I think the answers to both questions depend on your climate a bit, so you should give that. KarinL

I am in the process of laying an 85 foot long flagstone path in my back yard. I have done a few of these and there is definitely a trick to laying the flagstones level. The sequence of steps involved is important: First, establish the boundaries of the path. I use 15/8" plastic bender board and 12" plastic stakes made of the same material. Drive the stakes into the ground on outside of the border every three feet (half that distance around curves). Use decomposed granite for the base. A 2"-3" thick layer is fine. Spread the base layer over weed control fabric if you're concerned about weeds between the stones. Otherwise spread the decomposed granite directly onto compacted soil that is undisturbed (non-fill). Next, lay the largest stones first, more or less down the center of the path (no leveling is done yet), and fill in the rest of the path with medium flagstone pieces. The smallest pieces can be used to fill in wide gaps near the edges but I prefer trimming the medium stones with hammer and masons chisel. The goal is about a 1 1/2", uniform gap between the stones, which is filled in later with 3/8" decorative stone of a contrasting color. Or polymetric sand, or any small gravel, colored or natural of uniform size. The gravel locks the flagstones in place against each other and against the borders. But don't fill the gaps yet, we need to level the flagstones! first! LEVELING the FLAGSTONES: I did mention that I would share a trick to doing this step! At this point, since no effort was made to make the surfaces of the stones level with each other, the path is going to be uneven vertically. You can't leave it like this! Here is the $64 Answer: Using the decomposed granite you saved from step two (you did save half the base material, didn't you?), spread the remaining DC over the tops of the stone, completely burying the flagstones. Now level this layer with a straight 2x4 the entire length of the path by sliding the 2x4 along the tops of the edge material. Ideally the highest points of the stones will be at the same height as the edging, and the flagstones will be under a layer of decomposed granite. Here is the trick seen here for the first time: Level the flagstones one at a time by digging down to find the edge and lifting the stone up completely above the path and dump the DC into the depression the flagstone came out of. Replace the stone and pick up the next one and do the same thing. After lifting each stone, dumping the material, and replacing the stone in its exact location, the tops of all the flagstones will be at the same exact level. Now all you have to do is fix any stones that rock and set the stones into the base material with a rubber mallet. It is fast and easy, and the resulting degree of co-planarity is stunning. The trick works because when the base material is leveled on top of the flagstones, the volume of base material over each stone is the exact amount that is needed underneath the stones to bring the surfaces to the point where the gratest variation in surface elevation is across the surface of the individual stones--not between the the stone and its immediate neighbors! Now go out there and apply this technique to your next flagstone pathway project. The best part is that this work just as well on existing flagstone installations where the surface has become uneven, or was never level to begin with.

DH is a hardscape contractor, I showed him your pix. They are minor cracks from moving the stone during installation. No problem unless it bothers you aesthetically as in your second pix. Your PB could replace them, but it may cost a bit if there is no extra stone on site, that's only fair to him. I like the look, it is natural stone and imperfections add character. Looks like you will have a gorgeous pool. HTH Barb

Would try and discourage you from installing flagstone, especially with mortar, over a patio that is cracked and separated. If you mortar and the crack widens or deepens it will cause separation of the flagstone above. If you mortar or glue just the edges and set the middle in sand it may work for a short time but will almost assuredly disappoint you later, with the same 1/4 section that is now sunken, progressing in its failure. The 'best practice' in this situation is to fix the underlying problem which is the existing patio itself. Fixing probably means removing the patio and re-pouring a new one before you would apply any material topically. Good luck!