What causes ice dams on Boise gutters?
Heat escaping through the roof. It melts snow up high, the water runs down to the cold edge over your Boise gutters, and it refreezes into a ridge that traps everything behind it. Clogged gutters make it worse, but they are not the cause.
How does an ice dam actually form?
Four steps, in order. Break any one of them and the dam does not build.
| Step | What is happening |
|---|---|
| 1. Heat rises | Warm air leaks from the living space into the attic |
| 2. Snow melts | The roof deck over the attic warms and melts the snow above it |
| 3. Water refreezes | Meltwater reaches the eave, which is over open air and stays cold |
| 4. The dam grows | Ice builds at the edge and holds standing water behind it |
Why do Boise and Treasure Valley homes get them?
The valley gets its precipitation in the cold half of the year, which the National Weather Service office in Boise tracks through late fall, winter and spring. Add the freeze and thaw pattern the valley runs on, where a sunny afternoon melts what a cold night refreezes, and you get the exact conditions a dam needs: repeated melting at the top of the roof and repeated freezing at the edge.
The homes that dam every year usually share a few things. Older attics with thin or settled insulation. Can lights, bath fans and attic hatches leaking warm air straight into the roof space. A low-slope section or a north-facing eave that never sees sun. And a gutter that went into winter full.
Do clogged gutters cause ice dams?
Not on their own, and anyone telling you a cleaning is an ice dam cure is overselling it. What a clogged gutter does is remove the escape route. Debris holds water in the trough, that water is the first thing to freeze, and the block of ice it makes gives the dam a foundation to build on. Meltwater that would have run down a clear downspout instead stands at the edge and adds to the ridge.
So the cleaning is genuinely worth doing before the freeze, and it is not the whole answer. Both halves matter, and only one of them happens from a ladder.
What damage does an ice dam do?
Water under the shingles
Standing water behind the ridge works backward under the shingle courses, because shingles shed running water rather than seal against it. That is how ice at the edge of a roof ends up as a stain on a bedroom ceiling.
Soaked insulation and framing
Water that gets past the deck lands in the attic. Wet insulation loses most of its value and stays wet for a long time in winter, which keeps the framing and the ceiling below it damp well after the ice is gone.
Gutters pulled off the fascia
A trough full of ice is heavy and it expands as it grows. Hangers pull, seams open, and the front lip bends out. See gutter repair for what that costs to put right.
Rotting fascia and soffit
Water running behind a gutter instead of into it soaks the fascia board and works into the soffit. It shows up as staining and peeling paint first. Detail on the fascia and soffit page.
How do you prevent ice dams before winter?
In order of how much difference each one makes.
- Seal the air leaks into the attic. Recessed lights, the attic hatch, bath fan housings and plumbing penetrations. ENERGY STAR publishes homeowner guidance on sealing and insulating for exactly this reason.
- Bring attic insulation up to depth. A cold roof deck does not melt the snow sitting on it, and that is the whole game.
- Check that bath and kitchen fans vent outside, not into the attic. A fan dumping warm, wet air under the roof is a dam machine.
- Clear the gutters and every downspout in late fall. Meltwater needs a path out, and standing debris water is the first ice to form.
- Fix sagging sections before winter. A run out of pitch already holds water, so it starts the freeze with a head start.
- Extend downspouts away from the foundation so the water you did move does not refreeze in a sheet at the base of the house.
The first three are attic and building work. The last three are gutter work, and they are the part we handle. If you want the timing on the fall visit, it is on how often to clean gutters in Boise, and what a visit covers is on the gutter cleaning page.
What not to do when you find one
Do not chip at it. A hammer, a hatchet or a screwdriver on a roof edge takes shingles and gutter with the ice, and a ladder set on frozen ground is how people get hurt in January.
Do not throw rock salt into the trough. It does little against a real dam, and it is hard on aluminum, on fasteners and on the plants and paving the runoff reaches.
Do not pressure wash it off. Warm water on a frozen roof edge refreezes within the hour and you have added to the problem while soaking everything under it.
What is worth doing in the moment is pulling snow off the lower few feet of roof with a roof rake from the ground, if you can reach safely, and getting the ceiling stains looked at. Then fix the cause before next winter. Call (208) 203-4508 if you want the gutter half of it handled, and see the gutter cleaning cost page for how the work is estimated.
Ice dams and frozen gutters: common questions
What Boise homeowners ask after the first hard freeze.
What exactly is an ice dam on a roof?
Do gutters cause ice dams in the first place?
Will cleaning my gutters stop ice dams from forming?
Is it safe to chip an ice dam off with a hammer?
Should I put salt or ice melt in my gutters?
Why do icicles hang off my gutters every winter?
Can an ice dam damage the gutters themselves?
When should Boise gutters be cleared before winter?
Does adding attic insulation stop ice dams?
Do gutter guards prevent ice dams?
Get the gutters clear before the first freeze
A late fall cleaning gives the meltwater somewhere to go. The estimate is free.
(208) 203-4508