We were called to address this walkway which was starting to fall apart. Several stones were loose, including some at the edges that were becoming dangerous. In the image below you’ll see a flower pot blocking an area where a stone had completely flipped free. In my experience, it’s hard to fix mortared flagstone when it starts breaking down. Patching usually looks terrible and is only ever a short term fix. Mortared flagstone fails because water has gotten into the system; once in, the water is impossible to get out.
I was able to persuade the homeowners to switch to a drystone installation. We removed all the stone- after pressure washing it all first. We were able to brighten the colors and salvage enough stone to redo the walkway without purchase any new materials. Instead of rebuilding the mortared steps, we switched to sandstone slabs from Tennessee. The gray slabs blend wonderfully with the Pennsylvania flagstones. The steps are all single slabs, six feet across. It was a stroke of luck to find so many good stones at such a consistent thickness. The steps walk great.
We reset the flagstone in a bed of gravel. The steps and the surrounding soil hold the gravel so it will not squish out the sides. The gravel bed drains moisture away which will prolong the life of the stones. When we set dimensional flagstone like this, we abut the stones as closely as we can; there’s no need for a grout joint (because there’s no mortar or grout!) and I think it looks better.