Ice dam risk in Ann Arbor, MI
Ice dams in Ann Arbor, MI: how often
20.5 dam days a winter
High risk. On an ordinary house in Ann Arbor, conditions for an ice dam come on about 20.5 days in a normal winter, mostly in February and January.
Ann Arbor averages 59 inches of snow a year and about 64 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 Ann Arbor
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 10.3 | Moderate |
| Typical, some insulation, not air-sealed | 20.5 | High |
| Older or poorly insulated, warm attic | 33.3 | High |
Sealing and insulating a typical attic takes out about 10.2 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 33.3 days.
Preventing ice dams on a Ann Arbor 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.
Ann Arbor winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 16.6 | 19.9 | 7.4 |
| February | 17.1 | 20.0 | 6.9 |
| March | 7.3 | 8.2 | 1.9 |
| April | 2.4 | 0.6 | 0.1 |
| October | 0.1 | 0.0 | 0.0 |
| November | 3.9 | 2.4 | 0.4 |
| December | 11.9 | 12.7 | 3.9 |
Ann Arbor next to nearby towns
Ann Arbor ranks 27 of 90 Michigan cities here for ice-dam days, behind Sault Ste. Marie at 48.8 days.
- Ypsilanti, MI 6 mi, 20.5 days
- South Lyon, MI 13 mi, 18.3 days
- Wayne, MI 18 mi, 14.1 days
- Westland, MI 18 mi, 14.1 days
- Novi, MI 19 mi, 18.1 days
Where these numbers come from
NOAA 1991–2020 climate normals from Ann Arbor SE, 3.1 miles from Ann Arbor. 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.