I did a quick test and made a simple file. I don't model that much in Sketchup so perhaps others have different techniques. I just wanted to point out how easy it actually is.
If you want certain tiles to appear more than others, select it in the list and edit the probability, make sure together they make 100%, otherwise, check the manual.
I didn't use patches like he explained, I wanted more randomness. Click through the scenes and read the hints I added, hope you understand it. I put it together rather quickly.
Rooftile_test_Skatter.skp (1005.5 KB)