Why is my roof sagging?

This photo shows a transition point where two rooflines come together. The resulting valleys that are created here have to be constructed to withstand the extra snow load in these areas. In this case the valleys have sagged considerably and an abnormally heavy snowfall may actually cause the timbers to crack and fail. This type of failure may not be observable right away until the snow melts and rains into the attic and through the ceilings. Many older homes were constructed this way and I have seen other similar issues such as sway backs in the ridge line (similar to the saddledome). Often times this movement happened soon after the home was built and likely hasn’t gotten any worse. Having said that I always recommend having a qualified contractor or engineer access the situation to be sure it is still structurally sound.

P.S. Any inspector worth their salt should try their best to gain access to the attic space as it is so important to the whole picture. I don’t often walk in attics for many reasons but there are times when I do in order to get a closer look at a potential problem.