We are using irregular flagstone to build a new patio which will extend an existing concrete patio. The concrete is mostly covered. The "new" section of the patio has been framed and filled to level with tamped pea gravel. The original plan was to set flagstone in rock dust spread over existing concrete and new gravel to create a seamless patio. We considered using mortar under flagstone on the concrete and transitioning to rock dust over the gravel but I'm afraid of cracking due to settling. Either way rock dust will be swept into joints (no mortar).

Does anyone have experience with laying flagstone seamlessly on two different surfaces? The stone is 1.5-2 inch thickness and we have ample clearance for 2+ inches of dust on the concrete. The irregular stones are roughly 12x12, 12x18. We don't need to worry about heavy rains as the concrete area is well protected but the new section will not be covered. The drainage is already set away from the concrete.

Any suggestions are welcome! I seem to find lots of pros and cons to any type of installation. Please hurry, we're ready to set the stone!

I'm going to relay the stone on mortar and fill in between the stones with mortar.

At the edge of the patio and on each step, it looks like mortar has a rounded edge, so I was going to try slapping it on and trying to replace it with the same.

Under the stones, it looks pretty dusty, sandy, but I think it might just be the old mortar that was grinding underneath the stone. I'm guessing the patio is 40 maybe 50 years old.

Any suggestions would be appreciative.