Ask An Expert: Hurricane & Flood Events

Ask An Expert is a question-and-answer column designed to address common questions related to forensic investigation and property damage. Each month we’ll feature one or two questions submitted by you – our readers and customers – and provide detailed, easy-to-understand answers. Email your questions to [email protected] or submit your questions here.

Q: Is there a way to determine whether water intrusion was salt or fresh water, in the case where the homeowner does
not know?

Rich Grimshaw, P.E.: If salt water or brackish water are not nearby, then they can be ruled out. However, if they are nearby, and they are suspected of causing damage, any surfaces that were inundated can be tested to detect trace remains of minerals and salts.

Q: Can wind-damaged areas of EPDM be cut out and patched? 

Chris Scallion, P.E.: Yes. Adhered systems will require a sound substrate, which may necessitate partial insulation replacement. Repairs to mechanically attached or ballasted systems are generally straightforward.

Q: Is there a rule/standard that if water ponds for a certain period of time, then the roof can’t be repaired and must be replaced?

Chris Scallion, P.E.: No such guideline exists. Ideally, all low-slope roof products should provide positive drainage and prevent ponding water. With relatively low slopes, small areas of relatively shallow water may accumulate after a rainstorm. These areas should be dry within 24 to 48 hours. Most manufacturers have guidelines regarding the period of acceptable ponding water for warranty purposes, typically 48 hours. Localized repairs subjected to ponding water may not be eligible for manufacturer’s warranties, but spot repairs are seldom covered beyond an installer’s warranty. Ponding water is indicative of an underlying deficiency in the construction of the roof system and should be remedied during the next roof replacement. Ponding water results in a pressure head that can drive water into otherwise minor deficiencies in the roof system.  Minor leaks can become significant sources of water intrusion when subjected to ponding water over a prolonged period.

Q: How does wind damage a building? What is wind suction and how does it work?

Shane Conklin, P.E.: As wind passes over and around a building, two things happen. First, positive wind pressure applies to building components on the side(s) of the building that face the incoming wind (the “windward” direction). The windward wind load is essentially the force of the blowing wind pressing on the building. Second, negative wind pressure applies to building components on the side(s) of the building that face away from the incoming wind (the “leeward” direction). The negative wind pressure is also known as “suction.” The suction force will be applied to vertical surfaces such as walls, and also horizontal or sloped surfaces such as roofs. The suction force can be thought of as acting like a vacuum that pulls on a building and its components.

During an event with moderately strong winds, building materials such as asphalt shingles or vinyl siding may displace away from the building. Sometimes, windows break and the broken shards end up outside of the building. In such cases, a common misconception held by many people in the construction industry is that wind must have gotten under or behind the surface of the material to “blow” it out. However, acknowledging and understanding the concept of wind suction can explain how those materials were damaged. Wind does not only apply a direct blowing force on buildings, but it also induces a suction force. Depending on various factors, the suction force can be significant enough to cause damage to individual building components or the structure of the building itself.


You can discover more about Donan’s Catastrophe Response at donan.con/cat.
Learn more by attending an upcoming webinar or viewing other resources at 


About the Experts

To learn more about our experts, view their full professional profiles below.

Matthew Kenney, P.E., Technical Program Manager

Rich Grimshaw, P.E., Principal Consultant & Forensic Engineer

Chris Scallion, P.E., RRC, Forensic Engineer

Shane Conklin, P.E., Forensic Engineer


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