Slab-on-grade foundations are one of the most common foundations in the U.S. because they can be adapted to perform well in a wide range of climate zones and difficult soil conditions.
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Site Layout for Structural Slabs
Site Layout for Slab Foundations
When framing a floor system, there is a little latitude in both the timing and installation of the utilities. But when building on a slab, the utilities must be defined precisely before any concrete is poured.
When starting layout, make sure the plumber is the first trade on site. Let other subs work around him — it’s a lot -easier for an electrician to work around obstructions than it is for the plumber.
Before excavation begins, work through the plumbing plans to define all drain/waste and supply lines and vent stack locations, and review plans for all utilities running below the slab, such as electric, gas, and phone lines.
After the service lines have been installed, all plastic or copper lines should be properly bedded (clean fill, no rocks) and tested for leaks prior to backfilling. All copper should be checked for dents or abrasions and wrapped in split-foam insulation, or something similar, to protect it.
Install caution tape: Make sure every trade buries color-coded caution tape 12 in. below grade and in line with their work (Figure A). These plastic caution tapes are color coded: red for electric, blue for water, green for sewer, yellow for gas, and orange for phone.
Figure A: Utility Trench Infill
Mark location of new work: Use spray paint to color-code the location of new work after backfilling trenches. It’s easy to forget where lines run a few days after a trench has been backfilled. Don’t leave anything to memory: Before the concrete trucks show up, make notes either on the drawings or in a site log.
Slab Layout Checklist
Keep careful track of these critical layout details for structural slabs:
Benchmark: A slab excavation should be as level as possible. Set an elevation benchmark prior to excavation. Place the benchmark somewhere convenient and make sure everybody on the site knows where it is (Figure B).
Figure B: Structural Slab Layout and Formwork
Concrete formwork: Form monolithic slabs as shown in Figure C. Double-check forms, verifying that they are located properly, both in relation to the corner points the surveyor set prior to excavation and in relation to the elevation benchmark.
Figure C: Forming a Structural Slab
Reference lines: Mark on the edge of forms one reference point from which each subtrade can take dimensions and work. A string line across the forms is ideal. Locate this reference line at some feature of the building that cannot change, such as the longest bearing wall. Make sure that all trades clearly understand how this line corresponds to their work.
Edge of slab/wall: Take time to understand the -relationship between the edge of the form and the edge of the wall. Explain it to the plumber and other subs (Figure D).
Figure D: Slab/Wall Relationship
Penetrations: Double-check all penetrations before the slab is poured. These include toilet flanges, drain boxes, turn-ups for water, electric, gas, phone, and floor drains.