Ice dam risk in Anchorage, AK
Will your roof get ice dams in Anchorage, AK?
57.6 dam days a winter
Severe risk. On an ordinary house in Anchorage, conditions for an ice dam come on about 57.6 days in a normal winter, mostly in January and December.
Anchorage averages 59 inches of snow a year and about 154 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 Anchorage
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 23.1 | High |
| Typical, some insulation, not air-sealed | 57.6 | Severe |
| Older or poorly insulated, warm attic | 100.7 | Severe |
Sealing and insulating a typical attic takes out about 34.5 days a winter here. It drops the risk from severe to high. An older house with a warm attic should expect closer to 100.7 days.
Preventing ice dams on a Anchorage 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 621 kWh a winter in Anchorage, roughly $106 at 17 cents a kWh.
Which to spend on first: air sealing or heat cable.
Anchorage winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 10.3 | 30.0 | 13.0 |
| February | 7.6 | 27.7 | 10.7 |
| March | 7.2 | 28.1 | 8.6 |
| April | 1.6 | 10.4 | 2.0 |
| September | 0.1 | 0.0 | 0.0 |
| October | 5.7 | 5.8 | 1.3 |
| November | 10.7 | 23.4 | 9.7 |
| December | 16.1 | 28.6 | 12.3 |
Anchorage next to nearby towns
Anchorage ranks 2 of 4 Alaska cities here for ice-dam days, behind Fairbanks at 80.6 days.
- Wasilla, AK 29 mi, 43.5 days
- Fairbanks, AK 258 mi, 80.6 days
- Juneau, AK 558 mi, 16.3 days
- Lynden, WA 1340 mi, 0.6 days
- Ferndale, WA 1341 mi, 0.2 days
Where these numbers come from
NOAA 1991–2020 climate normals from Eagle Rvr 5 SE, 9.9 miles from Anchorage. 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.