Ice dam risk in Somerville, MA
How often ice dams form in Somerville
13.7 dam days a winter
Moderate risk. On an ordinary house in Somerville, conditions for an ice dam come on about 13.7 days in a normal winter, mostly in January and February.
Somerville averages 49 inches of snow a year and about 62 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 Somerville
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 8.8 | Moderate |
| Typical, some insulation, not air-sealed | 13.7 | Moderate |
| Older or poorly insulated, warm attic | 19.8 | High |
Sealing and insulating a typical attic takes out about 4.9 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 19.8 days.
Preventing ice dams on a Somerville 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.
Which to spend on first: air sealing or heat cable.
Somerville winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 14.3 | 18.1 | 5.0 |
| February | 14.4 | 17.8 | 4.5 |
| March | 9.0 | 13.4 | 2.0 |
| April | 1.6 | 1.0 | 0.0 |
| October | 0.2 | 0.0 | 0.0 |
| November | 0.7 | 1.2 | 0.1 |
| December | 9.0 | 10.8 | 2.1 |
Somerville next to nearby towns
Somerville ranks 32 of 58 Massachusetts cities here for ice-dam days, behind Pittsfield at 31.3 days.
- Cambridge, MA 1 mi, 13.7 days
- Medford, MA 2 mi, 13.7 days
- Everett, MA 3 mi, 13.7 days
- Malden, MA 3 mi, 13.7 days
- Chelsea, MA 4 mi, 13.7 days
Where these numbers come from
NOAA 1991–2020 climate normals from Boston, 5.1 miles from Somerville. 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.
- Snow-on-the-ground counts come from a second, nearby station, because the closest one does not record them.