No manufacturer recommends installing floor tiles in a basement that gets wet, since the tiles may come loose, curl and crack. The best bet is to try to correct the cause of the seepage before installing a different flooring. Start by checking the rain-gutter system for clogs or other defects.Q. We want to install vinyl sheet flooring on a wood floor with tongue-and-groove boards. The room is over an unheated crawl space, and we would also like to help insulate it. Should we install an underlayment, and will it be affected by expansion and contraction? -R. Lamplugh.

A. While underlayment is sometimes not necessary when vinyl flooring is installed over a very smooth surface such as existing vinyl, it should definitely be used this time. Without underlayment, the vinyl would probably telegraph or reveal defects in the underlying boards, such as cracks, low spots and ridges.The flooring dealer may recommend a specific type of underlayment, but generally if the wood floor is fairly smooth and the boards no more than about 31/2 inches wide, either one-quarter-inch hardboard or one-quarter-inch plywood can be used. If the floor boards are wider or uneven, three-eighths-inch plywood would be better. Underlayments are generally sold in 4-by-8-foot or 4-by-4-foot sheets.

Since the floor is over a crawl space, a sheet-plastic vapor barrier should be installed between the existing floor and the underlayment. The barrier and underlayment will have an insulating effect, especially by sealing out drafts and moisture.

Basic installation instructions can be provided by the dealer, but here are a couple of general tips:

Acclimate the underlayment by storing it in the room for several days before installation. Special underlayment nails, which have ringed shanks to improve their grip, should be used. The underlayment must be nailed not only at the edges but also throughout the body of the sheets on four-inch to six-inch centers. A slight space, usually one-32nd inch to one-eighth inch, should be left between panels and a gap of about one-quarter inch left at walls. The correct gap and nailing spaces for various underlayments may be specified on the panels or can be provided by the dealer. Joints of panels should be staggered to help avoid long gaps.

Acclimating the panels and proper spacing will help eliminate expansion-contraction problems.

Q. When my new house was built, I ordered a black-shingled roof. Will this make a difference in the attic and home temperatures? Will I need a power ventilator in the attic? -E. Conen.

A. Dark-colored surfaces absorb some heat, and light-colored surfaces reflect some, so roof color does make a difference. However, the ultimate effect on the home depends on many other factors. If attic insulation and ventilation are adequate, the effect on living comfort and fuel consumption should be small. I suggest asking the builder or roofing contractor to check the roof and attic in a year or two to determine whether additional ventilation or other measures are needed.

Q. My concrete patio is partially covered with mildew. It is unsightly and slippery when wet. How can I get rid of the mildew? -M. Hansley.

A. Scrub the patio with a strong solution of chlorine bleach, which should kill the mildew and eliminate the stains. Keep in mind that the bleach can harm any plants near the patio and should not be splashed on fabrics or other surfaces that are not colorfast. Rinse the patio thoroughly after the scrubbing, which will probably have to be repeated periodically.

Q. Our house has a full basement that is fully finished and carpeted. On several occasions, water entered the basement and soaked the carpet, which we had to remove, dry out and clean. Is there a floor tile I can install in place of the carpet that will not be affected by the water? -R. Ruszczak.

A. No manufacturer I know of recommends installing floor tiles in a basement that gets wet, since the tiles may come loose, curl and crack. The best bet is to try to correct the cause of the seepage before installing a different flooring.

Start by checking the rain-gutter system for clogs or other defects. Overflowing or leaking gutters and poor drainage around the house are frequently responsible for wet basements and are relatively easy to fix. Minor seepage through walls can also sometimes be cured by treating the insides of the walls with a special sealer or paint, such as Drylok or Thoroseal.

If the wet basement defies correction, probably the best bet is to stay with an easily removable carpet that can be lifted and taken outside for drying. Indoor-outdoor carpet that resists water damage is available.

(Readers' questions and comments are welcome and should be sent to Gene Austin, The Philadelphia Inquirer, Box 8263, Philadelphia, Pa. 19101. Questions cannot be answered personally.)