Recommended: Work with LDHP to address necessary elevating requirements. Designing foundations to withstand the loads and additional forces of flood waters. Anchoring buildings to their foundation.

Not Recommended: Not anchoring properly, or not strengthening the foundation.

When elevating a building, the recommended first floor elevation is usually determined by Base Flood Elevation (BFE) or a Design Flood Elevation (DFE) for the area. BFE is the height which experts have determined has a 1% chance of being reached or exceeded in any given year. This is based on research by the Federal Emergency Management Agency (FEMA) that considers past flood elevations, the magnitude of these floods, the flood level of local bodies of water, and other factors. FEMA’s recommendation is for the lowest horizontal structural member to exceed the BFE by a minimum of one foot. BFE is used by the National Flood Insurance Program and can be obtained from Flood Insurance Rate Maps.

The Wind Speed and Elevation Map from Louisiana State University allows users to determine the BFE, ood zone, and wind speed that must be considered when designing resiliant retro ts to meet building code. Image Credit: LSU Ag Center.

The Wind Speed and Elevation Map from Louisiana State University allows users to determine the BFE, ood zone, and wind speed that must be considered when designing resiliant retro ts to meet building code. Image Credit: LSU Ag Center.

House in New Orleans that was elevated after Hurricane Katrina. Image Credit LDHP

House in New Orleans that was elevated after Hurricane Katrina.
Image Credit LDHP

Building codes sometimes use the term DFE, which can be equal to or exceed minimum BFE requirements. Check with local planning officials to see whether BFE or DFE is used in your area. The Louisiana State University Ag Center has both online flood and wind maps which allow individuals to enter their address and find flood zones, wind speed, and ground elevations. Information can be different from neighbors’ and sometimes locations on the maps can be off. So, it is important to ensure that the point indicated on the map is correct. Engineering design is always required if the house is in a “V” (velocity) or coastal “A” (moderate waves) zone.

The iron beams and wire cables are in place in preparation for raising Yucca House at Melrose Plantation, Melrose, LA. The floor was already missing when the project began. Image Credit NPS

The iron beams and wire cables are in place in preparation for raising Yucca House at Melrose Plantation, Melrose, LA. The floor was already missing when the project began. Image Credit NPS

Foundations are designed to carry the load of a building and anchor it to the ground. During a disaster the foundation is affected by multiple forces. Floodwaters and debris in the floodwater place pressure on foundation walls, piers, piles, or posts. Water can also saturate soil creating additional forces on the foundation. Raising a building to elevate the first floor above BFE or DFE can also allow for strengthening of the foundation to better withstand these forces. When a foundation collapses or a building is washed off of its foundation, greater recovery efforts, additional expenses, or even catastrophic failure result.

Raising the elevation of a building lessens damage from flooding. The Louisiana Division of Historic Preservation and FEMA have information available on elevating homes. The key to getting the best results is to work with a professional architect or design consultant with experience working with historic buildings and a professional engineer to make sure the foundation is designed to withstand flood forces. Hiring an experienced contractor who has successfully completed similar projects will also increase chances of success.

Wood frame home with cribbing visible in the process of being raised after Hurricane Katrina. Image Credit NPS

Wood frame home with cribbing visible in the process of being raised after Hurricane Katrina. Image Credit NPS

When a home is elevated, steel beams are placed under the first floor to support the building and jacks are used to raise the house. A new foundation is constructed which should meet current building codes and be able to withstand the load of the home and forces from a flooding event. Exterior material such as brick or stucco veneer may need to be removed before raising the building. Additions to the original building may need to be raised separately. Heavier, taller, or complex shaped houses may make these projects more complicated and increase costs.

House in New Orleans, LA that was elevated after Hurricane Katrina. Image Credit LDHP

House in New Orleans, LA that was elevated after Hurricane Katrina. Image Credit LDHP

Houses that are already raised off the ground (buildings with a crawl space) are easier to lift above flood levels. Buildings constructed on a concrete slab are more difficult to elevate and may need additional support or a new floor. To raise a house built on piles, it must first be moved out of the way to allow access for machinery needed to drive, jet, or auger the new piles. New foundations should be designed by a professional structural engineer who will ensure that loads are accurately calculated and foundation wall and sill plate connections are designed correctly. The new foundation must meet current codes and structural systems must be tied together, from the roof to the foundation, to reduce risks from future events.

References

Disaster Mitigation for Historic Structures, 1000 Friends of Florida

Engineering Principles and Practices for Retrofitting Flood-Prone Residential Structures (Third Edition), FEMA

The History of Building Elevation in New Orleans, FEMA

Additional Resources

Home Builders Guide to Coastal Construction: Technical Fact Sheet Series, FEMA

Homeowners Guide to Retrofitting: Six Ways to Protect Your Home from Flooding, FEMA


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