Finding a winter roof leak due to ice dams on a slate roof.

This leak is showing up on the ceiling of the main house below the upper arrow and in between the lower arrows (pointing to the interior french doors connecting the main house to addition)

Some comments from professional roofers.

Axiom;

I am guessing that it is building up in the gutter and getting under perimeter metal.  Joe Sherwood Roofing   N. Michigan

Shangle Nailer;

I think it is a leak from an ice damn on the rear eaves of the main house. Getting in the wall up there and showing above the interior door into the addition. never shown before because there was no additon!   Richmond VA.

Rosco;

If there is a vapor barrier above deck it could be from the wall, I suppose. The skylight is not the source? Oshkosh WI.  Lakewood Roofing, Inc.

Tinner 666;

I see scads of ice around that DS and molding, and running down the wall. You haven't said it getting into the top of the window, just on the floor, I assume below and to the side of it.
Water seems to coming through the fascia from the BIG, at the DS, so I guess it's coming through the wall there.  Central Virginia  Albert's Specialty Roofing

Cerberus;

Was the weep system in the exterior brick-masonry wall covered up? Not that it would be the cause of a leak over the French doors, but certainly could show up along interior walls now.
Ice damming along gutter is a possibility, as is condensation dripping from the skylight. 

Is that built-in gutter that feeds that downspout? If so, I suspect they are frozen solid, and water has begun to back up and leak along the walls. And it also looks like there may be a water-table involved.     Houston Texas

jwoolfsroofing;

I'm guessing that the leak originates at dormer window(2nd floor)and runs between brick and framed wall and showing up on 1st floor ceiling where opening is to addition.   Barberton  Ohio   www.woolfsroofing.com

 Source of ice dam roof leak over new room addition.

As noted by some above, the source of the ice dam leak is the built in gutter and it's downspout. The gutter, eave and roof are warmed by the heat escaping from a poorly insulated house. The water from melting snow runs down the cold downspout and freezes into a solid block of ice inside the downspout. Having nowhere to go, the water in the gutter backs up and out of the seam of the gutters outlet / downspout connection. It then finds it's way behind and into the space between the exterior brick wall and interior plaster wall and comes out at the interior ceiling and new addition entry way.

This could have been prevented if the gutter outlet had been fabricated and installed to extend all the way through and below the soffit. There would still be a problem with ice in and backing out of the lowered outlet / downspout seam, but the water would be running down the outside of the house and not leaking inside.

 Built in gutter outlet / downspout connection joint inside the house soffit.