The slate has been removed from the mansard roof in preparation for something. The problem is that whatever is about to happen--indeed what has already happened--doesn't have a certificate of appropriateness from the Historic Preservation Commission.
In July 2012, the plans for this building came before the HPC, presented by Ward Hamilton of Olde Mohawk, who was doing lots of work for Eric Galloway at the time. Hamilton told the HPC that the slate on the roof would have to be removed in order to repair the flashing. That presumably is what's happening now. He predicted that much of the original slate would be lost in the process of removal, because the type of slate it was--Chapman slate from Pennsylvania--breaks down after about 80 years.
The HPC asked about re-creating the rosette detail of the original roof, but it was explained that the mansard roof would be all charcoal gray slate, just like 416 Warren Street and 501 Union Street, because it was the personal preference of Galloway.
Six years ago, the HPC voted to grant a certificate of appropriateness for 356 Union Street "conditional on the strong preference that the decorative elements [of the slate pattern] be retained." A certificate of appropriateness is only good for a year, so a new certificate of appropriateness should have been in place before the work on the roof of 356 Union Street commenced. There should now be a stop-work order, and a new certificate of appropriateness should be secured before things progress any further.
CORRECTION: Gossips has been advised that the slate has not been removed from the mansard roof at 356 Union Street. Rather it has been covered with plastic tarp to protect it while work is done around it.