Hello Aaron,
As Organon hinted above, you’ll need to extract the actual revit face to get the material id property. You can do that with Clockwork’s “RevitFace.FromDynamoSurface” node.
To get the correct faces, consider shooting rays from the centroid of the room to the exterior boundaries. You can do that with the “RayBounce.ByOriginDirection”.
If you use Clockwork’s “Room.Boundaries” node, you can get all the vertical elements(walls, partitions, facades) directly and limit the ray tracing to just horizontal elements(ceilings roofs).