Ice dam risk in Portsmouth, NH
Will your roof get ice dams in Portsmouth, NH?
19.6 dam days a winter
High risk. On an ordinary house in Portsmouth, conditions for an ice dam come on about 19.6 days in a normal winter, mostly in January and February.
Portsmouth averages 65 inches of snow a year and about 69 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 Portsmouth
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 12.0 | Moderate |
| Typical, some insulation, not air-sealed | 19.6 | High |
| Older or poorly insulated, warm attic | 29.2 | High |
Sealing and insulating a typical attic takes out about 7.7 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 29.2 days.
Preventing ice dams on a Portsmouth 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.
Portsmouth winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 17.1 | 21.0 | 7.2 |
| February | 15.8 | 18.8 | 5.7 |
| March | 13.9 | 13.5 | 2.9 |
| April | 2.3 | 1.4 | 0.1 |
| October | 0.1 | 0.0 | 0.0 |
| November | 1.6 | 1.6 | 0.2 |
| December | 14.2 | 12.2 | 3.4 |
Portsmouth next to nearby towns
Portsmouth ranks 8 of 11 New Hampshire cities here for ice-dam days, behind Claremont at 34.6 days.
- Dover, NH 10 mi, 18.6 days
- Somersworth, NH 14 mi, 18.6 days
- Amesbury, MA 17 mi, 20.5 days
- Newburyport, MA 18 mi, 20.7 days
- Rochester, NH 19 mi, 18.6 days
Where these numbers come from
NOAA 1991–2020 climate normals from Greenland, 3.6 miles from Portsmouth. 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.