Ice dam risk in Portland, ME
Will your roof get ice dams in Portland, ME?
22.4 dam days a winter
High risk. On an ordinary house in Portland, conditions for an ice dam come on about 22.4 days in a normal winter, mostly in January and February.
Portland averages 69 inches of snow a year and about 74 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 Portland
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 12.4 | Moderate |
| Typical, some insulation, not air-sealed | 22.4 | High |
| Older or poorly insulated, warm attic | 35.0 | High |
Sealing and insulating a typical attic takes out about 10.0 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 35.0 days.
Preventing ice dams on a Portland 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.
Portland winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 18.6 | 23.3 | 8.5 |
| February | 16.6 | 20.8 | 6.8 |
| March | 13.6 | 15.9 | 3.7 |
| April | 2.8 | 1.8 | 0.2 |
| October | 0.2 | 0.1 | 0.0 |
| November | 2.3 | 1.7 | 0.3 |
| December | 14.6 | 10.6 | 3.0 |
Portland next to nearby towns
Portland ranks 8 of 11 Maine cities here for ice-dam days, behind Waterville at 40.9 days.
- South Portland, ME 5 mi, 22.4 days
- Westbrook, ME 10 mi, 22.4 days
- Saco, ME 14 mi, 22.4 days
- Biddeford, ME 17 mi, 22.6 days
- Auburn, ME 31 mi, 31.3 days
Where these numbers come from
NOAA 1991–2020 climate normals from Portland Jetport, 6.0 miles from Portland. 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.