Ice dam risk in Marshalltown, IA
Will your roof get ice dams in Marshalltown, IA?
20.4 dam days a winter
High risk. On an ordinary house in Marshalltown, conditions for an ice dam come on about 20.4 days in a normal winter, mostly in January and February.
Marshalltown averages 25 inches of snow a year and about 59 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 Marshalltown
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 9.2 | Moderate |
| Typical, some insulation, not air-sealed | 20.4 | High |
| Older or poorly insulated, warm attic | 34.3 | High |
Sealing and insulating a typical attic takes out about 11.2 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 34.3 days.
Preventing ice dams on a Marshalltown 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.
Marshalltown winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 6.3 | 19.9 | 8.1 |
| February | 5.3 | 16.2 | 5.8 |
| March | 3.3 | 6.8 | 1.6 |
| April | 0.7 | 0.4 | 0.0 |
| October | 0.2 | 0.2 | 0.0 |
| November | 1.7 | 2.1 | 0.4 |
| December | 7.8 | 12.9 | 4.5 |
Marshalltown next to nearby towns
Marshalltown ranks 11 of 41 Iowa cities here for ice-dam days, behind Mason City at 28.7 days.
- Newton, IA 24 mi, 19.6 days
- Ames, IA 37 mi, 19.3 days
- Altoona, IA 40 mi, 17.4 days
- Cedar Falls, IA 41 mi, 22.2 days
- Ankeny, IA 41 mi, 17.4 days
Where these numbers come from
NOAA 1991–2020 climate normals from Marshalltown, 2.3 miles from Marshalltown. 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.