days
- Scottsbluff, NE 154 mi, 11.1 days
Where these numbers come from
NOAA 1991–2020 climate normals from Rapid City Wfo, 0.4 miles from Rapid City. 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.