Rising Above the Floods: The Slab

Patricia Skinner, Baker, Eugene  |  8/15/2016 5:43:09 PM

The Slab

Ground-level building slabs in Louisiana are from 4” thick for residences to 6”-8” thick for light commercial structures. A slab is poured as a single piece (monolithic) with thicker areas, called footings and grade beams, around the edges and under load-bearing interior walls. In residential construction, especially where there are no building codes or where codes are not enforced through an inspection system, slabs may not be reinforced very well.

The weight of the house, including walls, roof and interior furnishings, is transferred to the slab. The slab transfers the weight of the building, and its own weight, to the ground. When you lift a slab off the ground, you change the way the slab is supported. It takes a professional engineer and an experienced structural mover to do this correctly.

Take the slab, or leave it?

Your house was designed to be supported by a slab, so it is intuitive that an elevation method which retains the slab will have the least structural impact on the house itself. Brick veneer and fireplaces are typically held up by the slab. Floor covering, toilets, bathtubs and built-in cabinets are either attached to the slab or depend on it for support. When you take a house off its slab, you lose some structural items such as tile floors. Other items, such as toilets, have to be reinstalled. Everything must be removed from the house, and repairs will have to be made to holes in exterior and interior walls where bracing was used.

By contrast, a house raised on its slab is completely sealed against the elements. Furniture, appliances and personal belongings remain undisturbed. There’s no need to move or store anything. It’s even possible to maintain electrical service during construction if the contractor’s insurance allows and proper grounding is established.

In raising a slab, you’ll face a different set of problems than you face in taking a house off the slab. The method used in the Denham Springs project involves removing all the dirt from beneath the structure. This may seem extreme, but the full access gained in this process allows the contractor to provide support for the slab over its entire area, including multiple slabs, varying slab thickness and separate foundations for masonry fireplaces. The method does not rely on the inherent strength of the slab.

While your house remains completely intact, your yard is turned literally upside down. Some of the restoration expense can be avoided by removing and storing sod and valuable landscaping plants before construction. The impact of rain during construction can be greater when elevating a slab than when removing the house from a slab. This is especially true in Louisiana floodplains, as explained on page 13.

When the slab is raised, or when the house is removed from the slab, the insulating effects of the earth are lost. The extent of heat loss through the slabs elevated in this project has not been quantified, but it is recommended that homeowners add insulation to the bottom of the original slab.

Slab elevation contractors have a larger investment in equipment and machinery than do most traditional housemovers. Few contractors have slab elevation expertise. As a result, there is limited competition in the market and little incentive to hold costs down.

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