Ice dam risk in Harrisburg, SD
Will your roof get ice dams in Harrisburg, SD?
26.7 dam days a winter
High risk. On an ordinary house in Harrisburg, conditions for an ice dam come on about 26.7 days in a normal winter, mostly in January and December.
Harrisburg averages 36 inches of snow a year and about 76 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 Harrisburg
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 12.0 | Moderate |
| Typical, some insulation, not air-sealed | 26.7 | High |
| Older or poorly insulated, warm attic | 45.1 | Severe |
Sealing and insulating a typical attic takes out about 14.7 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 45.1 days.
What to do about ice dams in Harrisburg
- 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.
Harrisburg winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 7.0 | 23.5 | 9.7 |
| February | 8.5 | 17.1 | 6.3 |
| March | 4.7 | 10.5 | 2.5 |
| April | 2.8 | 1.9 | 0.2 |
| May | 0.1 | 0.1 | 0.0 |
| October | 0.8 | 0.6 | 0.0 |
| November | 4.8 | 5.0 | 1.2 |
| December | 7.4 | 17.7 | 6.7 |
Harrisburg next to nearby towns
Harrisburg ranks 5 of 14 South Dakota cities here for ice-dam days, behind Watertown at 35.4 days.
- Sioux Falls, SD 7 mi, 25.5 days
- Brandon, SD 13 mi, 25.5 days
- Vermillion, SD 46 mi, 23.8 days
- Yankton, SD 51 mi, 18.8 days
- Le Mars, IA 53 mi, 24.4 days
Where these numbers come from
NOAA 1991–2020 climate normals from Canton, 10.6 miles from Harrisburg. 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.