One of the most common materials for paving walks and patios is flagstone laid on a sand bed. It's beautiful, rustic surface that usually stands up to the elements for years.

But every now and them, troubles can crop up. Years of freeze-thaw cycles throw stones out of level. A set spring with temperatures that criss-cross the freezing point will tear up highways. So it's no surprise that the same conditions can throw flastones out of level. Other times, a stone breaks; or the sand between stones washes away and weeds take its place.

But fixing all these problems is simple.

First, examine the stones. Usually unevenness results when some stones are raised above the level of others, and occasionally a few stones will sink. You want to raise any low stones, and lower any that are too high.

To raise a stone, lift it out of place and spread a layer of sand under it. Even better than sand is stone dust. It packs down better and in time becomes almost as firm as concrete. Use enough sand or stone dust to raise the low stone to the level of those around it. Try to spread the sand evenly, or slightly higher around the edges than at the center. If the bed of sand is high near the center, the stone will rock on the high spot.

Dampen the sand with a mist from the garden hose, then replace the flagstone and check for fit. If necessary, add or subtract sand. Then put sand or stone dust into the joints around the stone, dampen it and pack into place with a peice of wood. Keep packing until the joints won't take any more.

Resetting a high stone requires the same procedure except that you remove excess bedding instead of adding it. After leveling the stone, pack the joints. These provide a firm bed that keeps the stone from shifting.

Broken stones are best replaced. Take a piece of the broken stone with you to your stone supplier and try to match it. (Garden centers, home centers, lumberyard and mason supply houses all have flagstone.) The replacement should be at least 1 1/2 inches thick for use on a sand bed. Thinner flags tend to break unless laid on mortar.

You may have to cut the replacement stone to fit its spot. The easiest way to do this is with an abrasive cutoff wheel in a portable circular saw.

If you don't have a saw, you can rent one, or do the job with a hammer and chisel. The chisel approach is slow and takes a bit of skill. Here's how it goes:

Using a mason's chisel and a two-pound hammer, score both sides of the stone along the desired line of cut. Sometimes you can get by with scoring only one side, but if you score both the break will be more accurate. Work on a level bed of sand while scoring.

Once you have a score line about 1/8 inch deep on both sides of the stone, the flag should break along the line. Place the stone on a piece of wood with the scrap part hanging over the edge. Tap the scrap until it drops off.

Once the stones are level again, spread sand or stone dust over the surface of the paving and sweep it into the joints. Dampen it slightly and pack it into place. A good tight pack will help discourage weeds. You can get a herbicide if weeds are a real problem, but don't use chemicals if your paving is near a well or vegetable garden.