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WSRCA INFORMATIONAL BULLETIN — No. 2017‐1W — Waterproofing Buildings in Special Flood Hazard Areas

Posted By WSRCA, Wednesday, November 8, 2017

Greetings to Members of Western States Roofing Contractors Association:


Over the last several years many of our WSRCA members have become more aware of the flooding hazards associated with low‐lying coastal areas and near river valleys where flooding may have historically occurred, and in certain areas that seem to flood with some regularity. However, recent climate events reportedly have caused many unexpected and “flash‐flood” events, even in areas otherwise known to the layperson as desert areas, which have caused millions of dollars of damage to nonwaterproofed buildings. Some of these flood events have occurred in fairly large metropolitan areas and some in densely populated urban areas of the U.S.


The Federal Emergency Management Agency (FEMA) has long been involved with regulating how buildings located in flood‐prone areas should be constructed, or repaired, and/or retrofitted. However, recently Factory Mutual (FM) published a new Factory Mutual Global Property Loss Prevention Data Sheet 1‐40 regarding floods, partial‐building waterproofing, etc. Some WSRCA members may be aware of FM’s 1‐40 published during 2016, but FEMA’s regulations and guidelines for Special Flood Hazard Areas (SFHA) may be less familiar to some of WSRCA’s members involved in aspects of waterproofing. This bulletin is intended to impart additional waterproofing and “flood‐proofing” information to WSRCA Members.


FEMA’s Special Flood Hazard Areas are locations that may not have regular flooding but are subject to significant flash flood events on a much less regular basis, and are associated with what are referred to as 50‐year, 100‐year, or 500‐year storm events.


When the quantity of flood water is primarily made‐up of rain water, as opposed to snow‐melt or other contributors, one of the main factors designers must consider is rainfall intensity. Rainfall intensity is typically expressed as inches of rain over a given time frame, and the U.S. Weather Bureau maintains rainfall records for the United States and most major cities in any particular state. The specific rainfall data is regularly updated by the National Oceanic and Atmospheric Administration (NOAA). The chance that an unusually heavy or intense rain may reoccur is referred to as a “reoccurrence interval” and is sometimes stated as a 50‐year or 100‐year chance of reoccurrence. The Table below provides examples of a 100‐year reoccurrence interval for several Western States and select cities in these listed states.


One such FEMA guideline document that provides detailed information on this Federal agency’s recommendations and requirements for residential construction and retrofit work, which can help WSRCA waterproofing professionals working in a SFHA is: Engineering Principles and Practices for retrofitting Flood‐Prone Residential Structures (Third Edition) FEMA P‐259 / January 2012. While the document is provided primarily for the design professional’s use, it is also helpful for the waterproofing contractor that may get involved in a retrofit or a new construction project in a location designated by FEMA as an SFHA, as our WSRCA members endeavor to understand the requirements outlined in these relatively new documents. A full copy of this FEMA document is available for download at


The requirements, outlined in the aforementioned document, for making changes or repairing buildings within these flood hazard zones are multi‐faceted. These requirements may impact waterproofing contractors who are asked to make an existing building either permanently or temporarily flood resistant. Permanent flood resistance is referred to by FEMA as “Dry Floodproofing” and is defined as: “Strengthening of existing foundations, floors, and walls to withstand flood forces while also making the structure watertight.” Temporary flood resistance is referred to as “Wet Floodproofing” and is defined as: “Making utilities, structural components, and contents flood‐ and waterresistant during periods of flooding within the structure.”


FEMA has strict requirements for new construction that must be met if a new building is to be located within a SFHA. These requirements must also be followed for existing buildings located in a SFHA that undergo substantial improvement or that are substantially damaged buildings undergoing repairs and waterproofing or dry floodproofing. These requirements can impact the contractor’s work in a variety of ways, but the important thing to be aware of is that there are special requirements and it is important for those involved in construction or repairs to be apprised of the requirements.


Background Information:

Whether your waterproofing project is commercial or residential, knowing whether there are any special requirements related to potential flooding, waterproofing, and/or floodproofing in the area in which the project is located is important for all members of the project team. FEMA provides information about the flood hazards in areas throughout the country via their FEMA Flood Map Service Center, which is available on line at


Factory Mutual Global also offers guidance for flood prevention and mitigation in Factory Mutual Global Property Loss Prevention Data Sheet 1‐40, Flood. (Register to receive Factory Mutual Global data sheets at Flood abatement solutions, in the form of FM Approved products, can be found in FM Approvals at


Factory Mutual Global relatively new Interactive Flood Map is also available, which is accessible online at‐and‐resources/global‐flood‐map. (Refer to Figures 3 and 4).




Also, as flood hazard management requirements become more stringent, some Jurisdictions are providing amendments to the Codes requiring that construction must comply with the minimum participating criteria of the National Flood Insurance Program (NFIP), the standard upon which the FEMA requirements are based. The latest edition of the International Building Code, which is becoming more and more widely adopted and/or adopted and amended by local jurisdictions, also contains requirements for construction within flood hazard areas. FEMA currently provides reference documents summarizing: the flood resistance provisions of the 2015, 2012, and 2009 International Codes (I‐Codes); the referenced standard from American Society of Civil Engineers (ASCE) 24, Flood Resistant Design and Construction; and requirements of the National Flood Insurance Program (NFIP), which can be downloaded from:‐code‐resources.


Notes of Interest:


Where Flood Waters May Inundate Part or All of the Main Floor of a Building:

Among the floodproofing information provided by FEMA, which may be of interest to WSRCA member waterproofing contractors, designer, and manufacturers, is a statement related to how much of a building should be allowed to be made waterproof. In areas where flood waters can be expected to inundate part or all of the ground floor of a building, and dry floodproofing is allowed, FEMA has stated that waterproofing should not extend up more than a height of three feet, (without an engineering analysis) due to the danger of structural failure from excessive hydrostatic pressure and other flood‐related forces (e.g., from heavy floating debris, such as fallen trees etc., being pushed along by flowing flood waters).


Buildings Constructed with Crawlspaces:

Where buildings are constructed with crawlspaces under the floor, FEMA requires items or materials that could be damaged by flood waters to be located elsewhere, and it requires flood openings in the foundation walls so that high flood water is free to flow through the crawlspace without exerting further water pressure/force on foundation walls. For new buildings or substantially damaged or improved buildings with crawl spaces, flood openings are also required under the NFIP.


Dry Floodproofing:

FEMA advises us that dry floodproofing measures may be best described as “a combination of operations plans, adjustments, alterations, and/or additions to buildings that lower the potential for flood damage by reducing the frequency of floodwaters that enter the structure.” (Engineering Principles and Practices for Retrofitting Flood‐Prone Structures, (Third Edition) FEMA P‐259 / January 2012, 5D‐1)


Note: FEMA cautions that dry floodproofing should be considered for short duration of flooding (e.g., of a few hours), and that a structural engineer should be called on to evaluate the building to determine if the wall and floor assemblies can resist the hydrostatic and other flood‐related loads the waterproofing may add to an otherwise non‐waterproofed existing building.


Examples of dry floodproofing modifications to existing buildings indicated as general guidelines in the FEMA document include: 


 Use of waterproofing membranes, flexible membrane flashings, sealants, and weather stripping gaskets to reduce seepage of floodwaters through walls (See Figure. 5) and wall penetrations;


 Installation of watertight shields; sealants and flashings for doors and windows, and other wall openings;


 Reinforcement of walls to withstand floodwater pressures and impact forces generated by flood‐water pushed floating debris;


 Installation of drainage collection systems and sump pumps to control potential water entering interior levels, collect seepage, and manage hydrostatic pressures on the slab and walls;


 Installation of check valves to prevent the backflow of floodwaters or sewage flows through drains; and


 Anchoring of the building to resist flotation, and lateral movement.



As with all building codes, standards, and governmental requirements, the language and requirements can change from edition to edition, which may occur with FEMA floodproofing‐related documents, and the new Factory Mutual Global document. As such, it is imperative to remain up to date on the current set of Codes and verify the current codes and regulatory requirements that are in effect for your specific waterproofing project. It is also important to be aware of and understand what regulations and requirements are in place for existing buildings versus new construction, and the different requirements related to substantial improvement or damage repair to existing buildings as it relates to floodproofing and waterproofing. Waterproofing for dry flood proofing can be different from the normal waterproofing of structures (e.g., below grade) to prevent the entry of ground water.


In Closing:

Thank you for your participation in the western waterproofing industry. We trust the above information aids you by providing resources that can better equip you when you and/or your company may be involved in construction and waterproofing‐related work in flood prone or flood hazard areas. Thank you for your support of Western States Roofing Contractors Association, and our active efforts to strengthen and advance technology and science in our industry, as well as to promote the art of quality roofing and waterproofing.



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