The Inside Outside Guys: The truth about ice dams
The Guys once heard an “expert” on the radio make a comment to the effect that “if your house has ice dams, you had a bad builder”.
We disagree with that blanket assumption. Given the right weather conditions here in the Midwest, virtually any building can experience ice forming on the lower edge or eave of a roof.
Michigan is considered an “extreme weathering” state which means the combination of heat, extreme cold, high winds, UV from the sun and moisture in every configuration possible make it more difficult to protect exterior materials and the homes they enclose.
The problem with ridges of ice forming at the eave or in the valleys is that the ice can trap or dam melt water behind it.
Shingled roofs are intended to shed water. When water is “dammed” or trapped on the roof it can work its way back up under successive rows of shingles and into the attic space and house below.
This may show up as a leak in the ceiling or just as often a water intrusion around a window or door in the sidewall.
Additionally, ice forming on the heavy texture of a shingle can tear the shingle when the ice falls off the roof. This is one of the reasons we always tell homeowners not to chop the ice on the roof.
So why does that ice form? We have all seen snow build up on a roof above an attic space. Heat is lost from the house below into the attic space, and then through the roof. Heat moving through the roof melts the snow on the roof and water begins to run down the roof.
Right where the roof rests on the sidewalls and the overhangs begin we have the “hottest” and “coldest” parts of the roof. The greatest amount of heat loss from the house takes place at the point where the roof rests on the sidewalls. This is because there is very little space in which to put good thermal insulation. Just below that space, where the overhangs begin, is the very coldest part of the roof because there is no attic below, just cold air moving around three sides of the overhang.
So melt water from the roof hits this very cold spot and re-freezes. As ice builds up, a dam is created.
Another dynamic that plays into this is the sun helping to melt that roof snow on a very frigid day. In the right conditions, the sun and very cold temperatures can create ideal conditions for the formation of ice at the eaves on virtually any type of building.
Builders, and the Building Code, have tried to address this issue in part by installing “eaves flashing” at the lower edge of the roof. This is a layer of heavy material under the shingles that serves as a secondary water shed. The material typically extends up the roof two feet vertically beyond the sidewall. If water gets under the shingles, it will be on top of the eaves flashing and the house will be protected.
Quality heat tapes installed at the eave and controlled by a switch at ground level can help to minimize ice formation in specific areas of the roof and help get that melt water off the roof.
In northern Michigan, where the snow loads can be thick and heavy, builders may install an exposed metal flashing in place of shingles along the entire eave. This metal creates an impermeable and slippery surface where ice would normally form. Copper and aluminum are popular choices for this.
Mike Kearns, from Kearns Brothers Roofing, explains: “Good roofing companies today will generally provide a secondary water shed below the shingles at all points on the roof to provide added protection from the conditions we describe. Those companies should also provide additional protection in the valleys where ice may also form.”
Remember, the roof is a system where good insulation, proper ventilation, good materials and great practice must work together to keep you dry. You may get ice dams, but they don’t have to cause leaks.
For more home improvement advice, listen to the Inside Outside Guys every Saturday and Sunday on News/Talk 760 WJR-AM from 10 a.m. to noon or contact us at Insideoutsideguys.com with your questions.