What causes ice dams, and can then be stopped?

Ice dam in Holland, NY

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What causes ice dams, and can then be stopped?

Ice dams are thick, evil build-ups of ice along a home’s roofline.  They can be problematic in some homes, and in others they’re nothing more than thick glaciers of ice that sit harmlessly until the spring warmth kicks in.  

The problem is that ice dams can damage roofing materials and can leak water into a home’s walls and ceilings.  They can cause real monetary damage in some situations.  

The purpose of this article is to inform you of what causes ice dams, if they can truly be prevented and how to get rid of them.

How to identify ice dams

Most people will know an ice dam when they see one.  If you look along the roofline of your house and see an edge of ice that’s a couple inches thick or more, then you have an ice dam.  In the Buffalo, NY area it’s not unusual to see an ice dam that’s over 6” in depth.  Many ice dams also feature icicles that make for a more interesting appearance.

What damage can ice dams do?

The melting and freezing of water that forms ice dams has a way of getting water into places where it doesn’t belong.  This “freeze-thaw” cycle of ice dams can lead to real damage to a home.  Ice dams a

The life cycle of an ice dam can sometimes look like this:

  • Snow collects on the roof
  • This snow melts, and water finds its way under shingles
  • Ice forms under shingles, melts and then damages roof sheathing
  • Water melts, then freezes between gutters and fascia board, pushing gutters away from the house
  • Melted water drips onto ceilings, damaging drywall or plaster
  • Water finds a path into walls, staining the interior surfaces

The most common harm caused to homes from ice dams seems to be premature shingle wear, bent damaged gutters and water-damaged roof sheathing.  In extreme situations there can be water damaged wall and ceiling drywall. 

In most homes there are ways to at least reduce the potential for ice dams to develop.

What causes ice dams?

Ice dams are caused by snow landing on a roof, then melting and refreezing.  Snowier winters make ice dams worse.

This freeze-thaw cycle is caused by several things:

  • Solar radiation from the sun heating up the roof and snow
  • Heat escaping from inside of the house
  • The ambient outdoor air temperature rising above freezing

There are many factors that can contribute to ice dams, but there are only a couple sure-fire ways of preventing them from developing in the first place.  Most ice dam situations are caused by a combination of factors.  Let’s look at some of the contributing factors to ice dam “growth”.

Attic Bypasses

An attic bypass is a pathway between the heated inside air of a home, and the attic.  These bypasses let warm air heat up the roof, allowing snow to melt.  Melting snow will freeze during colder temperatures, resulting in the beginning of an ice dam.  One example of an attic bypass is a visible gap around a recessed light, another is a “chase” used for plumbing vent pipes.

Missing Vapor Barrier

Vapor barriers are required by residential building code in cold climates, and are intended to keep moisture from moving from the interior of a home, which has relatively humid air during the winter, to unconditioned spaces such as attics.  Homes without vapor barriers allow more heat to escape into the attic, many times leading to ice dam issues.

Insufficient Insulation

Modern building codes typically require an R-49 level of insulation between the top floor of a home and the attic.  This equates to 14” of fiberglass insulation, or 7” of closed-cell spray foam.  

Having this level of insulation in a house would go a long way toward minimizing ice dam potential, but building code inspectors aren’t always picky people when it comes to inspecting attics.  Attics are tough places to maneuver, and it’s quick for a code inspector to check off a box. 

Ice dams typically occur at a home’s lower roof edge, and insulation or ventilation issues in that area of the attic are usually part of the cause.  Insulating the narrow bottom corners of attic spaces is always difficult, and it shows in the prevalence of ice dams in these areas.

If you are building a new home, consider using “heel” trusses as a way to cram more insulation in that area (trusses are a quick way to frame a roof).  Heel trusses raise the lower corner areas, allowing a lot more insulation to fit into that area.  More insulation = less chance of ice dams.

Disconnected Exhaust Vents

Bathroom, laundry and kitchen exhaust fans should alway send their air outside of the house, and never in the attic.  Allowing the damp, warm air from these fans to linger in the attic is a recipe for mold, ice dams and condensation damage to the roof structure.

Bathroom Exhaust Vents Near the Soffit

Some home builders choose to route bathroom exhaust fans out of the home’s soffit.  This is an easy way to do things, and might seem OK.  But the problem in doing this is that warm air being sent from the bathroom to the soffit will rise, and many times will go straight back up into the soffit, warming the roof and contributing to the dreaded freeze-thaw cycle.  

Bad Attic Ventilation

Almost all unfinished attics are designed as “unconditioned” spaces, meaning that there is no heating or air conditioning provided in these spaces.  Unconditioned attics require substantial ventilation in order to prevent several issues:

  1. Condensation forming on the underside of sheathing and rafters
  2. Mold growth
  3. Damage to roof structure

Ventilation in most houses includes soffit vents, ridge vents, gable vents, roof surface vents and sometimes powered ventilation fans. 

Poor ventilation will lead to areas of the roof that are slightly warmer than others, potentially leading to ice dams.  The more attic ventilation present, the closer the attic and roof will be to the outdoor temperature, reducing the chances of ice dams developing.

Think about ice dams when replacing your roof

When it’s time to replace your shingles or other roof covering, you won’t really be able to stop ice dams, but you can take action to minimize the damage they can cause. 

Consider having your roofer install a high-quality waterproof membrane on the entire roof surface.  A common product is Grace Ice and Water Shield.  This membrane is designed to seal the roof sheathing to the extent that even nails used for installing shingles can’t penetrate it.  

Waterproof membranes won’t stop ice dams from forming, but at least they should stop any moisture that gets under shingles from finding its way inside of the house.

What causes ice dams, and can then be stopped? 1

Should ice dams be removed?

Home inspectors in places like Orchard Park, Hamburg and Ellicottville, NY frequently encounter ice dams during the wintertime.  A good question to ask when home shopping is if ice dams should be left as they are, or if it’s necessary to remove them.  

Large ice dams can cause ongoing damage and water intrusion, so many people want to remove them when they get thick.  Is this a good idea?  

Most times I would say that it’s better to leave ice dams in place, and to wait until they’re gone to take steps to prevent them from coming back.  

In my opinion, unless an ice dam presents a safety issue, then it’s better to let them melt naturally.  For example, if an ice dam is over a sidewalk, patio or driveway then it would make sense to have it removed in order to avoid a dangerous situation.

How can ice dams be removed?

If you’ve decided that an ice dam needs to go, how should it be removed?

Some people choose to climb on their roof with a pick, then start hitting the ice dam.  

THIS IS A TERRIBLE IDEA!

What are the better options?

Option 1

If the roofline is low enough to avoid using a ladder,  throw ice melter on the ice dam.  It may take several rounds of melter, but this is a cheap and safe way of doing it.

Option 2

Hire someone to steam away the ice dam.  This is probably the best choice, but since it can be extremely risky, make sure that whoever you hire carries liability and worker’s compensation insurance.  

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About the Author:

Bradley Beck

Bradley is owner of Alto Home Inspection, LLC.  He lives just south of Orchard Park, in Western New York’s Southtowns, and inspects homes throughout the Buffalo area, and is NY Licensed Home Inspector #16000086029, NY Certified Mold Assessor #MA01313.  He also serves as Technical Director of Alto Home Inspection’s Radon Laboratory.