preparing the finished dirt elevation for a new concrete slab

by Terry
(Colorado)

Question:

I have my footers and stem walls poured, I am back filling the dirt and packing it as I go. What is the finished elevation of the dirt for under the slab?

Answer:

Hi Terry,

If your finished floor called out on plans is measured from top of concrete, the top of compacted dirt should be top of finished concrete slab less the slab thickness.

For example, if your top of slab is 100'-0" with 4 inch thick slab - the top of compacted dirt should be 99'-8".

If the elevations shown on plans are measured from top of finished tile or carpet the thickness of tile or carpet need to be added to the thickness of slab and subtract from the top of finished floor elevations to get your top of compacted dirt elevation.

Detail 5 below is an example of footing, stem wall, and slab of a basement. For structural reason the slabs at the very least should be at the top of the footings as shown in the detail.

If your slab is for a transformer pad or dumpster pad you can use the civil site plans to determine the top of slab and minus the slab thickness for your top of compacted dirt elevation.

Hope this helps
Tonga