Floodproofing Methods and Standards: NFIP, ASCE 24, and FEMA

Floodproofing methods and standards fall into two federally recognized approaches: dry floodproofing, which seals a non-residential building against water entry, and wet floodproofing, which allows floodwater to flow through enclosed areas below the base flood elevation so pressure equalizes on both sides of the walls. Both approaches must meet the minimum criteria in 44 CFR § 60.3, the design requirements in ASCE 24, and the material and certification guidance in FEMA’s Technical Bulletins before a community will approve construction or an insurer will credit the work in your premium.

Dry Floodproofing

Dry floodproofing makes a building’s exterior walls substantially watertight. Waterproof membranes or specialized coatings go on exterior surfaces, and every opening — doors, windows, utility penetrations — receives flood shields, permanent gates, or sealed closures rated for the pressure of rising water. Engineers calculate hydrostatic and hydrodynamic loads so the reinforced foundation resists both moving water and the steady push of standing water without shifting or floating.

Under 44 CFR § 60.3, this method is available only for non-residential structures. The regulation gives non-residential buildings a choice: elevate the lowest floor to or above the base flood elevation (BFE), or make the structure watertight below that level with walls and structural components capable of handling flood forces.¹ Residential buildings must elevate. FEMA does not permit residential dry floodproofing except in communities granted a specific exception under 44 CFR § 60.6, and those exceptions are rare.

The regulation’s minimum is watertightness up to the BFE. For insurance rating purposes, though, FEMA only gives premium credit when dry floodproofing is certified to at least one foot above the BFE.² Falling short of that extra foot means a compliant building that still pays significantly more for flood insurance.

Passive Systems vs. Active Systems

Passive systems protect the building at all times without human intervention. Permanent flood walls, sealed utility penetrations, and automatic backflow valves belong to this category. Active systems require someone to deploy removable flood shields, close manual gates, stage pumps, or connect backup generators before floodwater arrives.

FEMA strongly prefers passive measures because they don’t depend on a trained person being available when a storm hits. When active systems are used, the building owner must develop a flood emergency operations plan approved by local officials. That plan has to spell out who deploys the system, the notification chain, storage locations for all shields and barriers, installation instructions, backup power procedures, evacuation routes, and a schedule for annual training drills.² Under ASCE 24, the plan must be permanently posted in at least two clearly visible locations inside the building. Active systems also require a minimum of 12 hours of flood warning time. If your area doesn’t reliably provide that lead time, active floodproofing will not qualify.

Wet Floodproofing

Wet floodproofing takes the opposite approach. Instead of keeping water out, it lets floodwater flow freely into and out of enclosed lower areas. Equalizing pressure on both sides of the walls prevents the crushing force that can buckle a sealed foundation. This method is standard for crawlspaces, garages, and storage areas beneath elevated buildings in both residential and commercial construction.

The design must include permanent flood openings, commonly called flood vents, that allow water to enter and drain automatically. Federal regulations require at least two openings with a combined net area of at least one square inch for every square foot of enclosed space subject to flooding.¹ The bottom of every opening must sit no higher than one foot above the adjacent exterior grade. Any mechanical, electrical, or plumbing equipment in these areas must be elevated above the flood level or otherwise protected from water contact.

Engineered vs. Non-Engineered Flood Vents

Non-engineered openings are simple cutouts in foundation walls that meet the one-square-inch-per-square-foot ratio. Screens, louvers, or other obstructions reduce the effective open area, so a standard 16-by-8-inch air vent does not actually provide 128 square inches of flood opening. Standard temperature-activated or manually closed foundation vents are unacceptable unless they are permanently fixed in the open position.

Engineered flood vents are designed, tested, and certified to open automatically during a flood. They must carry ICC-ES certification (specifically AC-364 for mechanically operated units), provide a minimum unobstructed opening of three inches to allow debris to pass, and permit water to flow in both directions. Their rated coverage area is typically much larger than their physical size, so fewer units protect the same enclosed space. Upfront cost is higher; reliability and automatic operation are the payoff.

Flood-Damage Resistant Materials

Every material used below the base flood elevation must survive direct, prolonged contact with floodwater without significant damage. FEMA’s Technical Bulletin 2 (updated in 2025) classifies building materials as either “Acceptable” or “Unacceptable” for use in flood-prone areas, replacing the older five-class rating system.³

Acceptable structural materials include concrete (cast-in-place or precast), concrete block, brick, cast stone in waterproof mortar, cement board, structural glazed clay tile, marine-grade or preservative-treated plywood, and solid decay-resistant wood. Acceptable finish materials include ceramic and porcelain tile set in mortar, metal cabinets, glass blocks, closed-cell spray foam insulation, epoxy flooring, rubber sheet flooring with chemical-set adhesives, and latex or oil-based waterproof paints.³

Standard drywall, fiberglass batt insulation, particleboard, and carpet fail the test. Installing them below the BFE is one of the fastest ways to accumulate unnecessary damage after a flood, and it can jeopardize compliance. If a contractor suggests standard paper-faced gypsum board for a below-BFE wall, that is a red flag. Only non-paper-faced or fiber-reinforced gypsum products qualify.

The Federal Rules Behind the Standards

The regulatory framework operates on three levels. 44 CFR § 60.3 sets the NFIP minimum floodplain management criteria that communities must adopt to participate in the program.¹ These are federal floors; your local community may impose stricter requirements.

ASCE 24, published by the American Society of Civil Engineers, provides detailed flood-resistant design and construction requirements intended to meet or exceed NFIP standards.⁴ It is adopted by reference in the International Building Code and most state building codes, which means compliance is often legally required even where a local floodplain ordinance only references NFIP minimums. ASCE 24 frequently sets a higher bar than the CFR, specifying Design Flood Elevations that may exceed the BFE.

FEMA’s Technical Bulletins translate the regulations into practical engineering guidance. Bulletin 1 covers flood openings in foundation walls, Bulletin 2 addresses flood-damage resistant materials, and Bulletin 3 lays out the requirements for dry floodproofing non-residential and mixed-use buildings.⁵ The bulletins are not themselves regulations, but they represent FEMA’s official interpretation and usually serve as the tie-breaker during a compliance review.

When Renovations Trigger Full Compliance: The 50% Rule

Under the NFIP, if you repair or improve a building and the cost equals or exceeds 50% of the structure’s market value (not counting the land), the project qualifies as a “substantial improvement” and the entire building must be brought into compliance with current floodplain standards as if it were new construction.⁶ The same threshold applies after flood damage: if restoring the building to its pre-damage condition would hit 50% of its market value, that is “substantial damage” and full compliance is mandatory.

For a non-residential building, full compliance means either elevating the lowest floor to or above the BFE or implementing a certified dry floodproofing system. For a residential building, elevation is the only option. The local floodplain administrator makes the determination, and there is no grandfather clause. A 1960s building that has sat in a flood zone for decades faces modern construction standards the moment the 50% threshold is crossed.

Two narrow exclusions exist. Repairs required to correct pre-existing health, safety, or code violations identified by a local official don’t count toward the 50% threshold, as long as the work is the minimum necessary. Alterations to designated historic structures are also excluded, provided the changes don’t disqualify the building from its historic designation.⁶ Outside those two situations, every dollar of improvement counts.

Required Certifications

Any non-residential building using dry floodproofing instead of elevation must file a Floodproofing Certificate, currently FEMA Form FF-206-FY-22-153 (which replaced the former Form 086-0-34).⁷ A licensed professional engineer or architect must sign the form, certifying that the design and construction methods make the structure watertight and capable of resisting anticipated flood loads in accordance with accepted engineering practice. The certificate records the flood zone designation, the BFE at the site, and the elevation to which the building is floodproofed.

The form carries legal weight. Providing false information on any federal form falls under 18 U.S.C. § 1001, which makes materially false statements to a federal agency punishable by up to five years in prison.⁸ Fines can reach $250,000 for individuals under general federal sentencing provisions. An engineer who signs without verifying the construction also faces professional license consequences.

In most jurisdictions, an Elevation Certificate (FEMA Form FF-206-FY-22-152) is required alongside the Floodproofing Certificate. The Elevation Certificate documents the actual elevation of the building’s lowest floor relative to the BFE, while the Floodproofing Certificate addresses the watertight design of the structure itself.⁷ Building codes typically require the Elevation Certificate before any vertical construction above the lowest floor. Both documents feed the insurance underwriting process and must be provided to your flood insurance agent to set your premium.

Ongoing Operation, Maintenance, and Local Approval

Installing a floodproofing system is not a one-time project. FEMA post-disaster assessments consistently find that poorly maintained systems fail when they are needed most. Degraded gaskets on flood shields, corroded backup sump pumps, and cracked coatings that went unrepaired are among the most common failure points.²

Building owners should maintain an inventory of all floodproofing components and their locations, inspect flood shields and rubber seals at least annually, check waterproof wall coatings for cracks, and test sump pumps and backup power systems. For active systems, the required emergency operations plan must schedule regular deployment drills — at minimum once per year — and provide for periodic review and update. After any actual flood event, an immediate inspection by a qualified professional determines whether components need repair or replacement before the next event.

Completed certification forms go to your local floodplain administrator or municipal building department. A pre-construction inspection covers the site plan, proposed materials, and design specifications. A post-construction inspection confirms that flood shields, vents, sealants, and structural reinforcements were installed as designed. Following a successful final inspection, the local official issues a compliance letter or certificate of occupancy. Keep copies of all approved plans, inspection reports, and certificates permanently. You will need them for insurance renewals, property sales, and any future renovation permits.

If your property cannot feasibly meet the standard requirements, you can request a variance from the local floodplain board. The bar is intentionally high: you must show that denial would create exceptional hardship unique to the land itself, and that the variance is the minimum relief necessary. Within a regulatory floodway, no variance is allowed if it would cause any increase in flood levels during a base flood event. The community must notify you in writing that building below the BFE will dramatically raise your flood insurance premiums, potentially as high as $25 per $100 of coverage.⁹

Insurance Credit and Costs

A properly certified dry floodproofed non-residential building can receive NFIP premium credit based on the level to which floodproofing is certified, but only if that level reaches at least one foot above the BFE.² Buildings certified to exactly the BFE or below pay higher rates. Communities that participate in FEMA’s Community Rating System can earn additional premium discounts from 5% to 45%, based on the community’s overall floodplain management efforts.¹⁰

Costs vary. Professional engineering fees for completing the Floodproofing Certificate alone typically start around $600 and can reach $2,000 or more for complex commercial buildings. Add waterproof membranes, flood shields, engineered vents, backup power, and flood-resistant materials, and a full commercial dry floodproofing project can run well into five figures. Municipal floodplain development permit fees generally range from $50 to several thousand dollars depending on the jurisdiction.

Individual property owners cannot apply directly for FEMA’s Building Resilient Infrastructure and Communities (BRIC) grants, but you may benefit from a project sponsored by your local government or tribal authority.¹¹ Your state or territorial hazard mitigation agency can tell you whether any flood mitigation projects in your area are accepting participants.

1
eCFR. 44 CFR 60.3 – Flood Plain Management Criteria for Flood-Prone Areas
2
Federal Emergency Management Agency (FEMA). NFIP Technical Bulletin 3 – Requirements for the Design and Certification of Dry Floodproofed Non-Residential and Mixed-Use Buildings
3
Federal Emergency Management Agency (FEMA). NFIP Technical Bulletin 2 – Flood Damage-Resistant Materials Requirements
4
American Society of Civil Engineers. Protecting Structures in Flood Areas – ASCE 24-24
5
FEMA. National Flood Insurance Program Technical Bulletins
6
FEMA (Federal Emergency Management Agency). NFIP Substantial Improvement/Substantial Damage Desk Reference
7
FEMA. National Flood Insurance Program Underwriting Forms
8
Office of the Law Revision Counsel. 18 USC 1001 – Statements or Entries Generally
9
FEMA (Federal Emergency Management Agency). NFIP Floodplain Management Requirements – Unit 7 – Ordinance Administration
10
FEMA. Community Rating System
11
FEMA. Building Resilient Infrastructure and Communities (BRIC) Program Funding Opportunity for Fiscal Years 2024-25