FrostList

When to Prevent Ice Dams in Seattle, WA

ON TRACK157 days until snow season (estimated)Dec 15

Ice-dam season in Seattle begins as snow settles near December 15, about 4" a year in the normals; air-seal and insulate the attic ahead of it. Plan for the early end: the one-in-ten date lands about 20 days before the median.

OUTLOOK

Typical first snow season (estimated) near Dec 15; local deadline about Nov 15. The live 10-day outlook loads here.

Local freeze dates for Seattle

ThresholdEarly (1-in-10)MedianLate (9-in-10)
32°F (light freeze)Nov 1Nov 17Dec 6
28°F (hard freeze)Nov 12Dec 2Dec 30
24°F (severe)Nov 21Dec 14Jan 29

NOAA station: Seattle Boeing Fld · 5.4 mi away · 20 ft elevation.

Numbers for Seattle come from Seattle Boeing Fld, 5.4 miles away at 20 feet, where the medians fall 32°F by Nov 17, 28°F by Dec 2, 24°F by Dec 14. Year to year the 28°F date has ranged from Nov 12 to Dec 30 — about 48 days apart. Spring's last 32°F freeze clears around Mar 13. Snowfall averages 4 inches a year, first reaching an inch near December.

Seattle usually sees its first 32°F night about Nov 17, with the first 28°F hard freeze close behind near Dec 2. That first freezing night has ranged from Nov 1 to Dec 6, roughly a 35-day spread. On the spring side, the last 32°F freeze clears around Mar 13 and as late as Apr 1 — the green light for reopening water and de-winterizing. Only about 4 inches of snow falls in a typical year, so cold protection outranks snow removal.

Your ice dams checklist

  1. Before snow flies, seal attic air leaks around lights, the attic hatch, and plumbing stacks so warm air stays out of the attic.
  2. Add insulation to bring the attic floor up to a deep, even blanket; a cold roof deck is what stops dams from forming.
    Helpful gear: Attic vent baffles — Recommended pick
  3. Confirm soffit and ridge vents are open and clear so outside air keeps the underside of the roof cold.
  4. After a storm drops four inches or more, rake the lower three to six feet of roof from the ground.
    Helpful gear: 21-foot roof rake — Recommended pick
  5. Keep a safe distance from the edge while raking and never climb an icy roof; work from the ground.
  6. If a dam forms, lay a calcium-chloride melt sock across it to open a drainage channel — do not chip at the ice.
    Helpful gear: Calcium chloride roof-melt socks — Recommended pick
  7. For a roof that dams every year, have heat cable installed at the eaves before the season starts.
    Helpful gear: Roof de-icing heat cable — Recommended pick
  8. Watch for long icicles and interior ceiling stains; both are early signs water is backing up under the shingles.

What to have on hand

21-foot roof rake
Telescoping rake that clears the lower roof edge from the ground.
Recommended pick
Calcium chloride roof-melt socks
Filled tubes laid across the eave to open a drainage channel.
Recommended pick
Roof de-icing heat cable
Zig-zag cable that keeps a melt path open at the eaves.
Recommended pick
Attic vent baffles
Chutes that keep soffit airflow open so the roof stays cold.
Recommended pick

What this means locally

Compared with nearby cities, Seattle's first-freeze date near Dec 15 sits close to Bellevue (Dec 15) and close to Kirkland (Dec 15). Washington's deadlines span Oct 16 to Dec 16 statewide — one date for all of Washington would be off by weeks for Seattle. Once you know Seattle's freeze date, use it to guard your pipes and ready your snow blower too.

Other winter jobs in Seattle

Every task below is dated to Seattle's own freeze and snow normals.

Sprinklers → Nov 22Frozen pipes → Nov 1Car battery → Dec 2Snow blower → Nov 24Sump pump → Mar 13RV → Nov 22Boat → Nov 25Motorcycle → Nov 17

See the full Seattle winter checklist, in order →

Frequently asked questions

What causes ice dams?
Heat escaping into the attic warms the roof deck and melts the underside of the snowpack. That meltwater runs down to the cold eave, where it refreezes into a ridge of ice. The dam then traps later melt, which can back up under the shingles. University Extension programs point to attic heat loss as the root cause.
How much snow on a roof causes ice dams?
There is no single number, but several inches of snow that lingers gives dams the material they need, especially when days rise above freezing and nights fall below. A roof that sheds snow quickly or stays uniformly cold rarely dams. Watch for snow that sticks around through a stretch of thaw-freeze weather.
Do heat cables prevent ice dams?
Heat cables do not fix the underlying attic-heat problem, but installed in a zig-zag at the eaves they can keep a melt path open so water drains instead of pooling. They work best as one part of a plan that also includes air sealing, insulation, and ventilation. Run them only when needed to save energy.
Is roof raking worth it?
Removing the lower three to six feet of snow from the ground after a storm takes away the material a dam forms from, and it is one of the safest do-it-yourself steps. Use a roof rake with an extension and keep clear of the edge. Never climb onto an icy roof to rake.
Will my insurance cover ice dam damage?
Many homeowner policies cover sudden interior water damage from an ice dam, but coverage and deductibles vary, and repeated damage may raise questions about maintenance. Document the damage with photos. Preventing dams through attic work is cheaper and less disruptive than filing repeat claims.
How do I know if I have an ice dam forming?
Look for a thick ridge of ice at the eaves, large icicles hanging from the gutters, and water stains on interior ceilings or the tops of exterior walls. Icicles alone are not proof, but combined with a warm attic and lingering roof snow they are a warning worth acting on.

Data: NOAA 1991–2020 normals via Seattle Boeing Fld, live outlook by Open-Meteo. Sources · Methodology. Last updated: July 11, 2026.