Ice dam risk in Philadelphia, PA
Will your roof get ice dams in Philadelphia, PA?
2.3 dam days a winter
Low risk. On an ordinary house in Philadelphia, conditions for an ice dam come on about 2.3 days in a normal winter, mostly in February and January.
Philadelphia averages 16 inches of snow a year and about 15 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 Philadelphia
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 1.7 | Low |
| Typical, some insulation, not air-sealed | 2.3 | Low |
| Older or poorly insulated, warm attic | 3.2 | Low |
The attic matters less in Philadelphia than in colder towns: there are only 0.6 days a winter between a typical attic and a sealed one.
What to do about ice dams in Philadelphia
- Check the gutters before the first snow and clear them, so meltwater has somewhere to go. If you see icicles thicker than a finger along the eave after a storm, that is the early sign to look at the attic.
Snowy places are not always the worst: why snowfall misleads.
Philadelphia winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 6.4 | 4.3 | 0.9 |
| February | 4.0 | 5.5 | 0.9 |
| March | 2.0 | 2.1 | 0.2 |
| April | 0.1 | 0.1 | 0.0 |
| November | 0.1 | 0.1 | 0.0 |
| December | 3.5 | 2.7 | 0.3 |
Philadelphia next to nearby towns
Philadelphia ranks 68 of 68 Pennsylvania cities here for ice-dam days, behind St. Marys at 26.9 days.
- Camden, NJ 5 mi, 2.3 days
- Collingswood, NJ 7 mi, 2.3 days
- Gloucester City, NJ 8 mi, 2.3 days
- Yeadon, PA 8 mi, 3.0 days
- Lansdowne, PA 9 mi, 3.0 days
Where these numbers come from
NOAA 1991–2020 climate normals from Phila Franklin Inst, 4.1 miles from Philadelphia. 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.