Q. One of our ceiling tiles was damaged recently and we want to replace it with a new one. We saved several of the tiles from the original job so we have a tile that will match. The trouble is that the tiles are tongue-and-groove so they interlock when installed. It seems this means we will have to take all the tiles out starting along one wall until we get to the damaged one. Not only is this a lot more work, but we don't have enough tiles to replace that many. Is there a simpler method you can suggest?

A: There is no need to replace more than one tile. Remove the damaged one carefully by making several cuts in the center, then cutting around the edges so you can pull the pieces off without damaging the tiles next to it. Use a sharp knife to cut the tongue edges off the new tile (the part that would be hidded when the tile is in place). Apply a liberal amount of adhesive around the rim of the new tile, then press firmly in position with both hands until it is level with the tiles around it.

Q: We recently moved into a house and the previous owners told us that the septic tank had just been cleaned. Neighbors told us they were having trouble with their septic tanks and they have been using various solutions, including one with yeast. Will you please tell us how to treat our septic tank problem?

A: As near as I can determine you don't really have a problem yet; you are anticipating one from the stories you have heard. Some people believe that various solutions will help the action of a septic tank, but most tests run by government agencies and others do not substantiate these claims. A septic tank needs no attention until it fills to the point where cleaning (emptying by scooping out the contents) is required. Theorectically, if soil drainage is adequate, and if grease traps are properly installed and functioning, a septic tank of adequate size could work forever without attention. If it gives repeated trouble, even after being emptied, the drainage field probably needs replacing.

Q.Heavy rain caused two window wells to fill up and spill over into our basement. Our house is 50 feet long, with downspouts at each end. The wells fill from the bottom up. I dug them deeper, which helped but did not solve the problem. Can you suggest something other than hailing out the wells?

A: Since the wells fill from the bottom it is ground water that is causing your problem. Short of putting an automatic pump in each well, the only practical solution is to install underground drain tiles or pipes at the bottom, sloping them away in trenches and emptying them into a dry well at least 10 feet away from the house foundation.

Questions about home repair problems should be addressed to Home Improvement Department, The New York Times, 229 W. 43d St., New York, N.Y. 10036. Questions of general interest will be answered, but unpublished letters will not be answered individually.