I had to get this gas line moved for inspection so the first step was poking a hole in the subfloor. This was a bit nerve wracking because the house is equipped with radiant heat; water pipes under the floor that 150 degree water is pumped through to heat the house. Hitting one of those pipes would be disastrous. I started with a 1/4" drill and put a stop collar on it at 3/4", the thickness of the subfloor plywood. I drilled that hole, then stuck my borescope down the hole to look for pipes. As I expected, since this portion of the floor was under the old hearth, pipes we well clear so I grabbed a 1 inch hole saw and drilled my hole.