Ice dam risk in Carlisle, PA
How often ice dams form in Carlisle
6.9 dam days a winter
Moderate risk. On an ordinary house in Carlisle, conditions for an ice dam come on about 6.9 days in a normal winter, mostly in February and January.
Carlisle averages 28 inches of snow a year and about 27 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 Carlisle
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 4.8 | Low |
| Typical, some insulation, not air-sealed | 6.9 | Moderate |
| Older or poorly insulated, warm attic | 9.4 | Moderate |
Sealing and insulating a typical attic takes out about 2.0 days a winter here. It drops the risk from moderate to low.
What to do about ice dams in Carlisle
- 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.
Carlisle winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 7.7 | 8.0 | 2.4 |
| February | 9.6 | 11.0 | 3.0 |
| March | 4.8 | 3.9 | 0.7 |
| April | 0.2 | 0.1 | 0.0 |
| October | 0.5 | 0.1 | 0.0 |
| November | 0.9 | 0.4 | 0.1 |
| December | 4.0 | 3.2 | 0.7 |
Carlisle next to nearby towns
Carlisle ranks 53 of 68 Pennsylvania cities here for ice-dam days, behind St. Marys at 26.9 days.
- Mechanicsburg, PA 10 mi, 8.0 days
- Harrisburg, PA 18 mi, 8.0 days
- Hanover, PA 29 mi, 7.6 days
- York, PA 30 mi, 5.8 days
- Chambersburg, PA 30 mi, 7.4 days
Where these numbers come from
NOAA 1991–2020 climate normals from Carlisle Wtp, 1.9 miles from Carlisle. 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.