Ice Dams on Iowa Roofs: Why They Form and How to Prevent Them

Winter roofs Updated 2026-10-22 · By Kasey, Waukee, Iowa · 6 sources

If you have lived through a Waukee or Ames winter, you have seen it: a thick ridge of ice along the eaves, a row of icicles, and sometimes a brown water stain on the ceiling a few days later. That ridge is an ice dam, and it is one of the most common winter roof problems in cold-climate states like Iowa.

This guide explains why ice dams form, using guidance from the U.S. Department of Energy's Building America Solution Center and ENERGY STAR, what actually prevents them, and what to do safely if you already have one. It is a good one to read in late fall, before the first heavy snow.

What an ice dam is

The Building America Solution Center (BASC), run for the U.S. Department of Energy by Pacific Northwest National Laboratory, defines an ice dam as an accumulation of ice that forms at the eave of a roof and prevents melted snow from draining off the roof. Some meltwater overflows the dam and makes icicles. BASC says those icicles are often a sign that more liquid water is building up behind the dam, where it can be drawn up beneath the shingles by capillary action and cause roof leaks. In extreme cases, enough water and ice can overwhelm the roof structure.

Why ice dams form

BASC says ice dam formation requires three things:

1. Snow on the roof
2. A poorly air-sealed and/or poorly insulated attic
3. Freezing temperatures

Here is the chain reaction. Heated air from the house escapes into the attic and warms the underside of the roof deck, melting the snow above. The meltwater runs down the roof until it reaches the cold eaves, which stick out past the warm walls, and refreezes. As melting and refreezing continue, the ice ridge grows into a dam, and a puddle of liquid water forms behind it. BASC notes ice dams can form with as little as 2 inches of snow on a roof.

ENERGY STAR describes it the same way: snow melts off a roof from an attic that is too warm and then re-freezes at the gutters, causing an ice dam that can damage the roof.

Signs to watch for this winter

You can check most of these from the ground or from inside:

• Thick ice ridges and long icicles along the eaves, especially over heated rooms
• Bare patches in the snow on the upper roof while the rest stays covered (BASC shows this as a sign that heat from the room below is warming the roof deck)
• Water stains or drips on ceilings or exterior walls near the eaves
• Frost or damp insulation in the attic

ENERGY STAR lists ice dams in the winter as a common symptom of homes that need attic air sealing, alongside drafty rooms, uneven temperatures, high heating bills and dust in rooms below the attic.

How to prevent ice dams

BASC says the three best ways to combat ice dams are to fully air seal the ceiling plane, thoroughly insulate the attic, and ventilate the roof. ENERGY STAR agrees on the order, recommending air sealing the attic before adding insulation.

1. Air seal first

BASC calls sealing air leaks from the living space into the attic the most important step. Common leaks it lists include gaps around electrical wiring, recessed can lights, plumbing stacks, furnace flues and chimneys, attic hatches and stairs, exhaust fan housings, top plates and attic kneewalls. ENERGY STAR's attic air sealing project uses caulk for small gaps and expanding foam for larger ones, with special high-temperature caulk and metal flashing around flues and chimneys.

2. Insulate the attic

Adequate insulation keeps heat from reaching the roof deck, BASC says. ENERGY STAR's insulation guidance includes two cautions: never lay insulation over soffit vents or non-IC-rated recessed lights, and keep insulation at least 3 inches away from recessed "can" lights unless they are rated IC (Insulated Ceiling). ENERGY STAR also recommends a professional if you find wet insulation, moldy rafters, bathroom or dryer vents exhausting into the attic, little ventilation, or knob-and-tube wiring.

3. Keep the attic ventilated

ENERGY STAR explains that a natural flow of outdoor air through the attic in winter helps keep it cold, which reduces ice damming. It says to never cover soffit vents with insulation and to use rafter vents (insulation baffles) so air can move in at the soffits and out at the ridge or gable vents. BASC recommends an air gap of at least 2 inches between the top of the insulation and the underside of the roof deck.

4. Ask about ice barrier when you re-roof

Building codes already account for ice dams. The City of Des Moines' residential shingle requirements, based on the International Residential Code, call for an ice barrier on asphalt shingle roofs that extends from the lowest roof edges to at least 24 inches inside the exterior wall line. If you are planning a replacement, ask your roofer how they will handle ice barrier, flashing and ventilation. Our Des Moines roof replacement guide covers the rest of that document.

ENERGY STAR also suggests checking with your utility before an air sealing or insulation project to see what incentives are available.

If you already have an ice dam

• Do not climb up or try to chip the ice off yourself. Ladders and icy roofs are a dangerous mix; leave rooftop ice to a professional.
• Protect the inside first. Move belongings away from drips, catch water, and photograph any damage.
• Call a registered roofer for safe ice removal or a leak repair, and see our guide to choosing a roofing contractor in Iowa.
• Talk to your insurer if water got inside. The Iowa Insurance Division's claims guide notes that maintenance items are not a covered peril under an insurance policy, so ask how your policy treats the damage.
• Fix the cause in spring: air sealing, insulation and ventilation, so the dam does not come back next winter.

Frequently asked questions

What causes ice dams on Iowa roofs?

The Department of Energy's Building America Solution Center says ice dams need snow on the roof, a poorly air-sealed or poorly insulated attic, and freezing temperatures. Warm attic air melts snow, and the water refreezes at the cold eaves.

Are icicles a bad sign?

Often. The Building America Solution Center says icicles are often a sign that more liquid water is building up behind an ice dam, where it can get under shingles and cause leaks.

Will more attic ventilation alone stop ice dams?

Ventilation helps keep the attic cold, according to ENERGY STAR, but the Building America Solution Center lists air sealing the ceiling as the most important step, followed by insulation and ventilation together.

How much snow does it take to form an ice dam?

The Building America Solution Center says ice dams can form with as little as 2 inches of snow on a roof.

Is ice dam damage covered by insurance?

It depends on your policy. The Iowa Insurance Division notes that maintenance items are not a covered peril, so call your insurer to ask how your policy handles interior water damage.