We've just moved into a detached house built in the early 70s. The previous owners were Spanish ans heated the place to 24oC constantly, even in Summer, and rarely opened the windows (which are single glazed). Since we moved in 6 weeks ago a number of cracks have started appearing in all of the upstairs ceilings. They seem to be pretty regular, as in around 60/70cm apart parallell to each other, with the occassional crossing crack. They look like they might be following the edges of the plasterboard... There are also cracks forming where the walls join the ceilings. Most of the cracks weren't there when we moved in, so I was wondering what could have caused them. We don't have anything stored in the partially boarded loft, so it can't be due to weight. There are no cracks in the exterior brickwork and as far as I can see there are no problems with the roof. Couldit be the condensation (currently a bit of a problem as windows can't really be left open easily and tehy're single glazed) and the reeceng fluctuations in temperature? Or should I start worrying and get a structural engineer or a builder to take a look?