Ice dam risk in Johnston, IA
How often ice dams form in Johnston
17.8 dam days a winter
High risk. On an ordinary house in Johnston, conditions for an ice dam come on about 17.8 days in a normal winter, mostly in January and February.
Johnston averages 38 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 Johnston
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 8.5 | Moderate |
| Typical, some insulation, not air-sealed | 17.8 | High |
| Older or poorly insulated, warm attic | 29.4 | High |
Sealing and insulating a typical attic takes out about 9.3 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 29.4 days.
What to do about ice dams in Johnston
- 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.
Johnston winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 9.7 | 17.9 | 6.8 |
| February | 10.5 | 16.9 | 5.7 |
| March | 5.0 | 5.8 | 1.2 |
| April | 1.6 | 0.5 | 0.0 |
| May | 0.2 | 0.1 | 0.0 |
| October | 0.5 | 0.3 | 0.0 |
| November | 2.2 | 1.8 | 0.3 |
| December | 8.3 | 11.3 | 3.7 |
Johnston next to nearby towns
Johnston ranks 19 of 41 Iowa cities here for ice-dam days, behind Mason City at 28.7 days.
- Grimes, IA 4 mi, 17.8 days
- Urbandale, IA 5 mi, 17.8 days
- Clive, IA 7 mi, 16.1 days
- Ankeny, IA 7 mi, 17.4 days
- Des Moines, IA 10 mi, 16.1 days
Where these numbers come from
NOAA 1991–2020 climate normals from Des Moines Wsfo-Johnston, 3.1 miles from Johnston. 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.