A Sagging Ridgeline

I visited a client last week who was asking about a portion of his home that had a sagging ridge line.    The roof is also the style that has two different roof slopes along the rafters.  It is an older home so my first thought was that it was not built using trusses and the rafters and or ridge beam was undersized.  I asked to go inside to see what internal effect this was having and was somewhat surprised to find a room with a vaulted ceiling which did not follow the roof line at all!   While this is not unusual and is often accomplished in current designs using a scissor truss, the age of the structure and the lack of a transfer of the exterior failure to the interior ceiling indicated this was not the case.

One of the advantages of a truss roof is that the roof load is transferred directly down on the perimeter walls.  When properly designed, the loads in a rafter roof design transfer the loads down as well, but when the ridge beam is undersized, it will sag causing the rafters to exert lateral pressure on the perimeter walls.  There weren’t any particular indications of this occurring.  And then there is the puzzle of the vaulted ceiling.  According the the owner, it was a later addition, so was a new ridge beam and rafter system put in below the roof to support the ceiling?  Is the ceiling hanging from the roof beam and rafters?  (Which would add to the stress on the roof.)  Unfortunately I had to admit that my X-ray vision was not up to the task of determining the internal workings of the roof and ceiling situation.

In recent years, particularly with the installation of some of the new residential metal roofs, I’ve seen sagging roofs being hidden under new roofs.  Sometimes this is accomplished using “sleeper” purlins that are shimmed to level the roof.  In some extreme cases new rafters are cut to lay on the roof at the proper slopes to correct the problem.  All too often I see this entire new system going on over the existing roofing.  Some of the concerns here are:

1. The failing roof is hidden and not corrected.  Out of sight, out of mind is not a solution.
2. The underlying failing roof is now under additional loading, potentially increasing the problem.  While the metal roof can at times reduce the potential snow load since snow won’t stick to the metal like it does to shingles, it is still an increase of the day to day material load on the underlying structure.
3. Often there is rot and deterioration associated with the sagging roof.  Under the additional roof, it may continue to deteriorate unobserved potentially leading to failure.

But back to my client’s problem.  I had to tell him that there wasn’t a quick fix.  Due to the hidden problems, some selective demolition would be needed to determine a solution.  That’s not generally something a client likes to hear, but it is the right thing to do.

Source:  Sagging Ridge image borrowed from Prugar Consulting,Inc.

Source:  Truss Load image borrowed from Next.cc