RENEE SMOKER, Indianapolis

A. The mold (which is a fungus) formed because the walls and floor are wet. They got wet from the leak and from condensation of water vapor on a cool surface. You cannot paint over it. First you have to kill the fungus by treating the walls and floor with a solution of one part bleach and three parts water. After this treatment, rinse thoroughly before painting, because paint and bleach do not mix. If you paint, use a solid color latex stain instead of paint. One or two coats will do; no primer is needed.

All this work will go for naught unless you ventilate that basement, to allow water vapor to escape. Open the windows and keep them open from April to October. If there are not enough windows or none at all, make some vents in the wall. Since your wall is concrete blocks, just chip out one block, as high up the wall as possible, and insert a louvered vent. Put several of these in; two in each wall is good, especially for cross-ventilation. They can be covered in winter.

Q. I have lichen on my roof, little light green-gray growths in several places. How can I remove it? Will it cause harm if left in place?

DAVE MYNOTT, Gloucester

A. I think it is best left as is, because it may be impossible to remove it from the asphalt shingles without wrecking the roof. I tried scraping lichen off a rock, and it was virtually impossible, or at least not worth trying. Left in place, it is unlikely to cause decay, unlike moss, algae or mildew, which could cause decay.

Just for the heck of it, try treating the lichen with straight liquid chlorine blaach and see what happens. I doubt if much will, because I tried that before I tried scraping it, and nothing seemed to affect the lichen. Even if the lichen is killed by the bleach, it still might be difficult to remove. Wear skin and eye protection when working with bleach.

One other thing that occurs to me that might work for you: Soak the lichen with vinegar. Repeat if you find it works to remove or kill the lichen.

Q. A maple dining set has a bad case of mold after sitting in a damp cellar this summer. Is there any way to remove it without harming the finish? Or is it necessary to have all pieces refinished professionally. I would like to clean it myself.

ELLIE CRONIN, Melrose

A. There is no need for refinishing, professionally or by you. Indeed you can remove the mold or mildew yourself without harming the finish. Make a mix of 1 part bleach and 3 parts water, and paint this on the areas where you see or suspect mold. The solution will kill the mold and remove it. And it would not hurt the furniture to apply the solution to all finished and unfinished surfaces.

After this treatment, get the pieces out of the cellar, or ventilate the cellar by opening windows. You want to keep the furniture dry; as it is, water vapor is condensing on the furniture and growing mold.

Another thing you might try is Lysol disinfectant spray, which claims to fight mildew and also tends to be a drying agent, reducing moisture on which mildew thrives. I'm going to check that out, but I think the contents are mostly alcohol, which is a drying agent. Alcohol also might kill mildew, but that's something I'm going to find out for myself.

Lichen. It is driving some people bananas. It is a light greenish-gray growth that appears on rocks, trees, and, most important, roofs. It is a combination of algae and fungus, is hard as nails, and does not respond to bleach, the usual treament for algae (the green growth seen on roofs, siding, and brick and paved patios) and fungus (mildew), seen in similar places. Lichen must be brushed off vigorously, so is impossible to remove from the rather soft asphalt shingles without destroying the shingles.

That was the problem of H.S. of North Andover, who asked the unanswerable question two weeks ago.

Well, H.S., you have your answer, but no one will be happy about it.

Allen Hill, a preservation architect of Winchester, called to say the lichen is encouraged by overhanging and nearby vegetation, and the only answer is to get rid of any trees around the house and put up new roof shingles. The trees provide shade and prolong moisture on a roof, making an ideal environment for the growth of lichen. And with new shingles and no nearby trees, the lichen is less likely to grow.

``I've seen lichen growing four or five inches high,'' said Hill. ``They look a little like model train trees. And moss, on Cape Cod particularly, can grow so big that it looks like the big shakos of British soldiers.''

Thank you, Allen Hill; you have scored again.

The solution is radical, so the homeowner might be able to avoid some cutting of trees by cutting back branches and see what happens.