Valve used(I believe) blendgrasstosnow001 for most displacements.(the image in hammer appears like the blend file images for "sawmill")
for the roofs and random snowpiles(such as in a corner) used snow_full001.
Many of the main roofs on the map that have those beautiful snow piles on top are unfortunately models..so are difficult to use in your own custom maps. :[
they used overlays on concrete/wood and other materials that are next to snow. There's three so far: footsteps, horizontal, and patch.
The horizontal is used on the sides of buildings that are next to snow.
Footsteps in areas where players would be exiting snow and patches for spots by snow piles, etc.
There 7 or 8 models of icicles with straight and angled, along with 3 different sizes of each.
for props they made many alternative skins for rocks and trees that have snow lightly placed.
also a blend file for the cobblestones.
searching viaduct in models or searching "snow" in materials is a good way to find these. ^_^
I'm currently working on a snow themed map so I needed to find these as well
~While on the subject of the models for the snow on the roofs...would it be possible to clip each individual pile of snow out of their current "cluster of roof top piles" so that each pile of snow for each rooftop was a seperate model? It'd be so much easier to make your custom maps have nice roofs. ;o;