Ice dam risk in Plymouth, MN
Will your roof get ice dams in Plymouth, MN?
37.3 dam days a winter
Severe risk. On an ordinary house in Plymouth, conditions for an ice dam come on about 37.3 days in a normal winter, mostly in January and February.
Plymouth averages 54 inches of snow a year and about 104 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 Plymouth
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 15.5 | High |
| Typical, some insulation, not air-sealed | 37.3 | Severe |
| Older or poorly insulated, warm attic | 64.6 | Severe |
Sealing and insulating a typical attic takes out about 21.9 days a winter here. It drops the risk from severe to high. An older house with a warm attic should expect closer to 64.6 days.
What to do about ice dams in Plymouth
- 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.
- Heat cable is worth pricing here even on a sealed house, along the eave and in the gutters and downspouts it drains into.
Running 60 feet of 5 W/ft cable whenever snow lies below freezing would use about 393 kWh a winter in Plymouth, roughly $67 at 17 cents a kWh.
Which to spend on first: air sealing or heat cable.
Plymouth winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 10.1 | 29.0 | 12.4 |
| February | 10.1 | 25.6 | 10.0 |
| March | 9.0 | 17.2 | 4.3 |
| April | 5.1 | 2.5 | 0.3 |
| October | 0.8 | 0.4 | 0.0 |
| November | 6.9 | 6.7 | 1.6 |
| December | 12.3 | 22.1 | 8.6 |
Plymouth next to nearby towns
Plymouth ranks 29 of 103 Minnesota cities here for ice-dam days, behind Hermantown at 52.9 days.
- New Hope, MN 4 mi, 37.3 days
- Crystal, MN 5 mi, 37.3 days
- Golden Valley, MN 6 mi, 37.3 days
- Minnetonka, MN 6 mi, 37.3 days
- Maple Grove, MN 6 mi, 37.3 days
Where these numbers come from
NOAA 1991–2020 climate normals from NEW Hope, 4.2 miles from Plymouth. 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.