Wind-driven Rainwater

September 15, 2008
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For some 30 years now, I have owned property on a beach on the Florida gulf coast. I have dealt with varying weather conditions affecting that property, including three mandatory evacuations before hurricanes. I have also engaged in wind- and water-damage restoration on my property and the property of others in coastal and inland areas for more than 38 years.

This experience has provided a unique restoration perspective on tropical storms, water spouts, tornadoes, hurricanes and other weather events in this and other areas. As recently as 2004, the property suffered extensive damage from wind-driven rain associated with Hurricane Ivan.

There seems to be legitimate confusion as to the nature of water damage resulting from wind-driven rain, as defined by the IICRC S500 Standard and Reference Guide for Professional Water Damage Restoration.

The Nature of Rainwater

Evaporating water is Category 1 water. It contains nothing but molecules of water (H2O). Therefore, rainwater associated with tropical storms or hurricanes is predominantly evaporated water from a variety of sources (e.g., lakes, rivers, oceans – some 86%). According to IICRC S500, atmospheric rainwater is defined as Category 1.

Evaporated water (water vapor) contains no contaminants or even salt. As water falls through the atmosphere, it can absorb minor amounts of carbon dioxide (CO2), making the water slightly acidic. Acid rain is created when smog forms in high-population areas, particularly during thermal inversions, and falling rainwater bonds with oxides of nitrogen or sulfur from combustion emissions in the air.

Evaporating water from the sea or other inland bodies is Category 1, unless it flows (floods) across the ground and picks up soil bacteria, fungi, insects, decaying organic matter, herbicides, pesticides and the like. Pathogenic content is the key.

Contaminants rendered airborne by wind remain relatively close to the ground. There are fewer windborne contaminants farther inland because larger particles settle out, especially as wind diminishes over land and ground flora minimizes additional aerosolization. In many cases, large debris is filtered out of water entering a structure by structural components themselves, such as roof decking, wall framing, insulation and drywall. As dusts and debris becomes wet from rain, it increases in weight. Therefore, it is not as easily aerosolized, and when it does become airborne, it settles out of the air more quickly.

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Microbial Growth Conditions

It takes some period of time, depending upon various conditions, before the stagnation of Category 1 or 2 water becomes “pathogenic” or “grossly unsanitary” if, indeed, it ever does. Germination, growth, amplification and dissemination of microorganisms is influenced by:
  • the presence of organic materials that provide food for microorganisms;
  • the presence of moisture or high humidity (typically >60%;
  • optimum temperature ranging approximately between 68-86°F (consider both day and night);
  • air circulation, which inhibits growth and amplification, and
  • elapsed time, typically two to three days for common indoor fungi under optimum conditions.


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Those who wish to define wind-driven rain as Category 3 water typically fail to quote the complete definition in the IICRC S500 which, when read in context, is pretty clear. The statement regarding wind-driven rain in IICRC S500 specifies “…contaminated water…such as wind-driven rain from hurricanes, tropical storms or other weather-related events. In other words, the “wind-driven rain” had to be contaminated – actually, “…grossly contaminated and can contain pathogenic, toxigenic or other harmful agents” – to be considered Category 3 water. Therefore, wind-driven rain that is not already significantly contaminated would not qualify as Category 3 water.

Moreover, it is obvious to even a casual observer that not all wind-driven rain from hurricanes and tropical storms is, according to the industry-consensus standard of care for water-damage restoration (IICRC S500), “grossly contaminated” and with “pathogenic agents.” People live, work and are routinely in contact with wind-driven rain during hurricanes, tropical storms and other weather-related events. Indeed, in most coastal areas, “wind-driven rain” occurs practically on a daily basis during spring and fall seasons. Moreover, one has only to watch the Weather Channel for a few days to see meteorologists and reporters standing in wind-driven rain, breathing, ingesting and otherwise becoming thoroughly soaked to understand the nature of that water.

I am unaware of any published information written by qualified authorities that categorizes tropical storm, hurricane, or wind-driven rainwater as Category 3 (black or grossly contaminated) water. As one of the drafters of the IICRC S500, it is my opinion that the correct interpretation of Category 3 water is, as noted above and as written in the standard itself, contaminated wind-driven rainwater. This would require that the wind-driven rain pick up quantities of organic matter, such as ground soil, vegetation or biological matter - or possibly even uncontained contaminants released from industrial work sites due to physical storm damage to the work site in sufficient quantity to present a public health hazard.

If a question about the Category of water arises in any water-restoration situation, prudent remediation measures involve appropriate engineering controls and PPE, plus IICRC S500 remediation procedures designed to mitigate the loss in a contaminated building. While these mitigation procedures are underway, an independent IEP hired by the property owner can sample and interpret the conditions that exist.

When restorers or remediators have cause to believe that there might be the possibility of a Category 3 water situation, they should comply with the OSHA General Duty Clause and recommend appropriate testing including water sampling and analysis. Testing allows restorers to determine whether the water that affected a building was contaminated and, if so, the extent of the contamination, e.g., is the contamination pathogenic or grossly contaminated.

Although there are varying levels of contamination in each of the IICRC S500 Categories of Water, I am not aware of any evidence that rainwater, whether falling or wind-driven, that affects most buildings is “…grossly contaminated,” nor that it “contain[s] pathogenic, toxigenic or other harmful agents.” No one in a geographical area affected by tropical storms or hurricanes suggests that the rainwater outside the buildings, indeed throughout a large geographical area, is Category 3 as it falls from the sky, nor am I aware of any published or even unpublished authority that purposes such a conclusion.

So just what are some of the conditions that might cause a restorer or remediator to conclude that water propelled by wind driven rain might have contaminated a structure to a Category 3 status? Recalling that falling rainwater is Category 1, consider the following:
  1. Proximity of the structure to a body of water. If the structure is situated on a beach, or even within a few blocks of the beach where a significant storm comes ashore, then the wind may contain saltwater spray, sand, silt, organic matter or other debris that would cause it to be Category 2 at a minimum; but even that scenario would not be considered Category 3 (grossly contaminated with pathogenic, toxigenic or other harmful agents) – at least initially. Photos 1-4 illustrate a structure damaged by wind and rain, and where significant quantities of debris entered the structure during a hurricane. It was in close proximity to a beach where a hurricane came ashore; note that windows and doors were either blown out or significantly damaged.
  2. Physical location of the structure. If the water-damaged building is located in proximity to a facility that processes HAZMAT (e.g., pesticides, dry solvents, fuels, heavy metals) and is also damaged by the storm, then it may be necessary to test the water-damaged building for the suspected HAZMAT. If the structure is located inland from the body of water, and especially if there is ground cover (vegetation) that reduces aerosolization of debris to some extent, then it is more likely that airborne debris (sand, vegetation, salt water) associated with the initial storm surge would precipitate and not be carried very far inland. Therefore, direct introduction of quantities of debris would be diminished, at least to some extent, especially considering that rain wets debris, thereby increasing its weight and making it more difficult to become or remain aerosolized.
  3. Building penetration point. Where did the water enter the building? Obviously, flood water – defined as rising water that overflows natural boundaries and passes across the ground in its way into structures is Category 3 water, as defined by government agencies and by the IICRC S500. However, evaporation from natural bodies of water is nothing more than molecules of H2O – Category 1. Even condensed water that comprises clouds and eventually becomes rainwater is pure water unless it combines with gases forming atmospheric pollution in some metropolitan areas causing it to become slightly acidic. If the wind, and water driven by that wind, broke out windows or doors and propelled quantities of debris into the structure, then, that also might be considered Category 2 at a minimum, and possibly even Category 3, depending on the type and amount of contamination contained in the “wind-driven rain” before entry. If the falling rainwater pools on a roof and enters the building through a broken roof cladding, then several questions should be asked:
    • What is the construction of the roof and materials used? Peaked roof construction may lessen the amount of water penetration, and the water’s condition is more likely to be Category 1. If the roof is flat and the water pools and enters, its condition may be Category 1 or 2 depending on several factors.
    • Has the roof experienced contamination from soil, algae associated with pooled water, bird droppings, or even animal carcasses?
    • Is the roof subject to rainfall that flushes soils and contaminants from its surface routinely? Consider that people in many countries collect rainwater from roof run-off in cisterns, which is then used for bathing and cleaning.
    • What about the effect of sunlight (UV, temperature) on roof components and soils?
    • How large is the roof area and how much water accumulated?
    • What if any diluting effect did the quantity of rain and water accumulation (pooling) have on its Category?
    • What was the outside and inside temperature at the time of water entry and for the elapsed time before response, as well as during drying?
    • How much time elapsed after water entry and before restoration was initiated?


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Bottom line, is all wind-driven rainwater automatically Category 2 or 3 as defined by the IICRC S500? Certainly not. Can it become Category 2 or 3 once it enters buildings, along with debris, and is allowed to stagnate at warm temperatures, with little air movement for prolonged periods? Quite possibly – either Category 3 or, more likely, Condition 3 mold contamination.

It is my opinion that the IICRC S500 definition of Category 3 water requires that, first and foremost, water be contaminated or otherwise contain pathogenic or toxigenic agents including water from wind-driven rain. There are multiple factors that must be considered by a trained and experienced restorer to determine the Category of water in a weather-related event.

When in doubt, testing by a qualified and experienced IEP should performed to establish levels of contamination. The building’s location, and response time, ambient temperature, and air circulation are critical factors in the decision-making process.

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