Long crack across whole of bedroom ceiling.


About a year ago a long crack appeared across the whole of the bedroom ceiling on the top floor of the house. Closer to the middle of the ceiling the crack seems wider approx .5mm-1mm. I haven't looking in the loft yet as it is murder to get to. Any ideas what might have caused this and whether it's a serious problem or just decorative.
When we bought the house the survey said that we needed wall ties to the back of the house (room is at the back) could this cause it?
Thanks CB
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Is this crack across the ceiling joists or along them? I assume it's along them.
Rob Graham
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Yes they are along the ceiling joists.
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Are there cracks in the outer walls on which the ceiling joists sit?
Are the walls that are parallel to the joists straight or is there a slight bow in them (horizontally)?
What I'm getting at is, are the walls spreading, as indicated in your original post? It sounds as though they are and therefore a tie front to back, across the joists, may be necessary. They may be spreading and pulling away from the other walls, producing cracks in those walls, or they may be bowing outwards in the middle while remaining properly attached to the other walls.
Rob
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The Survey did say that the back wall needed wall ties, the front has been done (it's a mid terrace). I think my best option is to get some quotes for wall ties and get this sorted. Thanks for your help
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"CB" wrote:

May be not if the crack and the joists are at right angles to the wall(s) that need ties. But if the crack and the joists are parallel to the wall(s) that need ties then there could be a link. If there is movement of the internal walls you can get cracks in the wall plaster, gaps between the wall plaster and ceiling, as well as cracks in the ceiling plaster. If there is movement of the external walls you can get cracks in the bricks or mortar joints on external walls.
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