Ice dam risk in Flagstaff, AZ
Ice dams in Flagstaff, AZ: how often
16.2 dam days a winter
High risk. On an ordinary house in Flagstaff, conditions for an ice dam come on about 16.2 days in a normal winter, mostly in January and December.
Flagstaff averages 90 inches of snow a year and about 62 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 Flagstaff
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 14.0 | Moderate |
| Typical, some insulation, not air-sealed | 16.2 | High |
| Older or poorly insulated, warm attic | 19.0 | High |
Sealing and insulating a typical attic takes out about 2.2 days a winter here. It drops the risk from high to moderate.
What to do about ice dams in Flagstaff
- 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.
Flagstaff winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 20.9 | 19.6 | 5.4 |
| February | 19.3 | 13.8 | 3.6 |
| March | 15.6 | 7.1 | 1.7 |
| April | 5.0 | 1.2 | 0.2 |
| May | 1.1 | 0.2 | 0.0 |
| October | 1.5 | 0.4 | 0.1 |
| November | 8.2 | 3.9 | 0.9 |
| December | 18.5 | 15.5 | 4.3 |
Flagstaff next to nearby towns
Flagstaff ranks 1 of 44 Arizona cities here for ice-dam days, the worst in the state.
- Cottonwood, AZ 36 mi, 0.0 days
- Camp Verde, AZ 45 mi, 0.0 days
- Chino Valley, AZ 54 mi, 0.8 days
- Prescott Valley, AZ 57 mi, 0.8 days
- Prescott, AZ 62 mi, 0.8 days
Where these numbers come from
NOAA 1991–2020 climate normals from Flagstaff Airport, 3.8 miles from Flagstaff. 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.