Ice dam risk in Boston, MA
How often ice dams form in Boston
10.4 dam days a winter
Moderate risk. On an ordinary house in Boston, conditions for an ice dam come on about 10.4 days in a normal winter, mostly in January and February.
Boston averages 49 inches of snow a year and about 46 days with snow on the ground. Most of it comes and goes within a few days. 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 Boston
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 6.6 | Moderate |
| Typical, some insulation, not air-sealed | 10.4 | Moderate |
| Older or poorly insulated, warm attic | 15.1 | High |
Sealing and insulating a typical attic takes out about 3.8 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 15.1 days.
Preventing ice dams on a Boston 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.
Boston winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 14.3 | 14.8 | 4.1 |
| February | 14.4 | 13.4 | 3.4 |
| March | 9.0 | 9.3 | 1.4 |
| April | 1.6 | 0.6 | 0.0 |
| October | 0.2 | 0.1 | 0.0 |
| November | 0.7 | 0.5 | 0.0 |
| December | 9.0 | 7.5 | 1.5 |
Boston next to nearby towns
Boston ranks 47 of 58 Massachusetts cities here for ice-dam days, behind Pittsfield at 31.3 days.
- Winthrop Town, MA 4 mi, 8.9 days
- Chelsea, MA 4 mi, 13.7 days
- Everett, MA 5 mi, 13.7 days
- Quincy, MA 5 mi, 11.2 days
- Somerville, MA 6 mi, 13.7 days
Where these numbers come from
NOAA 1991–2020 climate normals from Boston, 1.6 miles from Boston. 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.