Original by Bruce Hirst
To add sand texture to a slab roof, we're going to use the plaster itself as the sand. Real sand (to scale) would be very fine and powdery, just like plaster is.
To sprinkle the plaster evenly, I'm going to poke holes in a plastic cup using a nail. This will make sort of a "salt shaker" that we can shake the plaster through.
Spray the surface of the roof with water. If your cardboard curls, then you may want to attach it to something stiffer like a piece of wood.
Liberally shake on the plaster and cover the whole surface with it evenly.
Please note that I am using tan dental plaster to sprinkle on the roof. If you use white plaster, then you'll have to paint it before using a staining method.
Wait 5 minutes before you scrape the roof. Using a wide putty knife, gently scrape off the dry plaster on top. Stop when you get down to damp plaster, the scraping will sort of compact it down onto the roof.
Let it dry completely. Once dry, use a stiff brush to gently remove the remaining dry plaster on the surface. It will leave the texture you see here.