Hi Tom, One of the things I did to facilitate this layout, which I did for FAO Schwarz, in NYC, being changed-over from a wintery setting to a Spring/Summer seasonal appearance, which the store asked of me since they decided to go, for the first time, with a year-round layout, not just a Holiday layout, was to make these snowy roofs. They are plates of 1/4" or 3/8" material cut to be precise inserts onto the roofs, with the snow having been glued down permanently onto the roof inserts.The snow may look fluffy, but it is quite permanent. When I changed the panorama over to fair-weather, I removed the snowy inserts and had the original non-snowy black roofs which are part of the buildings.
When it was time to change the layout over to winter again...…. on went the snow-encrusted roofs.
FrankM, Layout Refinements