What is a good way to simulate a slate roof?
I would use imbossed paper or plastic sheets.
I did a search of Wathers site (HO scale) and came up with
Tichy Train Group, Slate Roofing Shingles
Kibri Roof Sections
Northeastern Scale Models, Assembled Laser-Cut Paper Shingles (3-1/2 x 8" Sheet) - Gray Slate
2 Pack of above
Maybe this can help;
http://www.2guyzandsumtrains.com/Content/pa=showpage/pid=31.html