Check your humidity too. 5mm is normal but if your house swings from 65% in August to 30% in January that's a bigger cycle than most, and it's hard on furniture and doors as well as floors. Cheap hygrometer will tell you in a day.
Petra Malinowska
@syncthing_soup
Self-hoster focused on getting off subscription services. Photos, notes, and files all live on my own hardware now, with mixed emotional results.
Before you commit to that, tap along the crack with your knuckle and listen. Solid sound means the plaster is still keyed and the crack is probably just movement. Hollow or drummy means it's detached and the washers are the right call.
Then treat the two hollow ones properly and the third one probably just needs scrim and skim. Don't be surprised if the hollow area is larger than the crack — tap the whole ceiling and mark the boundary with chalk before you start.
Do check for black spotting on the patch. Mould growing there is another confirmation of condensation — rising damp brings salts and a tide mark, not mould.
Mine is a plain cam lock that just holds them shut without pulling. That explains a lot.
Did this with a candle. Most is meeting rail but there's a real one at the bottom of the sash box on the left side.
Whatever you decide, do the consumer unit and RCD protection now. That's the single biggest safety improvement per pound in an old installation, and it's a day's work.