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Do you really want to take that all up (just the carpet) to start the investigation? Potentially just splits , wedges moved impacted, or (and I'll use the wrong word) the stringer has dropped or the wall 'moved'.
[Some twat had stuffed a load of gunk (plaster or caulk) on the top of the stringer , it's very easy to cut off (make good) and use appropriate product in a better quantity]
So do I not understand how staircases work? Are winders not fixed to the wall? Is the skirting board on the winders just floating and not like a stringer? I guess I don't understand how it can flex so much