Hello there. Not used drywall before. Trying to plasterboard ceiling, which in corner has the underneath of the concrete front step plus a steel. The soffit of the concrete/steel is virtually level with the underneath of the joists around it. SO the last board in this corner can screw into joists on two sides, but needs to be bonded to the concrete on the other - there would be anywhere from 1-5mm space for whatever I use before the board isn't plumb. Can drywall handle going this thin, or should I use something like gripfill. Cheers for any advice