How can I make my slippery flagstone steps safer?
If you haven’t done so already, consider installing a handrail. As for the flagstones, while there are coatings designed to cover concrete and other uniform surfaces and give them a non-slip layer, we don’t recommend them. The problem with applying these coatings is that flagstones are usually individual pieces. On such a surface, the coating would crack over time. Forget spray-on products—those won’t adhere well to flagstone either.
If a buildup of moss or algae is what makes your steps slippery when wet. Give them a good power wash, and expose them to plenty of sun by cutting back overhanging branches.
Still slicker than a Slip ’n Slide? Combine fine silica sand or anti-slip additive with a high-quality stone sealer that will penetrate the natural stone. Paint on one thin coat of sealer and immediately sprinkle the sand overtop. Let it dry for 24 hours and cover with another coat of sealer. Try a test step before doing the whole flight, to make sure the sand renders the surface gritty enough—and to make sure you like the look.