Q: Our home was built 60 years ago on a concrete slab with radiant floor heating. When we remodeled 25 years ago, we pulled up the shag carpeting and the contractor glued 6-inch-square parquet wood tiles directly to the slab.

We now have a 3-by-3-foot area in the middle of the den where the tiles are rising up off the slab. We still have original extra tiles. Is it possible to replace tongue-in-groove tiles in the middle of the room? What should we use to glue the new tiles down?

A: It will be a piece of cake to replace the loose flooring, but maybe not so easy to make sure it stays stuck.

Before you go to the trouble of patching the floor, find out why the old tiles are rising. We’re guessing the cause is a small leak in the radiant floor system that is allowing moisture to wick through the concrete and loosen the glue holding down the tiles. Fortunately, the damage appears to be limited, so fixing the problem shouldn’t be too difficult.

One would think a 60-year-old radiant heating system would be on its last legs. But the reality is that radiant heat is as old as the Roman baths. Roman heating systems were based on hypocausts (literally "heat from below"), a series of ducts under the floor and flues built into walls that allowed hot air or steam from fires to circulate, warming the floor and walls, which allowed heat to radiate into the rooms.

In a modern floor-heating system, warm water is circulated through plastic tubes called PEX that are laid into the floor. These modern pipes are virtually indestructible.

A previous generation of radiant floor heating was popular just after World War II. Levittown, the large New York development, used this heating system. Radiant floor systems then consisted of hot water from a boiler flowing through a series of steel or copper pipes formed into loops, connected to a manifold and embedded below the concrete slab.

A thin layer of sand covered the pipes to prevent contact with the concrete. Heat from the hot water radiated through the concrete and warmed the house.

Generally, these systems work very well and can last indefinitely — or in your case, 60 years. But sometimes the old copper or steel pipes spring a leak. So the first step in fixing your floor is to have it inspected by a heating, ventilating and air conditioning (HVAC) contractor familiar with radiant floor heating to determine whether the system is in good repair. If we’re correct, isolating and repairing or abandoning a leaking loop is what you’ll need to do.

Once the moisture problem has been corrected, remove the tiles that are popping up. Start from the middle and work to the edges. Plan on using more tiles than just the ones that are separating from the slab, and make sure you have enough. A few of the existing tiles may be salvageable.

Once the tiles are removed, let the floor dry for at least three weeks to allow the slab to acclimate. Then pick up a caulking gun and a tube of good-quality concrete-wood adhesive. Finally, glue down the new tiles. Begin by inserting the tongue of a replacement tile into the groove of an existing tile.

To fit the last row of tile, remove the tongue from each. Find something heavy to weigh down the patch overnight so the adhesive sets.

Six months or so down the road, if things are still looking good, consider calling in a floor contractor for a complete sanding and refinishing.

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