Ice dam risk in Somersworth, NH
How often ice dams form in Somersworth
18.6 dam days a winter
High risk. On an ordinary house in Somersworth, conditions for an ice dam come on about 18.6 days in a normal winter, mostly in January and February.
Somersworth averages 39 inches of snow a year and about 64 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 Somersworth
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 11.4 | Moderate |
| Typical, some insulation, not air-sealed | 18.6 | High |
| Older or poorly insulated, warm attic | 27.6 | High |
Sealing and insulating a typical attic takes out about 7.2 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 27.6 days.
What to do about ice dams in Somersworth
- 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.
Somersworth winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 10.8 | 18.6 | 6.5 |
| February | 9.1 | 18.1 | 5.6 |
| March | 6.8 | 13.8 | 3.1 |
| April | 1.8 | 0.9 | 0.1 |
| October | 0.2 | 0.0 | 0.0 |
| November | 1.3 | 1.3 | 0.2 |
| December | 8.9 | 11.0 | 3.1 |
Somersworth next to nearby towns
Somersworth ranks 11 of 11 New Hampshire cities here for ice-dam days, behind Claremont at 34.6 days.
- Dover, NH 5 mi, 18.6 days
- Rochester, NH 5 mi, 18.6 days
- Sanford, ME 13 mi, 29.0 days
- Portsmouth, NH 14 mi, 19.6 days
- Biddeford, ME 28 mi, 22.6 days
Where these numbers come from
NOAA 1991–2020 climate normals from Durham, 8.3 miles from Somersworth. 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.