Ice dam risk in Ankeny, IA
Ice dams in Ankeny, IA: how often
17.4 dam days a winter
High risk. On an ordinary house in Ankeny, conditions for an ice dam come on about 17.4 days in a normal winter, mostly in January and February.
Ankeny averages 25 inches of snow a year and about 52 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 Ankeny
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 8.5 | Moderate |
| Typical, some insulation, not air-sealed | 17.4 | High |
| Older or poorly insulated, warm attic | 28.4 | High |
Sealing and insulating a typical attic takes out about 8.8 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 28.4 days.
Preventing ice dams on a Ankeny 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.
Ankeny winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 6.2 | 18.0 | 7.0 |
| February | 8.0 | 16.0 | 5.5 |
| March | 3.7 | 5.8 | 1.3 |
| April | 0.7 | 0.4 | 0.0 |
| October | 0.6 | 0.3 | 0.0 |
| November | 1.0 | 1.4 | 0.3 |
| December | 4.5 | 9.6 | 3.3 |
Ankeny next to nearby towns
Ankeny ranks 21 of 41 Iowa cities here for ice-dam days, behind Mason City at 28.7 days.
- Johnston, IA 7 mi, 17.8 days
- Altoona, IA 8 mi, 17.4 days
- Pleasant Hill, IA 10 mi, 16.1 days
- Des Moines, IA 11 mi, 16.1 days
- Grimes, IA 11 mi, 17.8 days
Where these numbers come from
NOAA 1991–2020 climate normals from Ankeny, 2.5 miles from Ankeny. 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.