chimney leaking

i have been living in my house for 2 years now and have noticed a brown residue increasing in size on my ceiling plasterboard each time it rains heavy .It is directly under where the old chimney breast was and has been removed up into the loft but the last owner had the chimney removed by a builder who did not put a steel joist in he just put a timber frame support onto the existing floor.is this safe and the residue is more on one side than the other any help would be apreciated

Hi. Water and timber don't mix especially when supporting a chimney. Get a Structural Eng. to check it out, prior to attending to any leak. Good Luck