cement or DG as substrate for patio with quartzite flagstone

by Judith Bell
(Novato, CA, USA)

What can you advise me about the difference between laying the flagstone in decomposed granite vs using mortar and thin set on cement? We have several areas that are contiguous that will get covered in quartzite. Some areas now have concrete with aggregate, others are dirt.
The large area that is cement with aggregate has joints that are laid in a geometric pattern that the flagstone will not replicate. I know that if there are cracks, they'll come in the mortar joints. However, if I rip up the concrete and lay the flagstone in decomposed granite with a grout mixture that is sanded grout and decomposed granite fines that are sifted, wet down, tamped down, wet again, what problems might I anticipate? Do you recommend concrete over DG? Any recommendations? thanks in advance.
Judith