Q. We are expanding our house by adding a new kitchen and family area. The subfloor is plywood. I plan to install ceramic tile flooring myself. Can I install the tile directly over the plywood? What is the best type of adhesive for laying tile on plywood?

A. Although it has long been an accepted practice to glue tile to plywood subflooring, you might consider some enhancements to this type of subflooring that will assure better durability for your new tile floor. In some cases, tile bonded to plywood will tend to form cracks in the grout lines and even pop loose.

The problem is caused by the flexibility of the wood--and wood's sensitivity to moisture. Wood floors tend to flex and bend under weight. But tile doesn't flex because it is rock hard. Grout, which is essentially cement, also hardens. Together the tile and grout form an inflexible sheet on top of a more flexible wood subfloor.

To assure a strong subflooring that won't flex, a special layer of plywood at least 3/8-inch thick, called underlayment, should be installed over a 3/4-inch plywood subflooring. A better choice than plywood underlayment would be to use cement backer board, 5/16-inch thick, bonded to at least 3/4-inch plywood subflooring.

Cement board consists of a layered concrete bond between two fiberglass mats. It comes in 30-by-60-inch sheets and, though heavy, it is easy to cut by scoring with a utility knife. Its advantage over a plywood underlayment is the protection to moisture it provides. It is the recommended underlayment for kitchens and bathrooms. However, wood is sensitive to moisture in other areas of your home as well. It expands and contracts according to weather conditions and the relative humidity.

Because tile and grout don't expand or contract with moisture changes, in areas where humidity is high and moisture penetrates the wood, you can easily have problems with cracking grout and popping tiles that are installed over a wood base. Cement backer board will help prevent this. It acts as a buffer, allowing the wood base to move with changing moisture conditions, while the tile remains stationary. Bonding the cement board to the wood also helps stiffen the floor, providing a better bonding surface for a latex-cement adhesive.

A solid foundation is important no matter which underlayment you use. To ensure this, your floor joists should be 16 inches on center. Anything wider will require additional reinforcement, and it would be best to consult a tile-installation professional. Also, if the floor you are tiling is more than 24 feet long, you'll need to put special expansion joints in it to keep the grout from cracking from floor movement. Often, this is best left to professionals. Consult your tile dealer.

There are two basic types of adhesives that professional tile setters use. One, the organic type, is a thick glue you buy premixed in cans or tubs and trowel on. The other is a latex-cement mortar you buy in powdered form and mix yourself. Although both work well, the mortar performs better in wet areas and when you're bonding tile to cement backer board. It is not as easy to use as the premixed adhesives, but it is the product I would recommend.

It's important to read and follow manufacturer's directions concerning the best use for that particular adhesive as well as directions for mixing, application and drying time. Drying time is critical. Some premixed adhesives dry very slowly. It causes major problems if you grout the tiles before the adhesive has dried properly. And walking on tile too soon can ruin the bond.

Q. I have a common problem with mildew in the grout in the bathroom and shower. I have used various treatments over the years. Nothing seems to work. I recently had the entire shower regrouted and put a fan in the ceiling because our bathroom has a lot of moisture with nowhere to go after showers. The problem has returned. Do you have any suggestions?

A. You have taken all of the appropriate steps to combat this problem. I often recommend cleaning with a thick paste of scouring powder and hot water left on the stained areas for five to 10 minutes, combined with household bleach if necessary, or using undiluted bleach left on the grout for a least five minutes prior to rinsing with clear water. Installation of a ventilating fan is useful in flushing out the moisture and helping the bathroom dry faster. If you cut off the moisture supply, mildew can't grow.

Perhaps the fan you installed is not efficient enough for the task. The one step you haven't mentioned taking is application of a sealer on the grout lines. A quality sealer will help reduce mildew in showers and other damp areas by preventing water from collecting behind the tiles and in the grout itself. Good sealers, though somewhat expensive, will last for years. I have previously recommended using one of the silicone sealers.

Sealer is best applied soon after new grout has dried, before stains can penetrate the porous surface. Apply this type of sealer carefully, avoiding the surface of the tile. A small artist's brush works well. Any excess sealer that gets on the tile surface should be wiped off immediately.

Two coats of sealer can provide long-lasting protection.

Q. We recently purchased an older house, which has dingy linoleum flooring glued down around the perimeter but unglued in the center. We would like to update this flooring with vinyl, either tiles or sheet vinyl. Can we install the new flooring over the old? Any advice you have would be appreciated, as this is my first attempt at laying a floor.

A. Since the linoleum is not firmly attached to the subfloor, it should be removed. However, an older flooring such as yours should be tested for asbestos prior to removal. If the flooring contains asbestos, a licensed asbestos remover is required to do the work in most states. Another option would be to encapsulate the asbestos flooring by putting down an underlayment over the existing linoleum.

An underlayment may be required even if you remove the linoleum. The surface to which you are attaching the new flooring should be perfectly flat and smooth. A wood floor in good condition that has a subfloor underneath is suitable. Otherwise, you should first cover the floor with quarter-inch-thick plywood or hardboard underlayment, available at lumberyards and home centers. Smooth plywood is best because of its durability, but untempered hardboard is less expensive and will last as long as most resilient tiles. Because hardboard readily absorbs water, do not use it in such places as kitchens and bathrooms.

Acclimate the underlayment material to the temperature and humidity of the room by standing it on edge for at least two days in the room where you intend to install it. To secure the underlayment, you will need lots of ring-shank flooring nails. Drive in one every 4 inches across the sheets into joists. Stagger the sheet joints, beginning in the center of the room and arranging 4-by-4 or 4-by-8 panels so that four corners never meet. Space the panels about 1/32 inch apart--the thickness of a dime--to allow for expansion without buckling.

(Resilient vinyl flooring can be installed on any grade, making it suitable for installation directly on concrete slabs--except in some cases flooring will not be suitable for installation on concrete in contact with the ground because of migrating moisture problems.)

When purchasing resilient flooring, you can choose from several grades and price ranges. Better grades are very durable, resistant to abrasion and discoloration. Cushioned vinyl is soft underfoot, has a minimum of dirt-catching seams and comes in rolls up to 12 feet wide. Sheet vinyl is difficult to install and in most cases should be laid by professionals. Installation kits for do-it-yourselfers are available, but remember, a cutting mistake can ruin an entire roll, not just one tile.

For most do-it-yourselfers, the vinyl tiles are a better bet, less expensive and easier to install, with a wide selection of patterns and textures readily available. Many come with an adhesive backing, making them even easier to set. Most tiles come with instructions.

The first and most difficult step is laying out the tile positions on the floor. If the room is square or rectangular, start by snapping chalk lines between the midpoints of the walls so the lines make a right angle in the center of the room. Position a row of tiles--without adhesive--along the chalk lines starting from the center and creating an L extending from the center toward two sides of the room.

If the space between the last tile and the wall is less than half a tile wide, move the row toward the opposite wall a distance of half a tile. This avoids having to trim any tiles to less than half their width, which weakens them. Adjust the chalk lines accordingly if you move the tiles.

Lay a second row of tiles along the chalk lines, stretching to the remaining two walls. You may have to make another adjustment if the last tiles are less than half a tile wide--moving both rows of tile as a unit half a tile width toward one wall and re-marking the chalk lines.

When laying out an L-shaped room, it is best to treat the space as two rectangles, making adjustments at the right angle of the room to keep full tiles in the center and any tiles requiring trimming around the perimeter of the room.

In dealing with an L-shaped room or an irregularly shaped room, or when laying the tiles in a diagonal pattern, it is often best to make a scale drawing of the room on graph paper and experiment with the layouts drawn onto tracing paper placed on top. When you choose a layout, mark it onto the floor with chalked strings. Exact planning prior to setting the tiles is essential to a successful project.

When you are ready to lay the tiles, peel off the paper backing and carefully position the tile, kneeling on it to apply pressure. If the tile you choose does not have an adhesive backing, buy a brush-on adhesive recommended by the manufacturer. Follow instructions printed on the adhesive container. Lay full-size tiles in the areas aligned with the chalked lines, working one-quarter of the room at a time. Set the border tiles around the perimeter last.

To trim a border tile, put it on top of the tile it will rest against. Place a second tile over it and slide the second tile to the wall. Using a sharp utility knife, score the border tile by using the overlapping edge of the top tile as a straight edge. Then cut or break the border tile along the line. The exposed portion should fit the space. Do corners in a similar fashion: Place a corner tile over the last tile on one side of the corner; mark with a pencil. Repeat on the last tile on the other side and cut along marked lines where they intersect.

Send inquiries to Here's How, Copley News Service, P.O. Box 190, San Diego, Calif. 92112-0190, or send e-mail to copleysd@copleynews.com. Only questions of general interest can be answered in the column.