Why ice dams form
The U.S. Department of Energy's Energy Renovations: Insulation guide explains that ice dams happen when the outside air is below freezing but the roof surface is above freezing. The roof can be warmed by the sun, by leaky heating ducts in the attic, or by warm air leaking into the attic from the rooms below. Snow melts, refreezes along the roof's edge, and the water that collects behind that ridge of ice can back up under the shingles and into the attic. Icicles are a telltale sign that this is happening.
The Building America Solution Center's guide to attic air sealing, insulating and ventilating for ice dam prevention adds that ice dams can form with as little as 2 inches of snow on a roof, and that they need snow, freezing temperatures and an attic that is poorly air sealed or poorly insulated. Its companion guide on constructing roofs and attics for ice dam prevention notes that ice dams can also form in valleys, around chimneys, or anywhere else snow sits on a warmer patch of roof.
Stop warm-air leaks first
The Building America guide calls sealing air leaks from the living space into the attic the most important step, because it keeps warm air out of the attic in the first place. Common leak points it lists include gaps around wiring, light fixtures (especially recessed can lights), plumbing stacks, furnace flues and chimneys, attic hatches and stairs, dropped ceilings, exhaust fan housings, the tops of walls and ceiling drywall seams.
Insulation alone doesn't solve this. DOE's Consumer Guide to Air Sealing the Home points out that insulation slows the flow of heat but doesn't necessarily block air moving through cracks and gaps. It also says a do-it-yourself approach won't find the small, hard-to-detect leaks in an attic that may be buried under insulation, and that a professional energy assessor can use a blower door test and infrared camera imaging to locate leaks. Ask anyone quoting the job how they'll find and seal those leaks, not just how much insulation they'll add.
Make insulation continuous, especially at the edges
Once leaks are sealed, coverage matters more than depth in the middle of the attic. The construction guide calls adequate insulation critical in the perimeter area where the roof extends over the exterior walls, and the companion ice dam guide notes that full-depth insulation over the tops of the walls helps keep the roof deck above the eaves cooler. DOE's ice dam performance brief likewise says to insulate well, especially over the tops of the walls. In a vented attic, baffles should keep insulation out of the soffit vents so outdoor air can still flow up under the roof deck. Unvented attics and finished roof slopes are built differently, so ask for an approach designed for your roof rather than an attic-floor fix copied onto it.
Treat ventilation as a partner, not a substitute
In a vented attic, outdoor air moving from soffit vents at the eaves to vents at the ridge helps keep the roof deck cold. The construction guide warns against venting the exhaust side more than the intake side: if the upper vents move more air than the lower ones can supply, more heated air can be pulled up through leaks from the house. So adding roof vents without sealing the ceiling below isn't a fix by itself. The same guide notes that roofs with many intersecting levels, valleys and dormers are harder to air seal, insulate and ventilate, and DOE's insulation guide says complex roofs can make an unvented design worth considering.
Separate leak protection from fixing the cause
The construction guide says self-adhering roof protection membranes have become standard practice in ice-dam-prone regions and provide long-term protection against water getting in. That protects the house when water backs up, but it doesn't keep indoor heat from melting snow. When you compare proposals, ask which parts protect against backed-up water, which parts reduce melting, and how the finished air sealing and insulation coverage will be checked. A quote that only addresses the roof edge leaves the heat-loss question unanswered.
Stay safe while you deal with it
DOE's insulation guide warns that icicles from ice dams can fall on people, and that the water and ice that build up can leak through the roof, rot attic framing and even collapse a roof. Keep people away from hanging icicles, don't climb onto a snowy or icy roof, and don't chip at ice on the roof edge. If water is coming inside, have a qualified roofer assess it.
Air sealing has its own safety side. DOE's air sealing guide recommends going to a professional contractor for sealing around flues, vents and pipes associated with chimneys, furnaces and water heaters, and says the first step is working out how much fresh air the home needs and making sure combustion appliances such as furnaces, stoves and fireplaces are properly vented. Without that, harmful gases such as carbon monoxide can build up, which is one reason the guide suggests hiring a professional home energy auditor or certified HVAC contractor.
Philadelphia and Bucks County
If fixing the cause turns into reroofing or roof-deck work, ask the contractor whether it needs a permit and which office issues it in your city, borough or township. Our guide to when a Philadelphia rowhome roof replacement needs a permit covers the city side, and if your home may be historically designated, check Philadelphia's historic designation rules before exterior work. Dated photos of icicles, stains and leaks help any contractor understand the problem; our photo checklist for repair estimates covers what to capture, and our contractor hiring checklist for Philadelphia and Bucks County lists questions to ask before you sign.