Flooding is one of the most common and costly natural disasters to which our country is exposed. In the past five years, every state in the United States has flooded and 21 states have experienced frequent flooding events, according to Beverly Cigler in her book U.S. Floods: The Necessity of Mitigation. Altogether, from 1980 through 2019, the United States has sustained 258 weather and climate disasters, for which overall damages and costs reached or exceeded $1 billion, with the total cost of these 258 events exceeding $1.75 trillion.
More recently, Hurricane Harvey damaged or destroyed more than 200,000 homes and businesses, with massive flooding from rainfall that reached 30 to 50 inches across the storm's path. And Superstorm Sandy (considered a significant flooding event) caused more than 305,000 homes, decks and porches to be significantly damaged or destroyed.
The U.S. has sustained 258 weather and climate disasters from 1980 through 2019.
Unfortunately, homeowners insurance typically does not cover flood loss costs. While a National Flood Insurance Program (NFIP) policy covers up to a certain amount, there is less than a 50 percent "take-up" rate for these policies. For example, 80 percent of the homeowners with flooding caused by Superstorm Sandy did not have an NFIP policy.
Those who live in flood hazard areas are left with the following options:
- Relocate out of the flood hazard area
- Elevate the building above current design flood elevation requirements in anticipation of future events
- Build to the required guidelines to prevent flotation, collapse or permanent lateral displacement from flooding
According to a National Climate Assessment study, more than $1 trillion of property and structures in the United States are at risk of inundation from climate-caused sea level rise of 2 feet above current levels, which could happen as early as 2050. For more information, visit the National Climate Assesment website: nca2014.globalchange.gov
To improve the resiliency of buildings and structures in flood hazard areas, it's important to identify where the current design flood elevation (DFE) is on the structure because construction standards are different for the parts of the building, deck or porch at or below the design flood elevation. The DFE is always the base flood elevation (BFE) plus freeboard (a factor of safety).
Also, there are different criteria for the type of flood hazard area (A Zone, Coastal A Zone or V Zone) in which the building or structure is located.
To locate the flood category for a property, go to www.floodsmart.gov, click the FEMA Flood Map Service Center and enter the address. Local flood insurance rate maps can provide the BFE, and the freeboard is usually 1 or 2 feet for homes, but it may be higher depending on the adopted freeboard for the jurisdiction.
Whether the building or structure is following the guidelines of the NFIP Section 60.3(a)(3), the International Residential Code® Section R322.1.2, or ASCE 24-Flood Resistant Design and Construction Section 1.5.1 (ASCE 24 applies to all buildings and structures using the International Building Code®, and all buildings and structures in floodways), the following principles apply:
If a proposed building site is in a flood-prone or flood hazard area, all new construction and substantial improvements shall be:
- Designed (or modified) and adequately anchored to prevent flotation, collapse or lateral movement of the structure resulting from hydrodynamic and hydrostatic loads, including the effects of buoyancy
- Constructed with materials resistant to flood damage
- Constructed by methods and practices that minimize flood damage
Buildings and Homes
Providing a continuous load path is essential to resisting the hydrostatic, hydrodynamic, wave and impact forces from water and objects carried by the water. In addition, flooding often comes with high winds associated with strong thunderstorms and hurricanes, making all the links in the load path important. For flood-resistant construction, focus on links 6, 7 and 8 (see illustration below).
Inspectors can review numerous parts of the building when it comes to elevated or raised foundations, including whether or not HVAC units are elevated above the design flood elevations, whether floor framing is connected to the support beam and whether there's a connection from the support beam to the pile, pier or foundation.
Due to failures that occurred during Superstorm Sandy, where the connection was too close to the edge of the wood, there are new details recommending that the strap connect above the centerline axis of the joist and below the centerline axis of the beam. See details and pictures to the right.
In buildings and homes with solid foundations (not permitted in V Zones or Coastal A Zones when the design is governed by ASCE 24), inspectors may want to look for the following:
When a building has a solid foundation in an A Zone, check to see that water is able to flow through the building and meets the minimum requirement for openings. If they are non-engineered openings, the net open area shall be at least 1 square inch for each square foot of enclosed area, not less than 3 inches in any direction and account for louvers, screens, faceplates or other covers so as not to impede the floodwaters.
In addition, if the building has a crawl space or if you are able to get underneath the first floor joists, look to see that everything below the top of the first floor joists is connected to the wood member or foundation below.
Starting with the sill plate and the foundation, check to see if the sill plate is connected to the foundation and if the rim joists, floor joists or both are connected to the sill plate or foundation. The photos below show failures of the framing to the foundation.
Similar to elevated buildings, all the links in the load path must be strong enough to resist the flood waters. See diagram below.
Decks and Porches
Buildings (that is, structures that include two or more outside rigid walls and a roof) and their contents may be covered under a NFIP policy. However, property and belongings outside the building envelope are generally not covered. Therefore, when a deck or porch is destroyed by a flood, the building owner or homeowner would have to pay for the repair or rebuild.
As with homes and buildings, it is essential that decks and porches have a continuous load path to resist the hydrostatic, hydrodynamic, wave and impact forces from water and objects carried by the water.
For flood-resistant construction of decks and porches, focus on links 2, 5, 6, 7 and 8 (see diagram below).
In addition to supporting the download of the deck and occupants, the footing must resist the lateral load of occupant movement and flood waters, and must be deep enough to resist scour associated with floods. In V Zones, the referenced standard (ASCE24) states that supports shall extend 10 feet below mean water level to account for the potential of significant scour. However, because an inspector can't inspect for footing or support embedment, the first key point to look for is the post-to-footing or foundation.
Inadequate porch or deck post connections are common failures during a flood event. Once the posts are "washed away," there is nothing to prevent the porch or deck from collapsing.
Codes state that stairways and ramps shall be designed and constructed to break away without causing damage to the building or the structure, or shall be designed and constructed to resist flood loads and minimize transfer of flood loads.
Inspectors may want to check whether the stairs are connected to the structure, deck or porch to determine if they could cause damage if they separate during a flood event. In addition, for stairs to meet all the other requirements and all the flood-resistant construction guidelines, the most economical way is for them to be independent of the building or structure. See photos and labels below.
One of the most common violations for stairs is the riser opening. The flood requirements are for the riser to be open or partially open to allow the flood waters to pass through the stairs. In addition to being open, they must not allow the passage of a 4-inch sphere. Typically, they are either completely open (see photos below) or completely closed (see photos below).
FEMA TB-2, which is referenced by the codes for flood-resistant materials, states that "performance of buildings that are exposed to flooding is, in part, a function of the fasteners and connectors used to put the components together. If corrosion occurs, buildings are less likely to withstand flood loads and other loads." So, if you can identify red rust on the connectors, fasteners or anchors, it should be reported.
Based on ASCE 24, exposed material below the design flood elevations shall be stainless steel or hot-dip galvanized after fabrication.
Inspecting homes, buildings, decks and porches to the requirements of flood-resistant construction will provide your client with the peace of mind that, after a flood, they'll be able to go back to their home or building with minimal repairs needed.
Later in 2020, Simpson Strong-Tie will debut two new online courses requirements for buildings, homes, decks and porches in flood hazard areas. Visit www.strongtie.com for more information.
There is much to know about the resiliency of buildings and structures in flood hazard areas. Inspectors may want to do their own research to gather more information on this complex subject.