The fireplace backs on to a stairwell. When the fire is lit each autumn, a horizontal 2mm crack opens up for about 5 feet either side of where the fire is located on the stairwell, enough to tear the wallpaper. We don't see it as a major problem just an annoying one.
The chimney is on brick construction and rendered. Could the bricks be expanding and lifting the wall somehow.
About 12 years ago we had alterations to the house and a self supporting roof was installed.
I think the answer is to fill in the crack whilst it is open with some hard filler so that when the wall cools and the crack starts to close and sit on the filler.
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