Slab on Grade
Slab-on-grade construction: the ecoHOME A to Z Slab building guide
...homes, see here .
INSTALLATION OF MECHANICAL RADON MITIGATION SYSTEMS:
Inside the perimeter of the slab, dig small trenches in the crushed gravel.
Install all mechanical equipment (plumbing, electricity, ventilation) and seal all above-ground extremities before pouring the cement.
For a video on radon barrier...