Note: This guest blog is by Lewis O’Leary, who has served as both a forensic investigator and restoration contractor since Hurricane Camille (1969). He has a degree in the fields of Mechanical Engineering and Architecture, is the Chairman of the Education Committee for the Building Envelope Science Institute, and is a design/build, General Contractor licensed in North Carolina. Mr. O’Leary also serves as a staff consultant for engineering firms involved with wind damage from hurricanes and tornadoes. He can be contacted at 919-577-0907 or via e-mail at firstname.lastname@example.org.
On July 9, 2017, the News & Observer (N&O), a Raleigh, North Carolina based newspaper, published a front-page article entitled Nearly 200,000 homes in NC are at risk from tropical winds. The article explains that nearly all “manufactured” (aka mobile) homes will be torn to pieces if they are exposed to a “strong” hurricane.
In an attempt to quantify this, I turned to the Enhanced Fujita Scale, which sets out 28 tables that describe what degree of damage would normally be caused to different objects by various ranges of wind speeds. Table 4, which lists the damage that can be expected for doublewide mobile homes, shows that older manufactured homes, will suffer “Complete destruction of roof and walls leaving undercarriage in place” at about 93 mph, 3-second gusts. Because meteorologists report hurricane wind speeds in terms of one-minute sustained units of measure, one would need to multiple the “sustained” values times 1.3 to get to the 3-second gust unit of measure. This is necessary to get the unit of measure that design engineers use. This means that the older manufactured homes would be destroyed at an approximate 72 mph, 1-minute sustained wind speed. This is below the threshold value for a category one hurricane. The surprising part here is that newer manufactured homes are also at risk. These will suffer catastrophic damage at about 85 mph, sustained winds, which correspond to a mid-range, category one hurricane. Topography plays a role in reducing free stream velocity. But if winds increase to a midrange category two, the ground clutter will only reduce wind speed to approximately this 85 mph, one-minute sustained wind speed number.
Only the most seasoned investigators understand that the risk of total destruction is not limited to just manufactured homes. Winds in this same speed range will also cause significant wide-spread damages to older homes and commercial buildings. In the case of Very new widespread roof leaks, very few investigators can spot latent (subtle) damages. This is especially true for when it comes to roofs.
The Insurance Institute for Business and Home Safety provided the data that the News & Observer used in its article. The industry took notice after Hurricane Andrew (1992) that structures of all kinds sustained damages beyond what the Fujita Scale had forecasted in 1971 at various wind speeds. Texas Tech’s Wind Science and Engineering Center had been studying the disparity between what structures were supposed to sustain and the damage actually sustained in high wind events long before Hurricane Andrew. In 2003, the scientists published their findings in a vastly improved version of the Fujita Scale, calling it the Enhanced Fujita Scale.
The Enhanced Fujita Scale shows that older structures are only good for about half the wind force for which they were designed. In 2010, the Insurance Institute for Business and Home Safety built three cosmetically identical pairs of homes in a wind tunnel and conducted three separate tests. In each case, one of the homes was built to the latest design standards and the other was built to earlier standards. The wind speed was steadily increased and in each of the three tests, the older code homes were destroyed before the new codes homes reached the threshold for visible damages.
The question of what caused damages beyond what a structure should have withstood arises with each severe wind event. When an older, but still very functional roof suddenly develops widespread leaks, the average investigator will find no obvious damage. There are two reasons for this. First, latent evidence is difficult to find if you are not looking for it. Second, the average investigator does not have the training to know what to look for.
As such, the average investigator will commonly attribute the leaks to wear & tear, defective design, or workmanship when the damages are all latent.
There is a distinct lack of training materials that specifically address and set out a protocol for identifying latent wind damages. In my experience, starting with Hurricane Camille (1969), investigators today commonly identify only about half of the total storm damage. Even fewer know how to assess whether damage is old or new.