Ice dam risk in Grafton, WI
Will your roof get ice dams in Grafton, WI?
19.2 dam days a winter
High risk. On an ordinary house in Grafton, conditions for an ice dam come on about 19.2 days in a normal winter, mostly in January and February.
Grafton averages 42 inches of snow a year and about 58 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 Grafton
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 8.9 | Moderate |
| Typical, some insulation, not air-sealed | 19.2 | High |
| Older or poorly insulated, warm attic | 32.1 | High |
Sealing and insulating a typical attic takes out about 10.3 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 32.1 days.
Preventing ice dams on a Grafton 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.
Grafton winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 13.2 | 21.0 | 8.0 |
| February | 10.7 | 17.0 | 5.7 |
| March | 6.0 | 7.6 | 1.7 |
| April | 0.8 | 0.5 | 0.0 |
| November | 0.9 | 1.3 | 0.2 |
| December | 10.7 | 10.9 | 3.4 |
Grafton next to nearby towns
Grafton ranks 78 of 98 Wisconsin cities here for ice-dam days, behind Wausau at 41.6 days.
- Cedarburg, WI 3 mi, 24.6 days
- Port Washington, WI 5 mi, 19.2 days
- Mequon, WI 8 mi, 24.6 days
- Brown Deer, WI 10 mi, 18.2 days
- Germantown, WI 11 mi, 24.6 days
Where these numbers come from
NOAA 1991–2020 climate normals from Port Washington, 6.6 miles from Grafton. 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.