Ice dam risk in Spearfish, SD
Ice dams in Spearfish, SD: how often
17.4 dam days a winter
High risk. On an ordinary house in Spearfish, conditions for an ice dam come on about 17.4 days in a normal winter, mostly in January and December.
Spearfish averages 71 inches of snow a year and about 69 days with snow on the ground. Snow that lies that long is what turns a little attic heat into a ridge of ice at the eave. 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 Spearfish
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 10.7 | Moderate |
| Typical, some insulation, not air-sealed | 17.4 | High |
| Older or poorly insulated, warm attic | 25.8 | High |
Sealing and insulating a typical attic takes out about 6.7 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 25.8 days.
Preventing ice dams on a Spearfish 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.
Spearfish winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 8.2 | 16.5 | 4.9 |
| February | 12.7 | 13.4 | 3.9 |
| March | 12.0 | 10.1 | 2.1 |
| April | 8.5 | 4.0 | 0.5 |
| May | 2.3 | 0.5 | 0.0 |
| September | 0.1 | 0.1 | 0.0 |
| October | 5.5 | 2.1 | 0.2 |
| November | 6.3 | 6.3 | 1.3 |
| December | 14.9 | 16.1 | 4.6 |
Spearfish next to nearby towns
Spearfish ranks 12 of 14 South Dakota cities here for ice-dam days, behind Watertown at 35.4 days.
- Rapid City, SD 41 mi, 11.8 days
- Box Elder, SD 45 mi, 13.9 days
- Gillette, WY 84 mi, 18.7 days
- Sheridan, WY 156 mi, 22.9 days
- Casper, WY 169 mi, 18.5 days
Where these numbers come from
NOAA 1991–2020 climate normals from Spearfish, 2.8 miles from Spearfish. 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.
- The nearest weather station sits at a noticeably different elevation from Spearfish, so the snow figures may run high or low for the town itself.