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

Sparkie K, March 2010

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!!!

Reubs, March 2010