The roof moss took forever! Well, it seemed like it, anyway. But to get believable looking moss, you have to get in between the shingles, and that takes a lot of patience.
The trick I'm using here is one I use a lot. I start with a bright color, in this case pink. I create a new layer, and paint (in pink) all of the areas where I want the moss to go.
Then I get a full page of a prepared texture, which happens to be a layer of grass that I've altered a bit. I lay the prepared texture field over my layer of pink markings. I use the Magic Wand to select "around" the pink markings, then hide that layer (or put it in my "NOT USED" folder of layers). Then I select the layer of grass, and simply Delete the unwanted areas. What I'm left with is "moss" only in the areas where I want it. To give the moss some depth and life, I use Layer Styles: Drop Shadow (or Outer Glow, making sure to set the layer type to Normal instead of Screen), and also some Bevel/Emboss.
I'm pretty sparing when it comes to Layer Styles, since any Photoshop user can detect them when they're used -- or abused.
Looking at the roof of the cottage next door, I can see that I could actually put more moss on the roof of the barn, so I'm going to go back and be more generous with it.
Then, ground textures. I've photographed a bunch of new grounds here in my new place (I rent a cabin on a farm in Oregon) and I'm eager to try them out.