How much topsoil has to be under sod

I wouldn't advise sodding OR seeding over concrete unless the concrete is 12 or more inches down. Ole SOL that warms everything else on this rock will definitely warm soil 1-2" thick over concrete one heck of a lot faster than over 12" of any kind of soil only. It will literally cook the bejesus out of sod and dehydrate plain old seed to just dust in a day or two. I speak from experience. I took out an old crumbling brick BBQ/fireplace for my father and was pooped at the end of the day. I saw that Dad had originally poured a 6" concrete base for this monster and I had no desire to try to break it up and haul it away. I dumped about 4" of new black pulverized top soil on top of it planted much seed and starter fertilizer and watered like Hell for a week. All I got was a nice black cooked dry square of 4" deep black topsoil. I put down less than day old thick fresh sod, fertilized it and watered like Hell for a week. It looked good and showed signs of taking hold. It was alright that season, but the next spring when everything else greened up I had this nice square brown patch of dead year old sod. After finally digging up and breaking up the concrete and hauling it away I put in more dirt (plain old field soil) planted seed with fertilizer and he had green grass forever.