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Re: Foundations - Floating Slab vs. Slab on GradeFrom: info@builderswebsource.com
Category: Foundations and Basements
Remote Name: 207.82.209.233
Date: 01 Nov 2002
Time: 08:29 PM
CommentsYou ask about the cost differences between a floating slab and a slab on grade. First, these terms are often used interchangeably and can mean different things to different people. Ultimately what dictates the TYPE of foundation you need is based on deliberate soils and geotechnical engineering reports. For example, in highly expansive soils, slabs must be specially engineered with piers or super structural reinforcement to withstand soil upheaval. Some people define a floating slab as one which is poured and isolated from the footings. This allows the slab to move relative to the footings. A monolithic slab is poured together with the footings. Upheaval could cause the slab to rupture if not properly reinforced. Since the costs depend on so many factors, such as slab thickness, size and spacing of steel reinforcement, you can only compare based on actual structural designs specific to your site. Since the foundation is the critical structural base of your home, never cut corners on this critical part. Do it right. Make sure you have good drainage as part of the plan. And hire a geotechnical firm to recommend a suitable foundation given the soil conditions in your area. It's money well spent. If you decide to go with a floating super-structural slab, the following excerpt from Geotech Engineering and Testing in Texas, a geotechnical services company, provides some good guidelines, "The risk on this type of foundation system can be reduced significantly if it is built and maintained with positive drainage and vegetation control. No piers are used in this type of foundation. Many of the lightly-loaded structures in the state of Texas are built on this type of foundation and are performing satisfactorily. In the areas where trees have been removed prior to construction and where expansive clays exists, these foundations must be significantly stiffened to minimize the potential differential movements as a result of subsoil heave due to tree removal. The beauty of this foundation system is that as long as the grade beams penetrate a minimum of six-inches into the competent natural soils or properly compacted structural fill, no compaction of subgrade soils are required. The subgrade soils should; however, be firm enough to support the floor slab loads during construction. The structural engineer should design the floor slabs such that they can span in between the grade beams. The subsoils within which the grade beams are placed must have a minimum shear strength of 1000 psf and a minimum degree of compaction of 95 percent standard proctor density (ASTM D 698-91) at a moisture content within ±2% optimum moisture content."
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