Ice dam risk in Lancaster, PA
How often ice dams form in Lancaster
5.3 dam days a winter
Moderate risk. On an ordinary house in Lancaster, conditions for an ice dam come on about 5.3 days in a normal winter, mostly in February and January.
Lancaster averages 21 inches of snow a year and about 22 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 Lancaster
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 3.7 | Low |
| Typical, some insulation, not air-sealed | 5.3 | Moderate |
| Older or poorly insulated, warm attic | 7.3 | Moderate |
The attic matters less in Lancaster than in colder towns: there are only 1.6 days a winter between a typical attic and a sealed one.
Preventing ice dams on a Lancaster roof
- 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.
More on when a roof rake is worth it.
Lancaster winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 6.1 | 6.8 | 1.9 |
| February | 7.4 | 9.2 | 2.3 |
| March | 3.4 | 3.4 | 0.5 |
| April | 0.2 | 0.0 | 0.0 |
| October | 0.3 | 0.0 | 0.0 |
| November | 0.6 | 0.2 | 0.0 |
| December | 3.4 | 2.4 | 0.5 |
Lancaster next to nearby towns
Lancaster ranks 60 of 68 Pennsylvania cities here for ice-dam days, behind St. Marys at 26.9 days.
- Columbia, PA 10 mi, 6.5 days
- Ephrata, PA 12 mi, 6.4 days
- Elizabethtown, PA 18 mi, 6.5 days
- Lebanon, PA 22 mi, 6.7 days
- York, PA 23 mi, 5.8 days
Where these numbers come from
NOAA 1991–2020 climate normals from Lancaster 2 NE Filt Plt, 1.5 miles from Lancaster. 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.