P 2.

10 BUILDING DAMAGE ISSUES IN TORNADOES

Timothy P. Marshall, P.E.*
Haag Engineering Company

1. INTRODUCTION

The assessment of property damage begins Objects susceptible to wind damage on or around a
immediately after a tornado. Homeowners, insurance building include television antennas, satellite dishes,
adjusters, contractors, engineers, and architects unanchored air conditioners, wooden fences, gutters,
examine buildings and their surroundings to determine storage sheds, carports, and yard items. As the wind
the extent of tornado damage. While catastrophic velocity increases, vinyl siding, roof coverings,
damage is easy to recognize, the more subtle signs of windows, and doors become susceptible to wind
building distress are not. Many types of building damage. Only the strongest winds can damage a
distress that are inherent to different construction properly designed and well-constructed building.
materials, are not recognized until after a tornado Marshall et al. (2002) describes the various failure
occurs. Inspectors often erroneously link these modes in buildings caused by high winds.
conditions to the storm. The purpose of this paper is to
summarize some of the more common building damage 3. THE ROOF COVERING
issues witnessed during the author's inspections of
thousands of structures after dozens of tornadoes since The roof covering is usually the first item on a
1980. building affected by strong winds. Obvious damage
In order to conduct an accurate damage includes tearing, breaking, and removal of the roof
assessment, it is important to have knowledge of covering especially along windward corners, eaves, and
building construction, building materials, and rakes where wind uplift forces are greatest. However,
environmental factors, as well as an understanding of there are a number of roof issues that arise after a
wind forces on buildings. Seaquist (1980) addresses tornado. Roofs can have inherent deficiencies due to
some of the inherent deficiencies in building manufacturing, installation, and weathering that can be
construction and how to recognize such problems. mistaken for storm damage. There are also myths and
Marshall (1992) and Minor (1982) have addressed misconceptions regarding how tornadoes affect roofing
some of the fundamental misperceptions with regard to systems. Thus, distinguishing between tornado
tornadoes and building damage. damage and pre-existing damage can be controversial.
In order to conduct an accurate assessment of roof
2. TORNADO - BUILDING INTERACTION damage, it is important to have knowledge of the
various roofing materials, installation procedures,
Tornadic winds encountering a building are deflected weathering effects, as well as an understanding of how
over and around it. Positive (inward) pressures are wind forces affect roofs. The National Roofing
applied to the windward walls and try to push the Contractors Association (NRCA, 1996) has published a
building off its foundation. Therefore, it is important that manual explaining the installation procedures for a
the building be anchored properly to its foundation to number of roof systems. Obviously, the performance of
resist these lateral forces. Negative (outward) a roof during a tornado depends greatly on the type of
pressures are applied to the side and leeward walls. product and how well it was installed. McCampbell
The resulting "suction" forces tend to peel away siding. (1991) presents a number of cases citing specific
Negative (uplift) pressures affect the roof especially problems with roof design and installation.
along windward eaves, roof corners, and leeward
ridges. These forces try to uplift and remove the roof 3.1 Wind damage issues to asphalt shingles
covering.
The roof is particularly susceptible to wind damage Asphalt shingles are composed of a mat material
since it is the highest building component above the that is either paper (organic) or glass-fiber (inorganic).
ground. Wind pressures on a building are not uniform The mat is saturated with an asphaltic mixture and
but increase with height above the ground. Tornado topped with granules. Roof shingles come in a variety
damage to a well-anchored building typically begins at of shapes, sizes, and patterns including three-tab, strip,
roof level and progresses downward and inward with and dimensional, and they are fastened to the roof deck
increasing wind velocities. Thus, the last place with nails or staples. It is important that the fasteners
structural damage to a building should occur is the be installed properly and placed in the correct positions
interior walls, floors, and foundation. in order to achieve the greatest wind resistance.
____________________________________________ As asphalt shingles age, the asphalt breaks down.
*Corresponding author address: Timothy P. Marshall, The extent of aging depends upon many factors
Haag Engineering Co., 2455 McIver Ln., Carrollton, TX including the quality of the asphalt, shingle color, roof
75006. email: timpmarshall@cs.com pitch, slope direction, and amount of attic ventilation.
Common deficiencies inherent with aged asphalt
shingles are blistering, flaking, cupping, clawing, and during the weathering process. Thus, more granules
general granule loss (Figures 1 and 2). In many are initially placed on the shingles than are needed to
instances, these anomalies are not discovered until cover the mat.
after the tornado, however, this does not mean they An asphalt shingle roof can leak water during a
were created or even aggravated by the storm. tornado. Many steep roof coverings like asphalt
shingles simply shed water and are not waterproof. As
a result, wind-driven rain can be forced beneath the
shingles and flashings and cause interior damage
without damaging the roof covering.
Wind uplift tends to remove asphalt shingles from
the roof. This can lead to a combination of disbonding,
creasing, and displacement of the shingles (Figure 3).
Fasteners secured to the roof deck are usually stronger
than the shingles themselves. Therefore, when
shingles are torn from a roof during high winds, the
fasteners usually remain in the roof deck. Windward
eaves, corners, ridges, and rakes are areas that are
most susceptible areas to wind damage because of
higher aerodynamic uplift pressures in these regions.

Figure 1. Asphalt shingle deficiencies not due to wind

effects: A) blisters, B) clawing, C) granule flaking, and
D) edge cupping.

Figure 3. Wind damage to asphalt shingles: A)

shingles removed from windward slope, B) windward
rake damage, C) loose and flipped shingles, and D)
Figure 2. Additional shingle deficiencies not caused by impact damage to shingles from flying debris.
wind effects: A) diagonal splitting, B) horizontal splitting,
C) shingle buckling, and D) elevated or protruding 3.2 Intentional mechanical damage to roofs
fasteners.
On occasion, some people have tried to simulate
Shingle blisters result from poor quality asphalt wind damage to roof coverings by lifting, tearing or
combined with excessive heat. Granule flaking stems removing the shingles. Such roof damage is normally
from asphalt shrinkage and subsequent delamination found on the periphery or outside the tornado damage
from the mat. Cupping and clawing results from asphalt path. The author has determined a number of factors
shrinkage on the top and bottom surfaces of the that can distinguish intentional damage from wind
shingles, respectively. Diagonal and horizontal splitting damage.
results from asphalt shrinkage and low tensile strength Intentional damage usually is concentrated in upper
of the mat. Elevated fasteners occur during the portions of the roof, away from roof edges, and
installation of the shingles and can buckle or protrude frequently avoids ridges. In contrast, wind-caused
through the overlying shingles. None of these damage usually is most severe at the roof edges and
conditions are caused or aggravated by tornado effects. ridges. Intentional roof damage typically involves
Asphalt shingles are not damaged by granule loss removal of a portion of the shingle and leaving pieces
during a tornado. The quantity of granules lost due to lying on the roof near where they had come from
wind or rain during the storm is a very small fraction of (Figure 4). In contrast, wind usually lifts loose debris
the total quantity of granules on the shingles and within from the windward slopes. Also, intentional roof
normal tolerances. Generally, about one-third the damage is sometimes found on roof slopes opposite the
weight of an asphalt shingle is granules such that the direction of the wind during the storm. In addition,
average residential roof has more than a ton of objects particularly susceptible to the wind (i.e.
granules. Granule loss occurs from the moment television antennas, satellite dishes, trees, etc.) are
shingles are manufactured, shipped, installed, and often found not damaged.
Figure 4. Intentional mechanical damage to roofs in Figure 6. Roof deck deficiencies not caused by wind
attempt to simulate wind damage: A) torn tab corners effects: A) warped decking beneath roof shingles, B)
upslope away from roof edge, B) closer view showing lack of space between deck boards, C) warped decking
tab pieces remaining on roof, C) uplifted shakes, and D) in attic, and D) decking not fastened to rafters.
closer view of uplifted shakes.
There needs to be sufficient spacing between the
After completing a general examination of the deck boards to allow for expansion and contraction.
building surroundings, the inspector should draw a roof Also, aluminum H-clips should be installed in the joints
plan diagram and plot the locations of the roof between the roof decking centered between the rafters.
damages. Any pattern or grouping of shingle marks If nail guns are used to install the decking, care must be
quickly becomes apparent in the diagram (Figure 5). utilized to make sure the roof deck is fastened to the
Note the roof damage occurred on all but one slope, rafters or joists. Otherwise, the roof deck can be
although the windward side of the roof was facing removed during the wind (Figure 7.)
southwest.

Figure 7. Examples of wind damage to roof decking:

A) Loss of entire roof deck since it was not fastened to
the trusses, B) entire deck sheet with shingles still
attached.

5. THE ROOF STRUCTURE

Figure 5. Plot of lifted and broken shingles on a roof The roof structure must support dead loads that
intentionally damaged to simulate the effects of wind. include the weight of the roof as well as live loads such
The letter "S" indicates an undamaged satellite dish. as people walking on the roof, snow, and code-
specified wind forces. However, the author has found
4. THE ROOF DECK many deficiencies with wood framed roof structures
including: warped purlins or braces, use of low grade
The roof deck must support the weight of the roof material with large knots, knot holes and cracks as well
covering as well as resist wind uplift loads. Therefore, as poor joinery. These deficiencies often are discovered
the roof deck must be fastened securely to underlying after the storm and erroneously attributed to high winds,
rafters or joists. Close inspection of the roof deck after low barometric pressure, etc (Figure 8). The cracks
a windstorm usually reveals various anomalies such as and gaps frequently contain dirt and cobwebs,
warped decking or decking that is not fastened properly indicating they were present prior to the windstorm.
to the roof structure (Figure 6). In some instances,
these deficiencies are erroneously linked to the storm.
Figure 8. Inherent deficiencies in the roof structure not Figure 10. Additional examples of wind damage to a
caused by wind: A) large knot with old crack, and B) roof structure: A) fractured rafter, B) uplift of rafter from
gap where rafter intersects ridge beam, C) bowed purlin the top plate, C) brace penetrated ceiling, and D)
from lack of support, and D) warped bracing from outward movement of soffit when tree impacted roof.
inadequate bracing.
6. GABLE ENDS
Wind damage to the roof structure can be subtle or
catastrophic depending on how well it is anchored. Most gable ends on residential buildings do not carry
Flexing of the roof can pull apart nailed connections roof loads. These triangular-shaped structures simply
where braces or collar beams are fastened to the "plug" or close off the ends of the attic. As a result,
rafters. Stronger uplift forces can pull apart toe-nailed gable ends tend to have minimal bracing and are quite
connections where rafters are attached to top plates. susceptible to wind damage. Positive acting wind
Rafters also can be broken at mid-span or where they pressures push the gable ends inward and break
are connected to the ridge beam, braces, or top plates. framing members or pulls apart the connections. In
In these instances, the exposed fracture surfaces are contrast, negative pressures pulls the gable ends
unweathered and absent of dirt, etc. The author has outward or removes them completely (Figure 11).
observed instances where a portion of the roof has
been uplifted along with the braces. Such roofs did not
return to their original positions, and braces penetrated
the ceilings. Downward forces also can overload the
roof structure, especially when struck by falling trees.
Rafters can be broken and nailed connections pulled
apart. Downward movement of the roof structure can
cause soffits to move outward from the tops of the walls
(Figures 9 and 10).

Figure 11. Wind damage to gable ends: A) outward

displacement of gable end, B) fracturing of plate
connections from positive pressure, C) breaking of
gable end stud, and D) pulling apart of nailed
connections.

7. EXTERIOR WALL STRUCTURE

Exterior walls are subjected to internal as well as

Figure 9. Wind damage to the roof structure: A) failed
external wind pressures. Inward acting pressures
nailed connections where purlins were attached to
push in the exterior walls. Well-designed buildings
rafters, B) broken brace, C) failed nailed connections
usually have shear walls (interior walls that are
between collar beams and rafters, and D) failure of
perpendicular to exterior walls) to transfer the wind
nailed connection where brace was attached to purlin.
forces. However, exterior walls without adequate shear Brick can be chipped either during shipping or
bracing are subject to catastrophic failures. Outward installation. Some bricks are actually tumbled to
acting pressures tend to pull out the nailed connections achieve a random chipped appearance.
at the top and bottom wall plates (Figure 12). Masonry walls should have wall ties to anchor the
Therefore, nailed connections should be designed to walls to the frame; however, the author has inspected
resist both inward and outward forces. many buildings that did not have such ties or did not
have them engage the masonry. Non-loadbearing brick
masonry walls not anchored to the building can be
flexed easily when pushed by hand. This does not
mean the wall was "loosened" by the wind; it just never
was anchored (Figure 14).

Figure 14. Brick masonry deficiencies: A) masonry wall

that could be flexed by hand, and B) removal of brick
Figure 12. Examples of wall failures due to wind: A) revealed wall ties (circled) did not engage masonry.
base of wall pushed outward, B) failure of bottom plate
to wooden floor connection, C) failure of bottom plate to Mortar is a mixture of Portland cement, sand, water,
concrete slab connection, and D) failure of wall stud to and lime. The lime is utilized to increase workability of
top plate connection. the mix. Masonry walls are particularly susceptible to
cracking due to differential (up and down) foundation
8. BRICK MASONRY movement. Drainage conditions, locations of trees and
downspouts, all affect the soil moisture content.
Bricks typically are molded from clay and dried in a Window and door openings are naturally weak points in
kiln. There are various types, grades, and qualities of the wall where the distress concentrates. Cracks and
brick. Certain deficiencies can occur during separations that open with height (V-shape) indicate
manufacturing. Shrinkage cracks can form in the brick settlement on either side of the crack whereas cracks
before the material dries. Cracks can also form in the tapering closed with height indicate settlement at the
brick as it absorbs moisture and is subjected freeze- base of the crack (Figure 15). Old masonry cracks
thaw effects. Eventually, brick faces detach and erode become discolored with time as they accumulate dirt,
away. These deficiencies may not be recognized until paint and debris. In contrast, recent cracks appear
after a tornado, and can be erroneously linked to the fresh and unweathered with broken pieces of masonry
storm (Figure 13). along the fractures. The Brick Institute of America
(1991) has a number of excellent technical bulletins on
masonry wall distress.

Figure 13. Brick deficiencies not caused by wind: A) Figure 15. Stair-step crack in brick masonry from
shrinkage crack, B) face separation, C) mortar over differential foundation movement. This distress was not
chipped brick, and D) spalling at a mineral spot. caused by the tornado.
 Frieze boards typically are installed along the tops of 9. INTERIOR WALLS AND DOORS
brick masonry walls to cover the gap between the
masonry and soffit. This decorative trim piece usually is Interior walls are frequently gypsum board or plaster.
mitered at the wall corners. Any rotation or side-to-side The gypsum board is attached to the underlying framing
movement of the wall can open the joints in the frieze with nails or screws and plaster is spread over lath that
boards at the wall corners. Analysis of the joints in the is fastened to the framing. These wall coverings are
frieze boards can give the inspector a history of wall brittle and record movement. Therefore, shifting of the
movement. Caulking, paint, cobwebs, etc. in the joints walls generates cracks emanating from wall and ceiling
indicates prior wall movement (Figure 16). joints as well as door and window frame corners.
These areas represent stress concentration points in
the wall. Examination of the wall cracks can reveal
whether they are new or old (Figure 18). Dirt or paint in
the cracks indicates prior movement. Generally, walls
and floors are rarely square, plumb, or level. This is
due to a number of factors including installation and soil
movement. Walls and ceilings can have a wavy
appearance if the underlying wall studs or ceiling joists
are bowed.

Figure 16. Open joints in frieze boards atop brick

masonry walls indicated prior movement as noted by:
A) caulking, B) cable spanning joint, C) cobwebs, and
D) paint.

Masonry walls are susceptible to wind damage

especially if they are non-loadbearing. Such "free-
standing" walls are pushed inward on the windward Figure 18. Wall distress not caused by wind: A)
sides and pulled outward on the leeward sides. hairline crack contains dirt, B) paint bridges cracks, C)
Movement of the roof structure or gable end can cracks patched and painted, and D) wavy appearance
fracture the top course in the masonry. Also, flying to gypsum board ceiling.
debris can strike the walls, breaking the brick and
cracking the masonry. Recent fractures in the masonry There are two ways interior wall and ceiling cracks
will be unweathered (Figure 17). can be caused by a tornado. One is by flexing of the
ceiling and wall framing due to differential pressures as
the tornado passes. The other is by increased
humidities in the building that occur if the electrical
power (air conditioning) is inoperative. As a result,
wooden doors and windows may become difficult to
open or close after the storm. Such wooden items
swell when the moisture content is increased. Luxford
(1955) conducted a series of moisture tests on gypsum
board nailed to wooden studs. Wall panels were
constructed in dry environments where the average
moisture content in the wood was six percent. He then
subjected the panels to a moist environment where the
moisture content rose to 19 percent. In most instances,
the nails withdrew, creating slight bulges in the
surrounding gypsum board.
Minor wall damage associated with the storm
involves nails backing out of the gypsum board or
Figure 17. Wind damage to brick masonry: A) impact hairline cracks in the wall coverings. Exposed gypsum
by flying debris, B) wall pulled outward, C) wall pushed board around the nails or cracks is not discolored.
inward, and D) movement in top course of masonry Such distress usually can be repaired. More significant
when roof was uplifted. damages involve the separations of ceiling and wall
joints. These occur when the roof joists are uplifted or a walls and are found in the northern portion of the U.S.
wall is pushed inward or outward (Figure 19). In such Houses on stacked brick or blocks are found typically in
instances, a detailed structural inspection is warranted. rural areas, especially in the southern United States.
Timber piles and masonry or concrete columns support
homes in coastal or flood-prone areas.
The foundation must bear the weight of the building
and also resist any movement of the soil beneath or
around it. Over the years, the author has found a
number of inherent problems in building foundations
ranging from inadequate support to distress from soil
movement. Many of these conditions are not identified
until after the tornado and then are incorrectly attributed
to the storm.
Figure 19. Tornado related damage to gypsum board:
A) protruding nail in wall and B) separation of 10.1 Concrete Foundations
ceiling/wall joint as roof was uplifted (note drapery
caught in joint). Concrete foundations tend to crack during or after
curing as they shrink. The extent of shrinkage cracking
The analysis of doors and door frames can give the depends primarily on water content of the mix and
inspector a history of interior wall movement. Doors placement of control joints. Environmental factors such
often are trimmed to fit within the door frames. Close as the rate of hydration and evaporation can affect the
examination can reveal evidence of planing or severity of shrinkage cracking. For these reasons,
abrasions on the doors. Doors binding against their shrinkage cracks are fairly common in concrete
frames may abrade the wood or paint. Relative foundations. Shrinkage cracks are usually small in
changes in the door level can be detected by analyzing width, less than an eighth of an inch, but can extend
the position where the door latch meets the striker several feet in length.
plate. Many buildings are constructed on thin concrete
slabs with shallow footings and therefore are
susceptible to differential foundation movements due to
cyclic moisture changes in the underlying soil or
settlement. Concrete slabs float on the ground and rise
and fall with expansion and contraction of the
underlying soil. As expansive soil dries around the
foundation perimeter, the perimeter of the building
settles relative to the center leading to distress in the
interior and exterior finishes. Mitered corners at frieze
boards open and interior doors bind. Cracks in plaster
and gypsum board frequently emanate from the corners
of door and window openings. Slab cracks can extend
through brittle finishes like ceramic tiles bonded to the
slab.
An examination of the crack interior often will
determine whether the crack is new or old. Cracks
become discolored with time as they accumulate dirt,
Figure 20. Interior door distress not caused by wind paint or debris (Figure 21). Edges of the cracks
effects: A) gap at top of door yet side of door fits well, become rounded with continued wear. Water entering
B) no relative movement between door latch and the cracks can lead to rusting of reinforcing steel and
painted striker plate, C) wear marks on door frame, and the resulting expansion can lead to spalling (removal of
D) evidence that top of door was planed. the concrete).
In contrast, cracks caused recently do not have
10. THE FOUNDATION these characteristics. Recent cracks appear fresh and
unweathered with broken pieces of the concrete along
The foundation is typically the last place in a building the fractures. Concrete slab cracks associated with
to experience distress from a tornado. There are six wind usually are found in the direct load path of a failed
common types of house foundations in the United building component (Figure 22).
States: 1) concrete slab, 2) pier and beam, 3) poured
concrete wall, 4) concrete masonry wall, 5) stacked
brick or block, and 6) timber piles or masonry and
concrete columns. In general, homes on concrete slab
foundations usually are found in the south and
southwest United States. Homes with basements or
crawl spaces usually have poured concrete or masonry
Figure 21. Cracks in concrete slab not caused by wind:
A) center crack and B) close-up view showing dirt in
crack.
Figure 23. Pier and beam foundation problems not
caused by wind: A) cantilevered beam, B) non-
continuous beam, C) tilted pier, and D) rotted piers.

11. SUMMARY

In this paper, the author has discussed certain

issues with assessing damage to a building after a
tornado. The information presented herein has been
assembled from more than twenty years of inspecting
buildings in the aftermath of tornadoes. Since most
buildings are not constructed perfectly, inherent
deficiencies usually can be found like walls that are not
plumb or square, and floors that are not level.
Seasonal soil movement causes cracks in brittle wall
Figure 22. Cracks in concrete slab caused by the materials and long term weathering causes roofs to
rotation of the steel column that failed in the wind. wear out.
Many people generally do not find these "less-than-
10.2 Pier and beam foundations perfect" conditions until after a tornado occurs when
they closely examine their surroundings. Cracks, gaps,
Many homes are constructed on pier and beam and other anomalies invariably will be found in certain
foundations. The piers can be constructed from a building components that have never been observed
variety of materials including brick, concrete, concrete before. In order to differentiate between inherent
masonry, or wood. Shallow piers rest on the ground building deficiencies from those caused by a tornado, a
surface or on small pads. Deep piers can extend 20 careful examination of the building is needed by a
feet or more depending on soil conditions. Homes on qualified individual. It is important to have knowledge of
pier and beam foundations can experience the same building construction, building materials, and
types of distress from differential foundation movement environmental factors, as well as an understanding of
as homes on concrete slabs. wind forces on buildings.
An examination of the crawl space can reveal A number of examples were presented herein
inherent deficiencies with the foundation such as illustrating the differences between inherent deficiencies
cantilevered or poorly supported beams, tilted piers, in buildings and those caused by strong winds. It is
wood rot, etc. (Figure 23). Such deficiencies were not hoped that damage assessors can utilize such
caused by winds during the tornado. It stands to information to better determine the extent of storm
reason that any significant shifting of the home on its related damage to a structure.
foundation would likely break pipes that extend through
the floor. Usually, homes on pier and beam 12. REFERENCES
foundations have little to no anchorage. Therefore, the
structural frame or floor is more apt to shift off the Brick Institute of America, 1991: Movement, Design,
foundation, leaving the foundation in tact. and Detailing of Movement Joints: Part II, Technical
Notes on Brick Construction, #18A, 8 pp.

Luxford, R. F., 1955: How to avoid nail popping in dry-

wall construction, Forest Products Laboratory, Report
No. 2036, Dept. of Agriculture, Madison, 5 pp.
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tornado damage. The Tornado: Its Structure, Dynamics,
Prediction, and Hazards, C. Church, D. Burgess, C.
Doswell, and R. Davies-Jones, Eds., Amer. Geophys.
Union, 495-499.

Marshall, T. P., 2002: Procedure for assessing wind

damage to wood-framed residences, Proceedings of
the Symposium on the F-Scale and Severe-Weather
Damage Assessment, American Meteorological
Society. Long Beach, CA. [CD-ROM]

McCampbell, B. Harrison, 1991: Problems in roofing

design, Butterworth-Heinemann Publishers, Boston,
227 pp.

Minor, J. E., 1982: Tornado technology and

professional practice, Proceedings of the American
Society of Civil Engineers, J. of the Struct. Div.,108,
ST11, Nov. 1982, 2411-2421.

Seaquist, Edgar O., 1980: Diagnosing and repairing

house structure problems, McGraw-Hill Book Company,
NY, 255 pp.

National Roofing Contractors Association, 1996:

Roofing and Waterproofing Manual. Fourth Ed., Vol. 1
and 2, 1857 pp.