Ice dam risk in Manchester, NH
How often ice dams form in Manchester
22.8 dam days a winter
High risk. On an ordinary house in Manchester, conditions for an ice dam come on about 22.8 days in a normal winter, mostly in January and February.
Manchester averages 53 inches of snow a year and about 71 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 Manchester
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 13.2 | Moderate |
| Typical, some insulation, not air-sealed | 22.8 | High |
| Older or poorly insulated, warm attic | 34.8 | High |
Sealing and insulating a typical attic takes out about 9.6 days a winter here. It drops the risk from high to moderate. An older house with a warm attic should expect closer to 34.8 days.
Preventing ice dams on a Manchester 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.
Manchester winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 15.9 | 25.2 | 9.2 |
| February | 17.6 | 20.4 | 6.8 |
| March | 6.7 | 13.5 | 3.4 |
| April | 0.9 | 0.8 | 0.1 |
| November | 1.1 | 1.2 | 0.2 |
| December | 10.6 | 10.3 | 3.1 |
Manchester next to nearby towns
Manchester ranks 7 of 11 New Hampshire cities here for ice-dam days, behind Claremont at 34.6 days.
- Nashua, NH 16 mi, 24.1 days
- Concord, NH 18 mi, 28.7 days
- Methuen, MA 21 mi, 20.5 days
- Haverhill, MA 23 mi, 20.5 days
- Lawrence, MA 24 mi, 20.5 days
Where these numbers come from
NOAA 1991–2020 climate normals from Massabesic Lake, 2.6 miles from Manchester. 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.