Ice dam risk in Johnstown, PA
Ice dams in Johnstown, PA: how often
20.5 dam days a winter
High risk. On an ordinary house in Johnstown, conditions for an ice dam come on about 20.5 days in a normal winter, mostly in January and February.
Johnstown averages 80 inches of snow a year and about 70 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 Johnstown
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 12.0 | Moderate |
| Typical, some insulation, not air-sealed | 20.5 | High |
| Older or poorly insulated, warm attic | 31.2 | High |
Sealing and insulating a typical attic takes out about 8.5 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 31.2 days.
What to do about ice dams in Johnstown
- 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.
Johnstown winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 24.0 | 21.1 | 7.2 |
| February | 19.3 | 19.0 | 6.0 |
| March | 14.1 | 11.3 | 2.6 |
| April | 2.6 | 1.2 | 0.1 |
| October | 0.7 | 0.3 | 0.0 |
| November | 4.9 | 3.5 | 0.7 |
| December | 14.1 | 13.4 | 4.0 |
Johnstown next to nearby towns
Johnstown ranks 2 of 68 Pennsylvania cities here for ice-dam days, behind St. Marys at 26.9 days.
- Indiana, PA 24 mi, 13.3 days
- Altoona, PA 30 mi, 14.6 days
- Greensburg, PA 33 mi, 9.2 days
- Murrysville, PA 40 mi, 8.8 days
- Monroeville, PA 45 mi, 8.8 days
Where these numbers come from
NOAA 1991–2020 climate normals from Ebensburg Sewage Plt, 13.9 miles from Johnstown. 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.