Ice dam risk in Ammon, ID
Will your roof get ice dams in Ammon, ID?
36.4 dam days a winter
Severe risk. On an ordinary house in Ammon, conditions for an ice dam come on about 36.4 days in a normal winter, mostly in January and December.
Ammon averages 66 inches of snow a year and about 111 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 Ammon
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 19.6 | High |
| Typical, some insulation, not air-sealed | 36.4 | Severe |
| Older or poorly insulated, warm attic | 57.5 | Severe |
Sealing and insulating a typical attic takes out about 16.9 days a winter here. It drops the risk from severe to high. An older house with a warm attic should expect closer to 57.5 days.
What to do about ice dams in Ammon
- 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 304 kWh a winter in Ammon, roughly $52 at 17 cents a kWh.
Which to spend on first: air sealing or heat cable.
Ammon winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 17.0 | 29.5 | 11.1 |
| February | 11.8 | 25.4 | 8.3 |
| March | 7.6 | 17.9 | 4.4 |
| April | 2.9 | 2.1 | 0.4 |
| May | 0.9 | 0.1 | 0.0 |
| June | 0.2 | 0.0 | 0.0 |
| October | 1.7 | 1.0 | 0.2 |
| November | 7.6 | 9.5 | 2.6 |
| December | 16.0 | 25.1 | 9.5 |
Ammon next to nearby towns
Ammon ranks 3 of 26 Idaho cities here for ice-dam days, behind Rexburg at 42.8 days.
- Idaho Falls, ID 4 mi, 36.4 days
- Rexburg, ID 25 mi, 42.8 days
- Blackfoot, ID 27 mi, 20.1 days
- Chubbuck, ID 45 mi, 20.8 days
- Pocatello, ID 49 mi, 9.4 days
Where these numbers come from
NOAA 1991–2020 climate normals from Idaho Falls 16 SE, 12.5 miles from Ammon. 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.
- The nearest weather station sits at a noticeably different elevation from Ammon, so the snow figures may run high or low for the town itself.