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Q. Last fall a contractor installed a flagstone patio for me. He carefully laid the flat stones on a bed of sand and then grouted the spaces between the stones with mortar. Well, the flagstone are separating, the mortar is cracking and the patio is a mess. The contractor wants to put a layer of cement over the sand and start over. Will this work? If I had hired you, how would you have installed my flagstone patio? - April W., Denver

A. Years ago the same thing happened to me. Like me way back then, you're going to have to start over.

First, remove and salvage the flagstone. Carefully chip off any mortar that is on the edges. Remove the sand and discard it unless you can be sure it is free of stones and dirt. If you can salvage it, much of it can be used to reinstall the flagstone.

At this point, you have two alternatives, although I am inclined to think that the first one is far better than the second.

This first method should produce a flagstone patio that can last 50 years or more without any cracks. Once you have determined the finish height of the patio, excavate soil so you can install a 4-inch-thick concrete slab, 3/4-inch-thick setting bed of cement mortar and the flagstones. Many flagstones are 3/4-inch-thick, so I would excavate to 5 1/2 to 6 inches. Compact the soil at the bottom of the excavated area.

Pour a concrete slab in the area that contains 1/2-inch steel reinforcing bars placed on 2-foot centers in both directions. This grid of steel needs to be suspended so that the steel ends up in the middle of the concrete slab. This steel has tremendous tensile strength and will prevent the slab from developing large cracks that cause the concrete to separate and tilt into separate planes.

To further strengthen the slab, dig vertical piers that are 6 inches in diameter and extend below the frost line in your area. These piers help give the patio added support. To prevent the patio and the piers from heaving if frost starts to sink into the soil, make the bottom of the piers flare to 8 inches in diameter.

When you pour the slab, it does not have to be finished smooth. This rough texture will help hold the flagstones in place when they are set in wet cement mortar.

Within 24 hours of pouring the concrete slab, start to install the flagstones. Use a mixture of Portland cement and sand to permanently attach the flagstones to the concrete. Mix one part of cement to three parts sand. Coarse sand is better for this purpose.

The mortar should be the consistency of regular bricklayer's mortar. Do not make it so dry that you have to pound the stones into place with a rubber mallet. If the mortar is just right, a small amount of pressure from your hands will float the flagstones into position. Once the flagstones have set for two days, you can fill the gaps between them with a similar mixture of medium sand and Portland cement.

A second method of installing the patio is to remove the current sand and allow it to dry or become just slightly damp. Blend one part Portland cement with three parts of this sand and reinstall the sand base. Before you install the sand back into the excavated area, generously water the soil so it is very wet. Be sure you install the sand at least 3 inches thick. Level the sand and place the flagstones onto the sand and tamp them into place. Use a garden hose and a nozzle to finely mist the flagstones so that some water flows between the stones into the sand. Do this for perhaps 10 minutes.

Wait at least one week before walking onto the patio to fill the gaps between the stones with the Portland cement and sand mortar mix. This waiting period will ensure that the sand and cement base has time to harden. The sand base will be immune to attack from ants and moles, but it will not be sufficiently strong to withstand serious frost heaving and/or other soil movement.

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