PATIO DOORS AND ROOM ADDITIONS

Maintenance reports verify that repairs to sliding patio doors within some room additions can be the most expensive.
The main reason is that the typical room addition is too often not built to the same standards as the original home and will eventually sag, shift and sink enough to make any sliding door repairs difficult at best.
When bare minimum foundation, footings, framing and support beams are used, (to underbid the competition) the stability of a room addition over the years may not meet the owner's expectations.
http://www.bhg.com/home-improvement/remodeling/additions/common-addition-problems/