Why the snow melts on the warm part of the roof
Warm attic air heats the underside of the deck, and the snow melts from below.
A roof that is warm in the middle and cold at the edge will melt its snow on the warm part and leave the eave under white snow. The meltwater runs down the roof and freezes when it reaches the cold eave, building a ridge that grows each time the snow melts above it.
The warmth is nearly always from the house. Heat leaks up through the ceiling, around the light fittings and the attic hatch, and into the space under the roof. Insulation that is thin at the outside wall lets the heat through faster, which is why the eave on the outside wall is the usual place for a dam.
- Warm attic air heats the deck from below
- Heat leaks through ceiling penetrations
- Thin insulation at the outside wall lets heat through
Why clearing the ice is not enough
Removing the ice this week does not stop the dam forming next week.
Clearing the ice gives relief for a few days. If the warm air is still leaking into the attic, the snow will melt again, and the meltwater will refreeze at the same eave. The ice is the result of the house losing heat, and only a repair of the heat loss stops the dam.
The work on the roof edge and the work in the attic are both recorded. The owner is told which one is needed first and which one can wait for the spring, because a dam that is cleared and not prevented is a repeat job.
- Clearing ice alone gives short relief
- Heat leak is the root cause
- Both repairs recorded for the owner
The eave, the barrier and the gutter
The eave is where meltwater stops, so the barrier and the flashing there are checked closely.
The ice barrier runs from the eave up the roof and is meant to keep water that gets under the shingles from entering the house. A barrier that is short, torn or laid on the wrong side of the deck will not hold the meltwater where the ice forms.
The gutter at the eave is checked as well. A gutter full of ice or set at the wrong slope holds water at the very edge. The crew checks the slope and the brackets, and it notes any gutter that will need work in the spring.
- Barrier checked for gaps and length
- Flashing at the wall line checked
- Gutter slope and brackets checked
What the owner can do before the next winter
Several steps reduce the risk and cost little to do.
Keeping the attic hatch sealed and insulation at the right depth reduces the heat that reaches the roof. Clearing the soffit vents at the eave lets cold air move under the deck, so the snow is not melted from below. The owner can also rake the snow from the lower part of the roof after heavy snowfall, using a tool that does not touch the shingles.
Snow should not be chipped at the eave. Chipping can tear the shingles and the barrier, and the damage is then hidden until the next thaw. The crew can advise on what is safe to do on the specific roof.
- Seal the attic hatch and check the insulation depth
- Keep the soffit vents clear
- Rake only from the ground or a safe platform
Insulation at the outside wall
The insulation at the outside wall decides how much heat reaches the eave.
The attic space above the outside walls is where the heat most easily reaches the roof. Insulation that has been pushed back from the eave, or that is thin where the wall meets the roof, lets the warm air travel up to the deck. The crew checks the depth at the eave with a rule and notes any place it is below the level the owner wants.
Baffles that hold the insulation back from the eave, so air can move from the soffit vent to the ridge, are checked as well. A missing baffle lets the insulation fall onto the vent and block the airflow the eave depends on.
- Insulation depth measured at the eave
- Baffles checked at each soffit vent
- Blocked airflow reported to owner
Heated cable and the honest limits of it
A heating cable keeps a channel open at the eave, and it does not stop the heat leaking into the attic.
Heating cable laid along the eave in a zigzag can keep meltwater moving to the gutter, and in some houses it is the only short-term measure a family can take during a hard week. It runs electricity at the eave in snow and ice, so the cable and its outlet are installed to the manufacturer and electrical rules, by someone qualified to do the wiring.
The cable is a relief, not a repair. It keeps the channel open while the attic heat is still leaking, so the owner should treat it as a bridge to the attic and eave work and not as the solution. The crew can say whether a cable is useful on the specific roof, and it does not promise that a cable will prevent a dam.
- Cable is a short-term channel only
- Electrical work by a qualified person
- Attic and eave repairs still needed
