Unroll waterproof membrane over the ceiling joists in the attic. Press the membrane over the joists and against the subfloor. Overlap membrane sections to prevent moisture from penetrating the seams.
Staple the membrane to the sides of the joists and the subfloor with an electric stapler. Apply waterproof tape to the seams.Trim the membrane as needed with a utility knife.
Apply polyurethane sealant to the interior surface of the ceiling with a 3-inch paintbrush. Allow the polyurethane to dry for at least eight hours. Apply a second coat to fully seal the ceiling drywall.