means the work, materials, and craft of covering and waterproofing the top of a building so the weather stays outside.

from From "roof," which traces back to Old English "hrof," meaning the top covering of a building — a word that has kept its shape remarkably well across more than a thousand years, with cousins in Old Norse "hrof" (a shed or boat-shelter). Add the "-ing" suffix, the old English machine for turning a thing into the activity of making or tending that thing, and "roof" becomes "roofing": the doing of the roof.