Our top floor is finished/heated. How to prevent ice dams?
The only way to prevent ice dams is to prevent the heat from transferring from the inside space through the ceiling and heating the underside of the roof.  If The heat in the indoor Space  transfers to the underside of the roof it melts the snow  which interns runs down to the eaves and then re-freezes causing ice dams. The only way to prevent heat transference to the roof is proper installation.