An ice dam is a ridge of ice along the lower edge of a roof. Meltwater backs up behind it, works under the shingles and shows up as stains on ceilings and walls, often weeks after the snow fell. Mercer County gets enough snow and enough freeze-thaw swings each winter for ice dams to be a familiar problem on older houses.
How an ice dam forms
- Heat escapes from the living space into the attic.
- The warm roof deck melts the snow above it, even on a freezing day.
- Meltwater runs down to the eaves, which sit over the cold overhang.
- The water refreezes at the eaves and the ridge of ice grows.
- New meltwater pools behind the ice and is pushed under the shingles.
The roof is where the damage shows, but the cause is in the attic.
Which houses see them
| House | Why ice dams happen |
|---|---|
| Capes with finished upper floors | Knee walls and sloped ceilings leave little room for insulation or airflow |
| Postwar ranches with small attics | Low pitch, thin insulation and blocked soffits |
| Older houses with board decking | Gaps in the decking and older roof layers let meltwater travel |
| Houses with recessed lights or attic ducts | Warm air leaks straight into the attic |
Ewing and Hamilton have many Capes and ranches from the postwar decades; see the Hamilton and Ewing pages for the housing in each.
What to plan into a replacement
- Ice-barrier membrane at the eaves. A self-adhering membrane bonded to the deck, running from the edge up past the line of the warm wall, and in valleys.
- Drip edge on every edge, with the membrane lapped correctly over or under it as the manufacturer specifies.
- Ventilation that works. Clear soffit intake, baffles over the insulation, and a continuous ridge vent, so the roof deck stays close to outdoor temperature.
- Air sealing and insulation. Seal the attic floor around light fixtures, pipes, chimneys and the attic hatch, then add insulation. This part may be done by an insulation contractor rather than the roofer.
- Bath fans ducted outdoors, never into the attic.
The asphalt shingle page covers the full layer-by-layer order of a shingle replacement.
Timing a winter replacement. Roofs can be replaced in cold weather, but shingle sealant strips need warmth to bond, and crews lose days to snow and ice. Ask how the contractor handles a cold-weather installation, and see getting ready for replacement day for the preparation.
Low-slope and rowhouse roofs
Trenton's rowhouses mostly have low-slope membrane roofs. They do not form a classic ice dam, but frozen drains, scuppers and gutters can trap meltwater on the roof. A replacement should include clear drainage paths and well-sealed seams at the party walls. See rowhouse and flat roofs.
Before you sign
Ask the roofer to list the eave membrane, ventilation and any attic work in the written contract, and check the contractor's 13VH registration number. The registration guide walks through it.
Ready to talk to a Mercer County roofer?
Mon–Fri 8:30 am – 7 pm, Sat 9 am – 4 pm ET
Questions & answers
Is an ice barrier required on a new roof in New Jersey?
New Jersey's residential code is based on the International Residential Code, which calls for an ice barrier at the eaves in areas with a history of ice forming there. Ask your roofer and your municipal construction office how that applies to your house, and get the membrane written into the contract either way.
Do flat rowhouse roofs get ice dams?
Not in the classic form, but snow and ice can block drains, scuppers and gutters on low-slope roofs, and standing meltwater then finds weak seams. On a rowhouse, plan drainage and membrane seams carefully; see rowhouse and flat roofs.
Will a new roof alone stop ice dams?
Not by itself. Ice dams are driven by heat escaping into the attic. A new roof with an eave membrane limits the leaks, but air sealing, insulation and ventilation are what stop the ice from building up in the first place.