Ice dam risk in Jamestown, ND
Will your roof get ice dams in Jamestown, ND?
45.7 dam days a winter
Severe risk. On an ordinary house in Jamestown, conditions for an ice dam come on about 45.7 days in a normal winter, mostly in January and February.
Jamestown averages 46 inches of snow a year and about 119 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 Jamestown
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 17.3 | High |
| Typical, some insulation, not air-sealed | 45.7 | Severe |
| Older or poorly insulated, warm attic | 81.3 | Severe |
Sealing and insulating a typical attic takes out about 28.5 days a winter here. It drops the risk from severe to high. An older house with a warm attic should expect closer to 81.3 days.
Preventing ice dams on a Jamestown 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.
- Heat cable is worth pricing here even on a sealed house, along the eave and in the gutters and downspouts it drains into.
Running 60 feet of 5 W/ft cable whenever snow lies below freezing would use about 512 kWh a winter in Jamestown, roughly $87 at 17 cents a kWh.
Which to spend on first: air sealing or heat cable.
Jamestown winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 11.4 | 29.7 | 13.2 |
| February | 7.4 | 25.5 | 10.7 |
| March | 6.5 | 21.5 | 7.0 |
| April | 2.5 | 5.3 | 0.9 |
| May | 0.4 | 0.2 | 0.0 |
| October | 1.9 | 1.0 | 0.1 |
| November | 4.2 | 11.2 | 3.6 |
| December | 11.8 | 24.0 | 10.2 |
Jamestown next to nearby towns
Jamestown ranks 1 of 9 North Dakota cities here for ice-dam days, the worst in the state.
- West Fargo, ND 85 mi, 41.7 days
- Fargo, ND 88 mi, 43.6 days
- Moorhead, MN 92 mi, 43.6 days
- Bismarck, ND 98 mi, 33.8 days
- Aberdeen, SD 100 mi, 30.2 days
Where these numbers come from
NOAA 1991–2020 climate normals from Jamestown State Hosp, 1.5 miles from Jamestown. 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.
- Snow-on-the-ground counts come from a second, nearby station, because the closest one does not record them.