Ice dam risk in Concord, NH
Will your roof get ice dams in Concord, NH?
28.7 dam days a winter
High risk. On an ordinary house in Concord, conditions for an ice dam come on about 28.7 days in a normal winter, mostly in January and February.
Concord averages 68 inches of snow a year and about 92 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 Concord
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 16.1 | High |
| Typical, some insulation, not air-sealed | 28.7 | High |
| Older or poorly insulated, warm attic | 44.3 | Severe |
Sealing and insulating a typical attic takes out about 12.5 days a winter here. It does not move the risk band, but it shortens every dam. An older house with a warm attic should expect closer to 44.3 days.
Preventing ice dams on a Concord 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.
- Heat cable is worth pricing here even on a sealed house, along the eave and in the gutters and downspouts it drains into.
Running 60 feet of 5 W/ft cable whenever snow lies below freezing would use about 225 kWh a winter in Concord, roughly $38 at 17 cents a kWh.
Which to spend on first: air sealing or heat cable.
Concord winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 17.1 | 25.4 | 9.6 |
| February | 16.9 | 24.3 | 8.2 |
| March | 13.6 | 19.3 | 4.8 |
| April | 2.5 | 3.1 | 0.4 |
| October | 0.8 | 0.1 | 0.0 |
| November | 2.5 | 2.8 | 0.5 |
| December | 14.3 | 16.7 | 5.1 |
Concord next to nearby towns
Concord ranks 4 of 11 New Hampshire cities here for ice-dam days, behind Claremont at 34.6 days.
- Manchester, NH 18 mi, 22.8 days
- Laconia, NH 24 mi, 29.5 days
- Rochester, NH 30 mi, 18.6 days
- Nashua, NH 34 mi, 24.1 days
- Somersworth, NH 34 mi, 18.6 days
Where these numbers come from
NOAA 1991–2020 climate normals from Concord, 3.5 miles from Concord. 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.