I have a long horizontal crack about 30cm (1'0") from ceiling on a south facing upstairs bedroom wall. I can't see why it cracks, nothing apparent on down stairs walls. How can I make a good repair on it. The wall is breeze block, sand & cement rendered with finish plaster. The house was built in 1959
As a builder I would remove the plaster 1" either side of crack. You will probably find the crack is a gap in the mortar... Remove some of the joint about 5" long break up some slate (either buy one from a builders merchents or jump in a skip) into sizes approx 1" x 4" and with a big hammer, taking care not to cut yourself on the slate, hammer the slate into the newly formed hole... Slate is incompressable and will nip up the joint... replaster!!!