Ice dam risk in St. Marys, PA
Ice dams in St. Marys, PA: how often
26.9 dam days a winter
High risk. On an ordinary house in St. Marys, conditions for an ice dam come on about 26.9 days in a normal winter, mostly in January and February.
St. Marys averages 66 inches of snow a year and about 81 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 St. Marys
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 12.9 | Moderate |
| Typical, some insulation, not air-sealed | 26.9 | High |
| Older or poorly insulated, warm attic | 44.4 | Severe |
Sealing and insulating a typical attic takes out about 14.0 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 44.4 days.
Preventing ice dams on a St. Marys 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.
- 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.
St. Marys winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 17.8 | 23.3 | 8.9 |
| February | 15.4 | 21.8 | 7.8 |
| March | 10.3 | 12.9 | 3.4 |
| April | 2.0 | 1.4 | 0.2 |
| May | 0.2 | 0.1 | 0.0 |
| October | 0.5 | 0.4 | 0.0 |
| November | 5.6 | 4.6 | 1.0 |
| December | 14.2 | 16.9 | 5.6 |
St. Marys next to nearby towns
St. Marys ranks 1 of 68 Pennsylvania cities here for ice-dam days, the worst in the state.
- Olean, NY 44 mi, 22.7 days
- State College, PA 57 mi, 12.5 days
- Jamestown, NY 58 mi, 25.4 days
- Altoona, PA 66 mi, 14.6 days
- Indiana, PA 66 mi, 13.3 days
Where these numbers come from
NOAA 1991–2020 climate normals from Glen Hazel 2 NE Dam, 8.4 miles from St. Marys. 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.