roofstone

noun2

1. •
   The stone or rock layer immediately above a coal seam.

   • The intention of the props is for the security of the workmen to keep up the roofstone; they have no effect in resisting the crush of the super-incumbent strata. — 1830
2. •
   A large, usually flat stone slab placed atop the side-stones (orthostats) of a megalithic burial chamber—such as in portal tombs, wedge tombs, or gallery graves—to form a roof covering the interior space.