Ice dam risk in Altoona, PA
How often ice dams form in Altoona
14.6 dam days a winter
Moderate risk. On an ordinary house in Altoona, conditions for an ice dam come on about 14.6 days in a normal winter, mostly in January and February.
Altoona averages 38 inches of snow a year and about 50 days with snow on the ground. Most of it comes and goes within a few days. 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 Altoona
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 8.0 | Moderate |
| Typical, some insulation, not air-sealed | 14.6 | Moderate |
| Older or poorly insulated, warm attic | 22.7 | High |
Sealing and insulating a typical attic takes out about 6.5 days a winter here. It does not move the risk band, but it shortens every dam. An older house with a warm attic should expect closer to 22.7 days.
Preventing ice dams on a Altoona 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.
Which to spend on first: air sealing or heat cable.
Altoona winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 10.1 | 15.8 | 5.4 |
| February | 11.0 | 15.1 | 4.7 |
| March | 7.6 | 7.5 | 1.5 |
| April | 0.6 | 0.2 | 0.0 |
| November | 1.2 | 1.5 | 0.2 |
| December | 7.6 | 9.5 | 2.7 |
Altoona next to nearby towns
Altoona ranks 13 of 68 Pennsylvania cities here for ice-dam days, behind St. Marys at 26.9 days.
- Johnstown, PA 30 mi, 20.5 days
- State College, PA 35 mi, 12.5 days
- Indiana, PA 40 mi, 13.3 days
- Chambersburg, PA 56 mi, 7.4 days
- Greensburg, PA 62 mi, 9.2 days
Where these numbers come from
NOAA 1991–2020 climate normals from Altoona 3 W, 3.6 miles from Altoona. 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.