Using Plastic Sheeting to Keep Floodwater Out
Plastic sheeting is a versatile flood barrier. Using plastic sheeting in a sandbag levee improves the levee’s performance significantly.
Dry floodproofing seals a building permanently so water will not enter. Plastic sheeting can be used against the outside walls to provide similar temporary protection. The plastic can be supported on structures built out from the building, so landscaping around the building is not displaced and the building does not feel the force of water on its walls.
It is necessary to block doors, windows, small drains and other openings and to prevent sewer backup when you make a temporary flood barrier with plastic sheeting.
When plastic is supported on the building, the wall is subjected to the unbalanced force of water on one side. Properly constructed walls in good condition should be able to withstand
the pressure of three feet of water, but buildings poorly constructed or suffering from decay or termite damage may not.
Depending on the duration of flooding and the ease with which water flows through the soil, the slab may be exposed to buoyant (upward) force. In a flooded building, this is balanced by the weight of water above the slab, but when flood water is excluded, the unbalanced buoyant force may cause damage. Wrapping is most suited to areas with clay soils, where floods are short in duration and where flood levels are below 3 feet.
Using a temporary wrap avoids any moisture problems that may be associated with permanent coatings, films and other dry floodproofing materials.
Buildings on piers or pilings are more difficult to wrap than buildings on slabs. Plastic supported on barriers other than the wall can be used to protect such buildings.
The principal materials required to wrap a slab-on-grade building are 6-mil polyethylene, plywood and some 2x4 lumber, and sump pump(s). Cost increases significantly if you hold the plastic away from the building on a temporary structure and even more if you have to pay for labor.
You can make a support system out of wood, aluminum, steel or other material. It must be strong enough and anchored well enough to withstand the force of the floodwater. Consult an engineer and confirm the adequacy of the design and materials you plan to use.
Commercially constructed barrier systems with heavyweight plastic and a steel support framework come in heights ranging from 3 feet to 10 feet. For homes, something in the 3-foot range would be appropriate and safe. Find manufacturers on the Internet to get quotes, knowing what length of barrier you need. Some systems can be rented for a fee that includes installation and post-flood removal.