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.
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.
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.
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.
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.
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