Ice dam risk in Williston, ND
Ice dams in Williston, ND: how often
36.3 dam days a winter
Severe risk. On an ordinary house in Williston, conditions for an ice dam come on about 36.3 days in a normal winter, mostly in January and February.
Williston averages 48 inches of snow a year and about 97 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 Williston
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 15.4 | High |
| Typical, some insulation, not air-sealed | 36.3 | Severe |
| Older or poorly insulated, warm attic | 62.4 | Severe |
Sealing and insulating a typical attic takes out about 20.9 days a winter here. It drops the risk from severe to high. An older house with a warm attic should expect closer to 62.4 days.
Preventing ice dams on a Williston 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 376 kWh a winter in Williston, roughly $64 at 17 cents a kWh.
Which to spend on first: air sealing or heat cable.
Williston winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 11.2 | 27.8 | 11.7 |
| February | 6.7 | 20.9 | 8.3 |
| March | 6.4 | 14.5 | 4.4 |
| April | 3.7 | 2.0 | 0.3 |
| May | 0.9 | 0.2 | 0.0 |
| October | 2.6 | 1.7 | 0.2 |
| November | 6.3 | 9.8 | 3.1 |
| December | 10.4 | 20.0 | 8.2 |
Williston next to nearby towns
Williston ranks 6 of 9 North Dakota cities here for ice-dam days, behind Jamestown at 45.7 days.
- Dickinson, ND 98 mi, 23.8 days
- Minot, ND 109 mi, 42.0 days
- Mandan, ND 160 mi, 33.8 days
- Bismarck, ND 165 mi, 33.8 days
- Jamestown, ND 247 mi, 45.7 days
Where these numbers come from
NOAA 1991–2020 climate normals from Williston Sloulin Field, 1.2 miles from Williston. 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.