Ice dam risk in St. Louis, MO
Ice dams in St. Louis, MO: how often
3.6 dam days a winter
Low risk. On an ordinary house in St. Louis, conditions for an ice dam come on about 3.6 days in a normal winter, mostly in January and December.
St. Louis averages 14 inches of snow a year and about 15 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 St. Louis
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 2.4 | Low |
| Typical, some insulation, not air-sealed | 3.6 | Low |
| Older or poorly insulated, warm attic | 5.0 | Low |
The attic matters less in St. Louis than in colder towns: there are only 1.2 days a winter between a typical attic and a sealed one.
Preventing ice dams on a St. Louis roof
- Check the gutters before the first snow and clear them, so meltwater has somewhere to go. If you see icicles thicker than a finger along the eave after a storm, that is the early sign to look at the attic.
Snowy places are not always the worst: why snowfall misleads.
St. Louis winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 4.9 | 7.2 | 1.9 |
| February | 3.7 | 3.5 | 0.7 |
| March | 1.2 | 0.6 | 0.1 |
| April | 0.3 | 0.1 | 0.0 |
| November | 0.8 | 0.5 | 0.0 |
| December | 2.9 | 3.5 | 0.8 |
St. Louis next to nearby towns
St. Louis ranks 43 of 80 Missouri cities here for ice-dam days, behind Maryville at 9.3 days.
- Clayton, MO 5 mi, 3.7 days
- University City, MO 5 mi, 3.7 days
- Jennings, MO 6 mi, 3.7 days
- East St. Louis, IL 6 mi, 3.6 days
- Cahokia Heights, IL 7 mi, 3.6 days
Where these numbers come from
NOAA 1991–2020 climate normals from Cahokia, 5.5 miles from St. Louis. 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.