Structurally, sagging could indicate undersized beams, rafters or purlins, knots or weak points in timber, (sometimes caused by installers of services cutting for access), rotting or corrosion of structural members, or even wetting of timber allowing more bend, or a heavier load on the roof than it is designed for. Another possible cause is the failure of some fixings of the structure.
Check for leaks and openings. Check the structure. Check the materials. Check how long it has been sagging. Get an expert to look at it.
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