Discerning Tornado Damage After A Storm

May 13, 2011 By Guest Blogger Insurance

(Note: This Guest Blog is by Steven M. Thomas, president of Roof Leak Detection Company, Inc., a Certified Testing Laboratory located in South Florida which specializes in testing and consulting services for commercial and industrial properties).

Severe weather events such as tornados can result in significant property damage. Structural damage can take many forms – some are obvious and others are less visible. But they can all present significant safety risks and must be identified and addressed immediately.

The wind speeds generated by some tornadoes are so great that designing for these extreme winds is beyond the scope of building codes and engineering standards. Most buildings that have received some engineering attention, such as schools, and that are built in accordance with sound construction practices can usually withstand wind speeds specified by building codes. Meeting these code-specified wind speeds can provide sufficient resistance to Tornadic winds if the building is located on the outer edge of the tornado vortex. In addition, if a portion of the building is built to a higher tornado design standard, then both building and occupant survival are improved.

Wind creates inward and outward-acting pressures on building surfaces, depending on the orientation of the surface (e.g., flat, vertical, low-slope). As the wind moves over and around the building, the outward-acting pressure increases as the building geometry forces the wind to change direction. These pressure increases create uplift on parts of the building, forcing the building apart if it is too weak to resist the wind loads. When wind forces its way inside or creates an opening by breaking a window or penetrating the roof or walls, the pressures on the building increase even more.

Heavy building materials (reinforced masonry or concrete) that are well tied to all other building components often survive extreme winds. The weight of these materials helps resist uplift and lateral loads, and heavy materials often stop windborne debris that can increase damage to the building. However, heavy concrete roof panels and heavy masonry walls that are not adequately connected or reinforced have failed during severe winds. Lightweight roofing and siding materials such as gravel, insulation, shingles, roofing membranes, and brick veneer can also be a problem.

Building shapes that “catch” the wind, such as overhangs, canopies, and eaves, tend to fail and become “sails” in extreme winds. Flat roofs can be lifted off when the wind flows over them and increases the uplift pressure at the corners and edges of the roofs.

What signs to look for if your building is in a region where Tornadic activity has occurred. These items may be subtle and not easily visible to the un-trained eye.

  1. Fresh Structural Cracks, the color difference in the cracks should be significantly different from other natural cracking.
  2. Tornadoes are different than Hurricanes in that roof wind uplift damage is not usually isolated to the corners and perimeter. Close inspection of the entire roof surface should take place; look for tented areas of the roof membrane particularly on mechanically fastened roofing systems. When wind uplift damage occurs the roof membrane may not tear, however as the storm and high winds pass, the roofing system will settle back down, if it has not blown off. When this settling occurs in most instances the roof membrane will be wrinkled or tented in some form. If this condition exist, the result is most likely from a high wind event.
  3. Inspect all roof top appurtenances for damage. Most flashing components protrude thru the structural deck. If they have been weakened or damaged, these areas can allow massive amounts of water into the building. Pay close attention to any fresh cracking of the roof membrane or sealant around the flashing components.
  4. Debris impact strikes, if you have a smooth surface roofing system, the debris strike areas will be easily identifiable. If you have a ballasted or gravels surface roof the areas may be concealed due to the gravel being displaced. Particularly on the gravel/ballasted surface roof, if displaced gravel/ballast is observed the roofing system should be further evaluated by Professionals familiar with wind damage analysis. It is very likely that piles of gravel will accumulate in sections of the roof, furthermore if you have roof gravel in the parking lot of your complex, it is most likely that your roof sustained some type of damage.
  5. Hail is usually associated with Tornadoes and can severely impact the performance of a roofing system. Even pea sized hail can cause severe damage to many types of roofs. Look for spatter marks on the roof surface, the impact strikes will be easily identifiable on smooth surface or metal roofs. Even small sized hail impacts can cause micro-fracturing which will result in a reduction of service life to the roofing system.

Steven M. Thomas CRI-RC, CIT 4875
Roof Leak Detection Co., Inc.

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