Raising the grade below and around a building involves moving the structure temporarily while fill that is designed to be stable under conditions of flooding is transported onto the site and graded to a height above the floodplain.
The primary advantages of elevating a building by raising the grade are that the building remains in its original location and the building is completely protected from flood risks. Raising the grade is also a fairly maintenance-free solution, although the new landscape must be maintained and monitored to make sure there is not any severe erosion.
Although the building remains in its original location, the building’s relationship to the surrounding landscape and any significant landscape features is changed.
- While fill is brought on to the site and compacted and a new foundation for the building at the higher elevation is constructed, the building must be moved temporarily.
- While not a lot of fill may be necessary to raise the building the required amount above the floodplain, additional fill is needed to create slope stability between the new grade and the old grade. Moreover, if you don’t want the change in grade to be extremely obvious, even more fill is needed to create a more gradual slope between the new grade and old grade. The area over which fill must be spread may become quite large. If there is not sufficient area to do this, retaining walls may be required, which can be ugly, expensive, and obtrusive to neighbors.
- Fill can be challenging to acquire and trucking costs can be very expensive.
- To raise a building with a 25 ft x 25 ft footprint 3 feet above the current grade, with the maximum slope around the building for slope stability, approximately 12 truckloads of fill are required (12 cubic yards of fill per truckload).
- Raising the grade close to property lines can negatively impact local drainage patterns, which may increase flooding for your neighbors.
- FEMA does not permit elevation of buildings on fill in V zones.
- Southern Tier Central Regional Planning and Development Board: www.stcplanning.org/usr/Program_Areas/Flood_Mitigation/Floodproofing/FProof_02_Elevate.pdf
- Farnsworth Flood Mitigation Project: farnsworthhouse.org/option-a-elevate