For sake of discussion, let's say the gravel layer is 8 inches thick and you're going to install common 4-inch perforated drain tile pipe. You'd put in about 2 inches of gravel over the soil, then ...

Radon gas is going to pass through the coarse, open gravel, go into the white pipe and be transported to the roof via the green vertical pipe on the far foundation wall. (Tim Carter) Analysis by ...