What Ice Dams Actually Do to a Pennsylvania Roof

Southeastern Pennsylvania doesn't get the kind of winter that stays cold in one direction for months. It gets a hard freeze, then a couple of warm afternoons, then a refreeze overnight. That swing, more than the raw temperature, is what builds an ice dam.

How an ice dam actually forms

Snow on a roof melts from the bottom up, not the top down, because heat escaping from the attic warms the roof deck under the insulation. That meltwater runs down the slope until it reaches the overhang past the exterior wall, where there's no warm attic underneath anymore. The roof deck there is cold, the water refreezes, and a ridge of ice starts to build right at the edge of the roof. Each cycle adds to it. Eventually you've got a dam of ice sitting over the gutter line, and water backing up behind it has nowhere to go but sideways, under the shingles.

Once water gets under a shingle it doesn't care which way is down anymore. It follows the underlayment, finds a seam, a nail hole, a place where two courses overlap, and works its way in. That's usually when a homeowner notices a stain on a bedroom ceiling that has nothing obviously wrong above it.

Why the eaves take the damage first

The eave and the first few feet of roof past the exterior wall line are the coldest part of the roof deck in winter, because they sit past the heated part of the house. It's also usually where the roof pitch is shallowest and where valleys and gutters concentrate water. Put those together and the eave is where ice dams form first and where leaks show up first, even on a roof that's otherwise in decent shape.

Valleys, where two roof slopes meet, and areas around dormers and skylights make the problem worse, because they funnel water from a larger area into a smaller channel. An ice dam that forms in a valley can back water up much further than one along a straight run of eave, simply because there's more roof feeding into that one spot.

Many roofs in this area also have a water-resistant membrane installed under the shingles at the eave and in the valleys, specifically to give a roof some protection if a dam does form and water backs up. Whether a given roof has one, and how much of the eave it covers, is worth asking about directly if you're getting a repair or a new roof quoted, rather than assuming.

What to check after a hard freeze and thaw stretch

• Icicles hanging in a straight line off the gutter, rather than scattered, usually marks where a dam has formed
• A ridge of ice sitting on top of the gutter or right at the roof edge, thicker than what's on the rest of the roof
• A new stain, bubble, or soft spot on an interior ceiling near an exterior wall, especially under a valley or a dormer
• Shingles near the eave that look lifted, curled, or out of line with the rest of the course
• Water marks or dripping inside the attic near the eave, even if the ceiling below looks fine so far

What actually helps

The dam itself is a symptom. The cause is heat escaping into the attic space and warming the roof deck unevenly. Good attic insulation and ventilation, particularly at the soffit and ridge, keeps the underside of the roof closer to outdoor temperature, so there's less melt to refreeze at the eave in the first place. Clearing gutters before winter so meltwater has somewhere to go also matters more than people expect. None of that is a same day fix once a dam has already formed and ice is sitting on the roof. Chipping at it yourself with a hammer or an ice pick is a good way to crack a shingle that was otherwise fine.

A roof rake used from the ground to pull snow off the first few feet of roof after a storm can reduce how much melts and refreezes at the eave, and it's a safer way to cut the load than climbing up with a ladder in icy conditions. It only addresses the snow though, not ice that's already formed, and it won't fix an attic that's venting heat unevenly.

When it's worth a look

If you're seeing any of the signs above, or you just want eyes on the eaves before the next freeze and thaw stretch, it's worth having someone look at the roof rather than the ceiling. We do free roof inspections and we're set up to answer emergency storm damage calls around the clock. We also offer a $100 roofing and gutter tune up, which covers a look at the eaves, the gutters, and the areas where ice dams tend to start, ahead of the next hard freeze. Aronson Roofing and Siding has been doing this out of Willow Grove for more than thirty years, across Montgomery and Bucks County. Call (267) 688-3163 to have someone take a look.