Although diagnosing the causes behind structural and cosmetic defects lies beyond the scope of a home inspector’s duties, according to the Residential Standards of Practice, understanding some of those causes can help inspectors recognize certain defects more quickly so that he can report them and make appropriate recommendations to his clients.
One of those defects related to roofs is rafter sag or a bowed interior ceiling that is mistakenly attributed to a lack of collar ties.
Collar ties are designed to resist expansive or an outward movement of force that is usually accompanied by wall spread and ridge sagging. However, they do not prevent rafters from sagging or bowing downward in the middle. That’s caused by factors such as over-spanned, under-sized or over-spaced rafters. Such conditions are also sometimes caused by excessive roof load, such as too many layers of shingles or other roofing-covering material, or a change-out from composition shingles to tile, where the weight is greater than 6 pounds per square foot. Collar ties help prevent roof settlement but not a rafter from sagging in the middle.
(It is possible that when the slope of the roof descends to a lower slope/pitch, the load on the rafters’ span shifts somewhat from the seat cut toward the middle of the rafter.)
In the case of rafter sagging, the collar ties themselves become bowed or bent inward, since they are not designed to resist a compressive load. Collar ties resist the outward or expansive motion of the rafters, not compression.
It is the framing
members that are designed to withstand both compressive and expansive loads. A
lack of collar ties (typically 1x wood) would cause the ridge — not the
rafters — to sag, as well as cause the walls to spread. They react by simply
bowing as they resist the force of expansive or outward movement.
One of the most common mistakes that homeowners and contractors make in remodeling is that they remove the ceiling plaster and joists (to raise the ceiling and gain room volume, etc.), and thereby also remove the ceiling diaphragm, which is a supportive element and can be an integral seismic element of a building. This not only affects the roof framing and wall spread, but it removes a seismic resistive plane of the structure (the ceiling), regardless that lath and plaster or drywall doesn’t have much shear value.
What generally happens is that, after removing the ceiling, homeowners and contractors sometimes fail to do one of two things:
Another issue is that a homeowner or contractor may add drywall to the underside of the rafters, thereby increasing the load on them and causing ventilation problems, which can, in turn, cause condensation and moisture problems, resulting in mold growth, rafter rot, etc.
An inspector who observes a sagging rafter in an unfinished attic, a sagging or bowed ceiling, or, through infrared imaging, detects heat signatures that may indicate moisture above the ceiling, should note such details in his report and recommend further investigation by a qualified professional who can make any necessary repairs and/or structural corrections.