Internal wall cracks

Joined
26 Jun 2026
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HI, this maybe a really daft question but when do cracks in walls become structural? ive got what’s looks like a 1mm slightly diagonal 1.5 meter long crack in my internal plaster. it’s covered by wallpaper at the moment. It easily shows during the day in the warmer weather by creasing the wallpaper then mostly goes in the evening when it’s cooler outside. Its an upstairs, gable end cavity wall south facing so gets the sun all day. It starts roughly at the bottom of a windowsill up towards a chimney breast and looks like it follows up the side of the chimney a little.

The bottom of the crack steps out a tiny bit but I think the plaster has blown. it’s original 1950s, very thick and sounds hollow in places.

we had a much shorter similar crack looked at by a structural engineer a few months ago and he said that was due to old plaster and the heat. I feel like a really wasted his time and he was lovely so feel reluctant to call him again so soon

Theres also a 1mm wide horizontal crack at the top of the window on the same wall that’s been there a couple of years, though when looking at it yesterday it is longer than it was. window opens fine, there’s no cracks showing outside
 

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