On one piece garage doors usually there is a metal rod that runs the length of the door and there should be one on top and one on the bottom of the garage door and in the middle will be a triangular piece of metal and this rod is tightened up sometimes they are adjustable and what this does is acts like a strut for a wood door since wood doors will tend to warp from moisture and the elements. Sometimes these struts can work their way loose and even sometimes the triangular metal piece in the direct middle will sometimes come away from the door and cause the rod not to put any tension on the garage door so it will tend to warp when the door is in the up position because all of the weight is on the middle of the door when the door is up other than when it is down.
adjustable tension rods
These tension rods are most of the time adjustable and you can tell because at the end of each rod is a nut that you can tighten or loosen the tension and this is something you should keep an eye on when inspecting your garage door if the rods are loose than they need to be tightened up if they have good tension on them then they should be okay but if your door opens and the top of the door is sagging because all the weight is falling in the middle of the door.
are the rods tight?
If these rods pretty tight and your door is still sagging when it is in the up position then you might have another issue going on like the deterioration of the wood framing of the garage door. Not all one piece garage doors have these tie rods on them this is usually because those kind of one piece garage doors have a wood frame that is probably a lot stronger and thicker than one that would have these tie rods that means that the wood used in the frame is going to keep the door from warping or sagging just because of the extra strength that the garage door frame was constructed with. If you have a one piece garage door that is constructed of steel or it looks exactly like a sectional roll up garage door but it is all screwed together and made to be a one piece then you would need to use the same kind of struts on that door as you would use on a sectional roll up garage door usually one on the top and one on the bottom and this is only for 2 car garages that are at least 15 feet wide a small single garage door that is 8 or 9 feet wide would not need these kinds of struts.