How an ice dam forms
Warm air from the house leaks into the attic and warms the roof deck. Snow on the upper roof melts, and the water runs down until it reaches the eaves, which hang out past the warm walls and stay cold. There it freezes. The ridge of ice grows, and the next meltwater backs up behind it and works under the shingles.
Central Pennsylvania winters suit this well: snow followed by days that swing above and below freezing. Older houses in Midtown, Uptown and Camp Hill with little insulation, and split-levels and capes with low attics, tend to see it most.
Signs you have them
- Thick icicles along the gutters, especially over heated rooms.
- Ice built up on the lower few feet of the roof while the upper roof is bare.
- Water stains or peeling paint on ceilings and walls near the outside of the house.
- Damp insulation or frost on the underside of the roof deck in the attic.
What a new roof has to include
Pennsylvania's Uniform Construction Code is based on the International Residential Code, which calls for an ice barrier membrane at the eaves in places with a history of ice forming there. The barrier is a self-adhering underlayment that runs from the edge of the roof to at least 24 inches inside the exterior wall line. It keeps backed-up water out of the house even when a dam forms.
On low-slope sections, valleys and around chimneys, many roofers run the membrane further. Ask how far up the roof it will go on your house and put it in the written contract. See Pennsylvania roof permits for when the municipality gets involved.
Treating the cause, not just the edge
The membrane protects the house; it does not stop dams forming. That takes a cold roof deck, which comes from the attic, not the shingles.
- 1Seal air leaks into the attic around light fixtures, plumbing stacks, chimneys and the attic hatch.
- 2Bring attic insulation up to current levels where there is room.
- 3Make sure there is a clear air path from soffit vents to ridge or roof vents.
- 4Keep bathroom and kitchen fans ducted outside, not into the attic.
See also: Attic ventilation and condensation · Replacing a roof in winter
Questions
+Should I chip ice off the roof?
No. Hacking at ice damages shingles and is dangerous on a ladder. A roof rake used from the ground to pull snow off the lower few feet is safer.
+Do heat cables solve ice dams?
They can keep a channel open at the eaves, but they use power, wear out and treat the symptom. Air sealing and ventilation fix the cause.
+Will a metal roof stop ice dams?
Snow slides off metal more readily, which helps, but heat loss from the attic still matters. Plan snow guards over doors and walkways with a metal roof.