Ice dam risk in Derby, CT
How often ice dams form in Derby
7.4 dam days a winter
Moderate risk. On an ordinary house in Derby, conditions for an ice dam come on about 7.4 days in a normal winter, mostly in February and January.
Derby averages 34 inches of snow a year and about 34 days with snow on the ground. Plenty falls, but it tends to melt off between storms, which keeps the count lower than the snowfall suggests. That gap is why snowfall alone misleads.
What your attic does to that number in Derby
| Attic | Dam days | Risk |
|---|---|---|
| Air-sealed and well insulated | 5.0 | Moderate |
| Typical, some insulation, not air-sealed | 7.4 | Moderate |
| Older or poorly insulated, warm attic | 10.3 | Moderate |
Sealing and insulating a typical attic takes out about 2.4 days a winter here. It does not move the risk band, but it shortens every dam.
Preventing ice dams on a Derby 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.
Which to spend on first: air sealing or heat cable.
Derby winter, month by month
| Month | Snow (in) | Days on ground | Dam days |
|---|---|---|---|
| January | 8.5 | 10.7 | 2.8 |
| February | 10.7 | 11.5 | 2.8 |
| March | 7.0 | 6.0 | 0.8 |
| April | 0.9 | 0.2 | 0.0 |
| October | 0.1 | 0.0 | 0.0 |
| November | 0.9 | 0.3 | 0.0 |
| December | 5.5 | 4.9 | 0.9 |
Derby next to nearby towns
Derby ranks 19 of 21 Connecticut cities here for ice-dam days, behind Torrington at 23.6 days.
- Ansonia, CT 1 mi, 7.4 days
- Shelton, CT 3 mi, 7.4 days
- West Haven, CT 7 mi, 7.4 days
- Milford, CT 7 mi, 7.4 days
- New Haven, CT 8 mi, 7.4 days
Where these numbers come from
NOAA 1991–2020 climate normals from Igor I Sikorsky Mem Airport, 11.4 miles from Derby. 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.