Ice dam risk in Vermillion, SD
How often ice dams form in Vermillion
23.8 dam days a winter
High risk. On an ordinary house in Vermillion, conditions for an ice dam come on about 23.8 days in a normal winter, mostly in January and December.
Vermillion averages 31 inches of snow a year and about 68 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 Vermillion
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 11.1 | Moderate |
| Typical, some insulation, not air-sealed | 23.8 | High |
| Older or poorly insulated, warm attic | 39.6 | Severe |
Sealing and insulating a typical attic takes out about 12.7 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 39.6 days.
Preventing ice dams on a Vermillion 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.
Vermillion winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 5.2 | 19.7 | 8.0 |
| February | 6.0 | 15.0 | 5.6 |
| March | 4.5 | 9.3 | 2.3 |
| April | 2.9 | 1.6 | 0.2 |
| May | 0.1 | 0.1 | 0.0 |
| October | 0.7 | 0.5 | 0.0 |
| November | 5.0 | 4.6 | 1.2 |
| December | 6.1 | 17.4 | 6.5 |
Vermillion next to nearby towns
Vermillion ranks 9 of 14 South Dakota cities here for ice-dam days, behind Watertown at 35.4 days.
- Yankton, SD 25 mi, 18.8 days
- Sioux City, IA 34 mi, 19.2 days
- South Sioux City, NE 34 mi, 19.2 days
- Le Mars, IA 38 mi, 24.4 days
- Harrisburg, SD 46 mi, 26.7 days
Where these numbers come from
NOAA 1991–2020 climate normals from Centerville 6 SE, 18.1 miles from Vermillion. 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.