Replies

1. GBA Editor
   Martin Holladay | Aug 17, 2012 05:36am | #1

   Leo,
   You didn't say whether the perforated drain pipe in question is inside the footing perimeter or outside the footing perimeter.

   If it is inside, then I think you are OK. If it is outside, probably not. As I understand radon mitigation systems, the perforated pipe has to be under the slab.

2. 
   Leo Ketchum | Aug 17, 2012 07:55am | #2

   Thanks Martin, the drain pipe is on the inside and is under the slab but in places it is approximately 4' below the slab. This is the whole story; I have a foundation with 3 footing elevations, for a building that is being built on a hill with a walk out. Toward the walk out of the building the footing and drain is 4' below the slab as you walk in further the footing makes a transition to 2' below the slab and finally at the back of the building, deepest into the hill, the footing is just below the slab. There will be 6" aggregate below the foam-slab in the entire floor and surrounding all footing drain. But in those areas where the footing is deepest there will be filter cloth and compacted sand to make up distance between the drain pipe/ drain rock and the rock below the slab. So in these areas the drain pipe will be below, what we always call the "cake" of undisturbed soil on the interior of the building. Hopefully this all makes sense. I am wondering if the aggregate below the slab combined with the Interior drain pipe will still function as a vent for radon gas when only the back portion of the drainpipe is with-in the bed of 6" aggregate below the slab. All the rest is deep with the footing.

3. GBA Editor
   Martin Holladay | Aug 17, 2012 08:25am | #3

   Leo,
   First of all, a footing drain is different from a radon mitigation system. Where does your footing drain terminate? If it runs downhill to daylight -- that's probable, considering your hilly site -- then a radon mitigation fan will just pull air from the opening downhill from your house. What you want instead is a separate pipe under your slab, so that the fan will depressurize the air under your slab.

   I don't completely understand your description of your crushed stone, but if the radon mitigation pipe is installed in a layer of stone that connects with the stone that is located under every area of your slab, you should be good.

4. 
   Leo Ketchum | Aug 17, 2012 08:33am | #4

   You are correct about the daylight drain. The separate drain pipe in the crushed rock is what I will do.
   Thanks for your time.