Not only can high humidity and temperatures lead to sagging ceilings, if present for an extended period of time, these conditions can also lead to the growth of mold and mildew. To avoid these problems, select ceiling panels that contain a broad spectrum antimicrobial treatment on the front and back, which provide resistance against the growth of mold and mildew. If your ceiling drywall is sagging between joists, sometimes called pillowing, it’s probably on the top floor and attached to the roof trusses. If so, the installers probably used the incorrect thickness plasterboard. Thinner plasterboard can sag if it’s hung under roof trusses that are spaced more than 600 mm apart. It isn’t strong enough to handle the span, and the weight of the attic insulation just makes the sagging worse.

You only have two choices. Rip it out and replace it with thicker sheeting. Ripping out the old stuff is the most professional approach, but it’s a nightmare job. You have to pry out the sheet along the edges and pluck out all the old sheet screws. As if that weren’t enough fun, the attic insulation may collapse into the room.