Roof sheathing sags for the same reason any component of a home that spans between two bearing points sags.
Roof dip between rafters.
Structurally inadequate or deteriorated over time.
If the framing is sound and problems are not found elsewhere but the roof still sags the problem might have originated because of poor installation of the plywood sheathing or shingles.
And of course the amount of deflection.
From the out side you can see a dip or maybe it can be described as a valley in the roof.
If the roof sags between the rafters or trusses the most likely cause is sheathing that s too thin for the area it covers.
Rafter boards are fastened to the ridge board at the peak and the top plate where they join the wall.
My rafters are 16 oc and the roof is 4in12 plywood deck about 1 2.
If you notice that there is a visible dip in your roof chances are you have a problem with one or several rafters and you may soon be facing more issues like water leaks bowed out walls or your roof caving in on you.
Its about 2 3inches deep in the center.
An inspection in the attic and i do not see any cracked or sagging rafters.
The boards creating the slope of the roof are wider than lumber used to build trusses 2x8s 2x10s and 2x12s are most common in rafters vs.
It depends if the dip is between two rafters trusses or between eave and ridge.
The fix here is to replace the thin sheathing with thicker sturdier sheathing.
Remedies for a sagging roof.
If a roof dips in between rafters or trusses it s probably because the sheathing is too thin for the span.
Someone said to me that the rafter might just have been installed.
They are typically barely noticeable and not a real problem.
Because it was structurally inadequate when installed or has deteriorated over time.
Minor dips or lumps can also occur at hip ridges or valleys due to poor roof truss alignment or other framing problems.
You can simply have the old sheathing replaced with better material when it s time for new roofing.
Repairing a roof rafter in this condition is not a hard project but it can be difficult if there is not easy access.
In finished space insulation is placed between the rafter boards and drywall or other material is fastened to the face.
Removal of the lateral bracing necessary for a roof rafters that are framed without a ridge beam will make the ridge dip noticeably as in the photo below.
