My flagstone patio is developing cracks at the joints. I would like to know how to patch/seal them to avoid water from sipping excessively into the ground. I understand patching is not a water proof method.

What product can be used? How do match the color of the "grout"?

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  • Matching the colour can be hardest job, but hopefully it is a common colour at near by stores. I would remove all of the loose/crack grout and redo. The local stores should the proper sealing for that grout.
    – crip659
    Commented Jul 29, 2023 at 20:53

1 Answer 1


Treat it like any other failing grout situation - use a grout saw (or possibly small chisels and a mallet or hammer for some of this) to remove the failing grout, and re-grout the whole thing (and bam, matching the color is solved.)

You could also choose to replace the grout with polymeric sand, which might be a bit more tolerant of seasonal movement of the stones, since the base is obviously not preventing all movement.

  • Removing everything on a large patio is a big job. I only have cracks in a few places. Further, I expect more cracks over time. Thus I am looking for something to fill the cracks. How can alter the color of polymeric sand? Can it gets into the smaller cracks (1st picture)?
    – Quoc Vu
    Commented Jul 29, 2023 at 23:50

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