Q: We love our house, which was built in stages from the late 1920s to the early '30s, but we're having problems with the grout in our flagstone paths and terraces. Whenever we try to repair it, the replacement material stays in place for only about a year.

— Jory, Peekskill, NY

A: Roger Cook replies: Grout between flagstones (typically called mortar) fails when water gets into the joint and cycles of freezing and thawing in the winter eventually pop it out. Once it happens, your only choice is to dig out all the old crumbly stuff and replace it.

Work with an old screwdriver or small pointed iron bar to dig down to the solid concrete layer beneath the patio. I'd remove all the old grout in an area, good and bad, in order to minimize the number of places where old mortar meets new; these are weak points prone to water intrusion. Brush or blow out the debris — flushing it out with water usually ends up introducing more dirt.

After an area is prepared, mix up a batch of mortar — 3 parts sand to 1 part portland cement — to a pasty consistency. Fill the joints using a grout bag (it's faster than a trowel), then pack them tight with a jointer, a bent metal rod. With a damp rag, wipe away any mortar that slopped onto the surface of the stones.

Let the mortar set for about 15 minutes or so, then go back with the jointer and smooth all the joints. This is called "striking"; if you don't do it, the joints won't be waterproof and will eventually fail.

Now you know why I like walkways that are dry-laid in sand or stone dust. It's a lot easier to sweep a little sand or stone dust into the joints now and then than it is to regrout. But short of jackhammering your entire terrace and starting over, that's not an option in this case.
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