Ice dam risk in McHenry, IL
Will your roof get ice dams in McHenry, IL?
18.8 dam days a winter
High risk. On an ordinary house in McHenry, conditions for an ice dam come on about 18.8 days in a normal winter, mostly in January and February.
McHenry averages 35 inches of snow a year and about 55 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 McHenry
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 8.4 | Moderate |
| Typical, some insulation, not air-sealed | 18.8 | High |
| Older or poorly insulated, warm attic | 31.8 | High |
Sealing and insulating a typical attic takes out about 10.4 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 31.8 days.
Preventing ice dams on a McHenry 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.
McHenry winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 11.0 | 20.0 | 7.8 |
| February | 8.9 | 16.2 | 5.7 |
| March | 3.9 | 6.3 | 1.4 |
| April | 1.0 | 0.6 | 0.0 |
| October | 0.1 | 0.0 | 0.0 |
| November | 1.4 | 1.5 | 0.3 |
| December | 8.2 | 10.6 | 3.5 |
McHenry next to nearby towns
McHenry ranks 24 of 225 Illinois cities here for ice-dam days, behind Arlington Heights at 21.5 days.
- Crystal Lake, IL 7 mi, 16.8 days
- Woodstock, IL 8 mi, 16.8 days
- Fox Lake, IL 8 mi, 20.1 days
- Cary, IL 9 mi, 18.8 days
- Wauconda, IL 9 mi, 18.6 days
Where these numbers come from
NOAA 1991–2020 climate normals from Mc Henry -Wg Stratton L&d, 2.7 miles from McHenry. 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.