Can I suggest a method for slate?
If you cut strips of plasticard the depth of the slate, then cut ¾ across the strip to make the individual slate shingles. Glue to the roof from the bottom up (sounds obvious), do the same with the next strip and so on.
When you place the next strip above and find that you may have two slates that line up and are not staggered, move your strip along effectively missing one slate out to continue the staggered look. Then when the roof is complete cut an individual slate and glue it below the missing space. It gives a very realistic rustic look and I think it will compliment your layout.
Just an idea (if you get what I mean).
Layout looks great!
Best regards,
Mark