To the best of your knowledge was the timber framed house designed with adequately sized roof trusses to allow for easy conversion of the attic or did the builder have to remove the inner cords of the roof trusses to make space for the attic conversion. I am assuming the roof is constructed with roof trusses and not tradition cut roof timbers.
Under normal circumstances in a masonry constructed house the builder can remove the inner cords of the roof trusses and then fit steel universal beams to support the roof and the attic floor. The steel UB's would normally be spanning across the width of the house and resting on support pads built into the masonry wall. This option would not be suitable in almost all cases for a timber framed house and in effect the attic space in timber framed houses cannot be converted unless the roof trusses were first designed with that in mind or other major works are carried out to support the roof structure.
Good & correct point by Peter,
Yes it may be best to get a structural engineer to have a look at it.
Try & find out all of the following:
1. If its timber frame, who were the designers, sometimes you may see the company name stamped on the timbers. IJM is one such company, there are many.
2. Get hold of the timber framing companies drawings for your particular house.
3. What contractor originally built the house?
4. What contractor carried out the attic conversion
5. Did you have an engineer to sign off on the conversion
6. Are there any issues with the floor joists in the attic?
7. Was their an architect involved in the attic conversion
8. When was the conversion done etc
The more information, contact names, dates you have the better you can equip your engineer with. Your engineer will help you too with some of the above points.
If there are several houses in a housing estate, & they all look the same, it may be a good idea for your engineer to inspect, say a neighbours house too, that does not have the attic converted, if you can't get hold of the drawings straight away. This way he will see the type of timber frame construction
I know from experience that it is sometimes difficult to get hold of these drawings, again it would be down to your engineer.
The removal of the chord members (diagonals) which form part of the truss may have resulted in increasing the span of the truss rafters, thus causing the roof to sag.
Now, if you notice the line of the ridge tiles sagging, then this could also be a different situation too, ie foundation issue etc.