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.


Plumbers First

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.


Subslab Utilities

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


Water supply lines must be below the frostline outside the foundation perimeter, and should be protected with split foam insulation throughout. Clearly mark all utility lines with caution tape 12 in. below grade, and transfer these markings to the surface with spray paint using color-coded caution tape: Red = Electric; Blue = Water; Green = Sewer; Yellow = Gas; Orange = Phone.
Water supply lines must be below the frostline outside the foundation perimeter, and should be protected with split foam insulation throughout. Clearly mark all utility lines with caution tape 12 in. below grade, and transfer these markings to the surface with spray paint using color-coded caution tape: Red = Electric; Blue = Water; Green = Sewer; Yellow = Gas; Orange = Phone.

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


Check the elevation of forms against a benchmark. Layout lines for slabs should be clearly marked with spray paint for the excavator.
Check the elevation of forms against a benchmark. Layout lines for slabs should be clearly marked with spray paint for the excavator.

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


In slab-on-grade construction, excavate accurately; the trench walls typically act as footing forms. The slab’s subbase should be a minimum 4-in. compacted gravel base.
In slab-on-grade construction, excavate accurately; the trench walls typically act as footing forms. The slab’s subbase should be a minimum 4-in. compacted gravel base.

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


When measuring formwork, the layout crew must understand the relationship between the edge of the form and the edge of the wall. In this case, the interior face of framing is 7 in. from the outside edge of the form.
When measuring formwork, the layout crew must understand the relationship between the edge of the form and the edge of the wall. In this case, the interior face of framing is 7 in. from the outside edge of the form.

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.

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