Dry floodproofing: the alternative to elevating the building

It is FEMA Form FF-206-FY-22-153, "Dry Floodproofing Certificate for Non-Residential Structures", edition 2/23, which the Planning Board publishes in the flood insurance section of its Flood Portal. A dry floodproofed building is, in the form’s own words, a building designed and constructed to be watertight — substantially impermeable to the passage of floodwaters — below the base flood elevation, and with structural components capable of resisting hydrostatic and hydrodynamic loads and the effects of buoyancy.

The certification is required for non-residential structures that are dry floodproofed and for the dry floodproofed non-residential portions of mixed-use buildings. It does not serve residential purposes: the form warns that dry floodproofing a residential building does not alter a community’s floodplain management elevation requirements or affect the insurance rating, unless FEMA has issued that community an exception allowing dry floodproofed residential basements; and for the residential portions of mixed-use buildings the applicable form is 206-FY-21-122, the programme’s residential basement floodproofing certificate.

• Floodproofing to the base flood elevation (BFE): that is the National Flood Insurance Program’s minimum requirement.Verified against the official source
• Floodproofing to one (1) foot above the base flood elevation for the building to be considered for floodproofing credit, under the ASCE 24 standard.Verified against the official source
• In B, C or X zones, that the design elevation be at least two (2) feet above the natural highest adjacent grade (HAG) to be considered for that credit.Verified against the official source
• That Section II be signed by a registered professional engineer or architect licensed in the jurisdiction where the building is located.Verified against the official source
• That Section III be based on finished construction and be signed and sealed by a land surveyor, engineer or architect authorised by law to certify elevation information.Verified against the official source

• The calculations showing the structure is designed with structural components capable of resisting hydrostatic and hydrodynamic loads and the effects of buoyancy, and that it will be watertight and substantially impermeable to the passage of water.
• The map information: NFIP community name and number, map and panel number with suffix, FIRM index date, panel effective or revised date, flood zone or zones and the base flood elevation — or base flood depth in Zone AO — with the source and datum used.
• The design and as-built elevations: dry floodproofed elevation, lowest adjacent grade (LAG) and highest adjacent grade (HAG), stating in each case whether they are natural or finished, using the same datum as the base flood elevation.

1. 1

   Step 1: What it is and why it exists

   Under the National Flood Insurance Program, dry floodproofing non-residential buildings may be permitted as an alternative to elevating to or above the base flood elevation, or, for certain flood zones, the natural highest adjacent grade. A dry floodproofed building is one designed and constructed to be watertight — substantially impermeable to the passage of floodwaters — below the base flood elevation, and with structural components capable of resisting hydrostatic and hydrodynamic loads and the effects of buoyancy. A dry floodproofing design certification is required for dry floodproofed non-residential structures and for the dry floodproofed non-residential portions of mixed-use buildings, and this form is what is used for that certification.

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   Step 2: What must be considered before designing

   The form itself carries a planning warning worth reading before hiring anyone. Before a dry floodproofed building is designed, numerous planning considerations must be addressed: flood warning time, uses of the building, mode of entry to and exit from the building and the site in general, floodwater velocities, flood depths, debris impact potential, flood frequency, and any other state and local requirements. All of that to ensure dry floodproofing is a viable floodplain management measure at that site, and not a decision made by elimination.

3. 3

   Step 3: The three numbers that decide the credit

   Here is the part that moves the most money. The programme’s minimum requirement is to dry floodproof the building to the base flood elevation. However, to comply with the ASCE 24 flood resistant design and construction standard, one foot is subtracted from the dry floodproofed elevation; therefore the building must be dry floodproofed to one foot above the base flood elevation to be considered for floodproofing credit. And for B, C or X zones, the dry floodproofed design elevation must be at least two feet above the natural highest adjacent grade to be considered for that credit. Section III’s note repeats those numbers adding zone D to the B, C and X list; we reproduce both notes as printed and do not choose between them.

4. 4

   Step 4: Residential goes by another form

   The form says it plainly and prevents an expensive mistake. Dry floodproofing a residential building does not alter a community’s floodplain management elevation requirements or affect the insurance rating, unless the community has been issued an exception by FEMA allowing dry floodproofed residential basements. Permitting a dry floodproofed residential basement requires a separate certification specifying that the design complies with the local floodplain management ordinance. And for the residential portions of a mixed-use building, the applicable form is FEMA 206-FY-21-122, the programme’s residential basement floodproofing certificate.

5. 5

   Step 5: Section I: the map

   The first section captures the flood insurance rate map information: NFIP community name and identification number, county, map and panel number with suffix, FIRM index date and panel effective or revised date, the flood zones and the base flood elevation — in Zone AO the base flood depth is used — stating the source of that datum and the elevation datum. The section also asks whether the FIRM shows a Limit of Moderate Wave Action and, if so, whether the property is in the Coastal A Zone; whether the property is in a floodway, in which case the velocity at the building location must be given; and whether it is on an alluvial fan, with the depth and velocity at that location.

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   Step 6: Section II: the design, and who signs it

   The design certification is signed by a registered professional engineer or architect licensed in the jurisdiction where the building is located. The measures incorporated into the design to meet the performance criteria are briefly listed and the calculations attached showing the structure is designed with components capable of resisting hydrostatic and hydrodynamic loads and the effects of buoyancy, and that it will be watertight and substantially impermeable to the passage of water. The professional certifies that the structure, together with its utilities and sanitary facilities, will be watertight to the indicated dry floodproofed elevation, will be substantially impermeable to the passage of water and will perform in accordance with 44 CFR 60.3(c)(3); and that all structural components are capable of resisting hydrostatic and hydrodynamic flood forces, including buoyancy effects and anticipated debris impact forces, up to that elevation. It adds a practical detail: flood damage-resistant materials are used in all areas where seepage is intended to collect inside the dry floodproofed areas, up to at least four inches above the floor.

7. 7

   Step 7: Section III: as built, and the surveyor

   The third section certifies the actual elevation. It is signed and sealed by a land surveyor, engineer or architect authorised by law to certify elevation information — that is, the circle of signers is wider than Section II’s. The benchmark used and the vertical datum are recorded, and three elevations: the dry floodproofed elevation, which must be based on finished construction; the lowest adjacent grade next to the building, stating whether natural or finished; and the natural highest adjacent grade. The height of floodproofing above the lowest adjacent grade is also recorded. The datum for the building elevations must be the same as that used for the base flood elevation, and if a conversion factor was used its source must be described in the comments.

8. 8

   Step 8: In Puerto Rico, in metres

   It is a small line and very easy to miss. When asking for the elevations used in design, specifications and construction drawings, the form says: "In Puerto Rico only, enter meters". And Section III repeats it for the height of floodproofing above grade, with a separate box labelled "In Puerto Rico only" in metres. A template filled in feet can come back for this.

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   Step 9: The copies and the criminal warning

   The form asks that all its pages and all attachments be copied for three recipients: the community official, the insurance agent or company, and the building owner. And both certifications close with the same warning: the certifier states the information represents a true and accurate determination made using the available information and data, and understands that any false statement may be punishable by fine or imprisonment under Title 18, Section 1001 of the United States Code.

The certificate goes to the corresponding permit office or participating community and to the insurer, with the owner keeping a copy: the form itself asks that all pages and attachments be copied for those three. The Planning Board publishes it in the flood insurance section of its Flood Portal, alongside the elevation certificate and its preparation guide. The form does not publish where it is filed in Puerto Rico or with which office, and this guide does not invent it.

• Floodproofing only to the base flood elevation and expecting the credit: one foot above is needed.
• Forgetting the B, C or X zone rule: two feet above the natural highest adjacent grade.
• Using this form for a residential building, for which dry floodproofing does not alter the community’s requirements.
• Certifying Section III from drawings rather than from finished construction.
• Entering elevations in feet while in Puerto Rico, where the form asks for metres.
• Using a datum different from the base flood elevation’s, or a conversion factor without describing its source.
• Signing Section II without being a registered professional engineer or architect licensed in the building’s jurisdiction.
• Submitting the certification without the calculations showing resistance to loads and buoyancy.

These are the government pages this guide is based on.

• Junta de Planificación de Puerto Rico (JP)

  Junta de Planificación

  portaldeinundacion.jp.pr.gov

• Planning Board Flood Portal

  jp

  portaldeinundacion.jp.pr.gov