Ice dam risk in Lebanon, NH
How often ice dams form in Lebanon
30.4 dam days a winter
High risk. On an ordinary house in Lebanon, conditions for an ice dam come on about 30.4 days in a normal winter, mostly in January and February.
Lebanon averages 61 inches of snow a year and about 95 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 Lebanon
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 15.6 | High |
| Typical, some insulation, not air-sealed | 30.4 | High |
| Older or poorly insulated, warm attic | 48.9 | Severe |
Sealing and insulating a typical attic takes out about 14.8 days a winter here. It does not move the risk band, but it shortens every dam. An older house with a warm attic should expect closer to 48.9 days.
What to do about ice dams in Lebanon
- 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.
- Heat cable is worth pricing here even on a sealed house, along the eave and in the gutters and downspouts it drains into.
Running 60 feet of 5 W/ft cable whenever snow lies below freezing would use about 267 kWh a winter in Lebanon, roughly $45 at 17 cents a kWh.
Which to spend on first: air sealing or heat cable.
Lebanon winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 16.4 | 27.0 | 10.5 |
| February | 13.1 | 24.6 | 8.6 |
| March | 13.5 | 19.1 | 4.7 |
| April | 1.9 | 2.3 | 0.3 |
| October | 0.1 | 0.1 | 0.0 |
| November | 2.2 | 3.1 | 0.6 |
| December | 13.4 | 18.4 | 5.8 |
Lebanon next to nearby towns
Lebanon ranks 2 of 11 New Hampshire cities here for ice-dam days, behind Claremont at 34.6 days.
- Claremont, NH 18 mi, 34.6 days
- Rutland, VT 36 mi, 28.5 days
- Laconia, NH 39 mi, 29.5 days
- Concord, NH 45 mi, 28.7 days
- Keene, NH 47 mi, 27.8 days
Where these numbers come from
NOAA 1991–2020 climate normals from Hanover, 5.1 miles from Lebanon. 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.