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Radiant floor heat over an existing concrete pad.

Q. I want to put down 1/2" pex over an existing pad for radiant heat and pour concrete over it. In your answer to a similar question you indicated a minimum thickness of 2 1/2". Would that be thick enough overlay to do a stamped decorative concrete pattern such as ashlar slate?

A. If you put 2 1/2 inches over the tubing, which means much of the floor will be 3 inches thick, then you should be OK. Use a stamp with relatively shallow grooves. Certain patterns have deeper grooves than others. A slow-setting pea gravel stamp mix with fiber reinforcement is recommended. To minimize shrinkage, don't pour the concrete wetter than a 5 slump. A contractor or the ready mix company will know what I'm talking about. With thinner concrete, control joints should be closer together since there's greater chances for cracking, no more than 6-8 feet apart.