Why Ice Dams Happen & How to Prevent Them

Icicles might look magical but they signal that you have problems on your roof. Icicles mean melting snow on the roof is not able to flow freely off the roof, so the water freezes and causes ices dams. Ice dams are bad because the water can often find it's way into your home following the laws of gravity. Not only will you need to remove the snow and ice, you'll also have to repair or replace the roof deck, framing and insulation damaged by leaking water.

Icicles present a safety hazard as they ultimately melt and fall off the roof. Icicles can fall and hit someone so it's important to remove the icicles and reduce your risk by blocking the area with yellow hazard tape that you typically see around work zones.

There are lots of reasons why ice dams form and this article explains why they happen and how to reduce the possibility of ice dams occurring. We also explain the type of damage that can occur to your roof and the interior of your home, from an unfinished attic down to your living space.

This article is the beginning of a series on ice dams and if we don't answer your questions, please leave a comment below and we'll be sure to add the information you're looking for.

• Why Ice Dams Happen & How to Prevent Them … this article
• Pantyhose, A Quick Fix to Melt Ice Dams
• Solutions to Minimize Ice Dams

What Causes Ice Dams

Understanding how ice dams form requires understanding how air circulates in your home. Using the diagram to the right:

• Your heating system warms air in your living or “heated space”.
• Warm air rises and some heated air leaks into your attic.
• The roof over your attic gets warmer than the eaves that hang out past the house and don't have any warm air underneath.
• As snow melts on your roof and rolls down, some of the water freezes when it reaches the colder eaves. The freezing water builds up to form ice dams along the edge (eave) of your roof.
• The icicles are formed by water getting past the roof edge but freezing before it can fall to the ground. When icicles become visible, it tells you there is an ice dam or ridge of ice on the roof even if hidden by snow.

How Much Damage Can ice Dams Cause?

Ice dams can cause significant damage to your roof, your gutters and when the water finds ways into your home, you can find yourself dealing with lots of damage from the insulation in your attic to stains on ceilings and walls, and more. Here are the most common types of damage from ice dams:

• Ice is heavy and it can loosen gutters and downspouts, and even tear them off.
• When water freezes, it can shift roofing shingles so they no longer protecting your home.
• Melting water sitting behind an ice dam can cause roof sheathing to rot if it gets wet and can't dry or escape quickly.
• The most common insulation, batts or blown in fiberglass insulation when it gets wet, compresses and looses its insulating value so you need to replace it.
• Water that leaks through the ceiling and/or down the walls can stain or compromise the sheet rock (also called drywall). It may only need a stain block and painting or the drywall may need to be replaced … and in either case, you then have to pain.
• Water that pools inside the house can cause the collapse of a ceiling, which I've see just once where they discovered 5 layers of roofing going back to original barn board when home was built in 1890s.

Less common is the risk that the weight of snow and ice over an extended period, will cause a roof to cave in.  In New Hampshire where I live though, we're getting more snow and in the article, Rising Snow Piles Lead to Roof Collapse Concerns, they state “… the Fire Marshal's Office said when the state was last hit hard with snow in 2008, there were 223 building collapses across the state causing more than $10 million in damage.”

Reducing the Risk of Ice Dams

You can reduce the loss of conditioned (warmed in winter & cooled in the summer) by adding insulation to your walls and attic. Adding insulation to an unfinished attic is fairly easy while more insulation in your walls is something you want to address when replacing the siding on your home. You also want to seal around electrical outlets/switches and any type of opening into your home (letting outside air in) or into your attic like the attic stairs, ceiling fixtures, plumbing, etc.

Along with insulation, you need ventilation to remove warm, moist air from unfinished attics. Ideally you want the temperature of the air in the attic to be the same as outside. When your attic is cold, snow will melt evenly on the roof, reducing ice dams due to uneven melting. Ventilation requires proper venting for outside air to flow into the attic, typically from soffit vents and out through a ridge vent. Older homes often only have gable vents on either end of the attic so a new roof should include the addition of a ridge vent.

Common problems that we saw in my handyman business are homeowners blocking air flow through gable vents and insulation installed wrong so that it blocks air flow from the soffit vents.