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Ice Dams: Preventing Roof Leaks

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The cold, wintery weather currently hitting much of the country has created the perfect conditions for ice dams and roof leaks.  Have you noticed moisture on your ceiling since cold weather arrived this season?  All it takes is a little snow and ice and you could have water leaking into your home.  You can’t change the weather outside but there are things you can do when building to help prevent ice dams from causing leaks into your new home. 

What Are Ice Dams?

Ice dams can occur on roofs when snow and ice accumulate.  As in the photo above, the snow and ice is melting on the main part of the roof over the living area because of the heat escaping.  The portion of the roof over the eaves is still frozen due to the cold air below.  As the water runs down the main roof it hits the frozen area at the eave and builds up.  This water build-up, called ice dams, can make its way back up the roof and under the shingles.

Causes for Ice Dams

There are basically two main causes for roof leaks from ice dams.  First, shingles are designed to shed water off roofs from the top to the bottom.  Whenever water moves sideways or up a roof slope, due to wind driven rain or ice dams, it can make its way back up under the shingles. 

The other major cause for ice dams is poor attic ventilation and insulation.  The reason the roof area over the living space is melting is because warm air is escaping and accumulating in the space below the roof assembly.  If you have adequate insulation and ventilation of the space immediately below the roof deck, it should stay cold enough to prevent melting until the eaves melt along with it.

Prevention Measures

One way to help prevent roof leaks from ice dams is to add a second layer of defense under the shingle.  A rubber underlayment can be installed in the locations where ices dams are most frequent to prevent water intrusion.

The photo above shows how a self-adhering rubber underlayment has been installed at the eaves of a new home.  The rest of the roof has the standard, code compliant roof felt.  According to the 2012 International Residential Code For One and Two Family Dwellings, “In areas where there has been a history of ice forming along the eaves causing a backup of water, an ice barrier that consists of at least two layers of underlayment cemented together or of a self-adhering polymer modified bitumen sheet, shall be used in lieu of normal underlayment and extend from the lowest edges of all roof surfaces to a point at least 24 inches inside the exterior wall line of the building.”

The last part of the code requiring the rubber underlayment to be installed at least 24 inches inside the exterior wall line is most important.  By installing the rubber underlayment inside the exterior wall line, we provide an extra layer of protection for cases where common freeze and thaw cycles move the ice dam back up the roof.

The other prevention measure for your new home is to make sure to have plenty of ventilation and insulation under your roof.  Your architect, builder consultant, roofing contractor, and insulation contractor all should work together on this.  The actual configuration of the ventilation system will depend on your particular climate, vent type, home design, roof material…etc.

Cost

For most homes with asphalt shingles, the extra protection provided by installing rubber underlayment at the eaves will cost $100-$300.  This is a small price to pay to prevent the hassles associated with roof leaks.

Michael Luckado

Michael Luckado has built, remodeled, and repaired thousands of homes across the U.S. He co-founded ArmchairBuilder.com to help people save money and time creating their dream home.

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