Ice dam risk in Lisbon, WI
How often ice dams form in Lisbon
24.6 dam days a winter
High risk. On an ordinary house in Lisbon, conditions for an ice dam come on about 24.6 days in a normal winter, mostly in January and February.
Lisbon averages 54 inches of snow a year and about 70 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 Lisbon
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 10.7 | Moderate |
| Typical, some insulation, not air-sealed | 24.6 | High |
| Older or poorly insulated, warm attic | 42.0 | Severe |
Sealing and insulating a typical attic takes out about 13.9 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 42.0 days.
Preventing ice dams on a Lisbon 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.
Lisbon winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 16.5 | 22.5 | 9.2 |
| February | 16.0 | 19.1 | 7.2 |
| March | 6.5 | 9.3 | 2.4 |
| April | 2.0 | 1.1 | 0.1 |
| May | 0.3 | 0.0 | 0.0 |
| October | 0.3 | 0.1 | 0.0 |
| November | 2.4 | 2.6 | 0.5 |
| December | 10.3 | 15.0 | 5.2 |
Lisbon next to nearby towns
Lisbon ranks 56 of 98 Wisconsin cities here for ice-dam days, behind Wausau at 41.6 days.
- Sussex, WI 2 mi, 24.6 days
- Richfield, WI 6 mi, 24.6 days
- Hartland, WI 6 mi, 21.4 days
- Pewaukee, WI 6 mi, 21.4 days
- Menomonee Falls, WI 6 mi, 24.6 days
Where these numbers come from
NOAA 1991–2020 climate normals from Germantown Wastewater Utility, 8.6 miles from Lisbon. 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.