When someone else heats the top floor
If you own a two- or three-family here and live in it, or nearby, you already know the roof is your problem. What is less obvious is that the thing damaging it every winter is a thermostat you cannot reach.
The mechanism, in one paragraph
Heat rises out of the top flat into the space under the roof deck. Snow above that warm area melts; the water runs down the slope and reaches the overhang, which is over outside air and cold. It refreezes there, builds a ridge of ice, and the next melt backs up behind it and goes under the covering.
That is why one house on a street ices up and the one beside it does not, and why the difference is almost never the shingle.
What you can actually do about it
Most of it is not roofing, and most of it is cheaper than roofing. Sealing the gaps between the top ceiling and the roof space — the hatch, the light fixtures, the chases, the tops of partition walls — stops the heat arriving in the first place.
Then intake at the eave that the insulation has not buried, and exhaust sized to match it. Those two together are the system; one without the other makes things worse rather than better.
And when the roof is replaced, membrane at the eave running well past the wall line — past where the heated space actually ends on your building, not the minimum.
What is reasonable to raise with a tenant
Not the thermostat. Setting a tenant’s heat is neither realistic nor, in most cases, yours to do — and a cold flat produces its own problems.
What is reasonable is the rest: that the attic hatch stays closed and sealed, that bath fans get used and actually vent outside rather than into the roof space, and that anything blocking a heating vent gets moved. Those are small, and together they change how much moisture ends up under the deck.
Before you replace the roof
Have someone look in the attic first. On a building like this the finding that matters is often up there rather than on the roof, and it can change what the job needs to include — sometimes in your favour.
Next step: Why the eave is where it shows
Anything involving a tenancy has rules of its own, and this page is about the physics rather than the lease. Agreements with tenants are worth putting in writing and worth checking against the law that applies to you.