ROOF STRUCTURE FAILURES
Charles C. Roberts, Jr.
Roof failures are at the heart of many insurance claims. Improper design,
deterioration, overloading, inadequate repair, and weather related influences are
known contributors to the failure of a roof structure. Before launching into our
case studies, perhaps a refresher on roof structures is in order. Figure 1 shows
common roof structural elements incorporated into the design of many residential
and commercial buildings. The pitch roof truss is a popular design used in some
homes and many small commercial buildings. The bowstring truss is an older
classical design often found in buildings over 30-40 years old. The flat truss is
used extensively in small industry and is typically made of steel. The basic gable
frame is used throughout the home construction industry. The I-beam may be
found in virtually all types of construction and may be used as a column.
Figure 2 is an example of ridge sag on 60 year old brick home. Figure 3 is
a view inside the attic showing the rafters and the absence of collar beams and
joist connections to the rafters. Since the attic floor joists are parallel to the ridge
board, there was no structural benefit to the roof framing. The rafters were nailed
to small wood supports that engaged the masonry wall to prevent the rafters from
moving laterally with respect to the ridge. Over time, these supports had failed,
causing rafters to move laterally and resulting in the ridge settling in the shape
shown in Figure 2. This is an example of non-standard framing, characteristic of
old buildings, that is structurally deficient for the long term.
Figure 4 shows a catastrophic collapse of partially constructed pitched
roof trusses. Winds of approximately 50 MPH caused the collapse. Investigation
of the remains showed relatively few braces and in many places a single nail
joining many structural members. Structural elements under construction are
vulnerable to wind conditions since the full structure required to resist wind
loading has not been installed. Consequently, good construction techniques
dictate proper temporary bracing of the structure which was lacking in this case.
Figure 5 shows a cracked member in a wood truss in a large metal pole
building. The truss cracked during unloading of grain from the building. Figure 6
shows grain deposits adhering to the bottom cord. Apparently, the grain level was
nearly at the roof and above the truss system since it is loaded by an auger at the
ridge. The grain was in the bin several months before withdrawal, allowing
consolidation and adhesion to the roof trusses. When the grain was removed from
below, the weight of the grain that adhered to the structure overloaded the old
trusses, causing failure. This is a common problem with grain storage structures
because of the unpredictable nature of compacted grain when unloading.
Figure 7 is a view of a large grain bin where the concrete roof began to
deflect downward. Figure 8 shows an I-beam, which was a major support for the
roof, also deflecting downward. Beam connection had corroded over time and
allowed the beam end to slip out of position. This is an example of corrosion
taking its toll over a 40 year period.
Figure 9 is a view of a bow truss that had been reinforced using wood and
steel. The temporary column used to support the structure was not supporting the
piece of steel used in the repair. Consequently the steel that had been added to the
structure, performed no useful purpose but added to the load on the truss. This is
an example of a "handy man" repair performed by the building owner. These
repairs, not usually inspected by governing authorities, are often inadequate and
are a cause of roof failure.
When investigating roof losses, information on environmental conditions
at the time (wind, rain, etc) helps establish an initiating event to the collapse. In
many instances, a moderate wind can cause the collapse of a marginal structure.
Inspect the debris as soon as possible lest clean up efforts spoliate the evidence.
(see Claims Magazine June 1992) When analyzing the cause of failure, finite
element structural analysis computer programs are readily available to identify
weakness in structural design (see Claims Magazine June 1987).
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