Houston, we have a problem! How much of a problem depends largely on how large is the roof sag? And we are going to differentiate between a sag and a hole, as may have been caused by a tree or branch falling on the roof.
Generally, any sag is a strong indication of failure of the roof decking or supporting framing. If the sag is small – 2 feet in diameter or less – it could very well be the result of a localized failure of the roof decking from a small leak that has rotted the plywood decking. The sag would occur between the framing members (rafters). A larger sag could be the result of failure of the supporting framing, being the rafters and/or purlins and braces. Go to our article on roof framing for a quick explanation on the various framing members.
Diagnosis of the problem requires investigation from the inside of the attic in the area of the failure. As with any such venture into the attic, plan it for the early morning and if possible, bring along a flashlight and some sort of board to stand on. Roof leaks are covered in another article, and a quick review of that article will lead you in the right direction for finding the leak. Repair can be tricky and best left to a professional, as it will require work from the roof side. Larger sags resulting from framing failures should be fairly evident. You will be looking for cracked or split rafters, purlins or braces, or sagging framing members. In some instances, necessary braces may have been omitted and over a period of years the roof has “relaxed” and settled, resulting in a sag in the roof. Again, repairs can get complex and difficult and should be referred to a professional.