Seventy percent of the Earth is covered by water and 70 percent of U.S. homes are covered with composition-type shingles. This column concerns what can be done when the substance that covers most of the earth leaks through what covers most of our homes.

Some roof leaks can be eliminated with a shingle patch. Replacing a shingle that has been removed by wind or has been damaged by other elements is really quite simple.

Here's how. The shingle or shingles above the missing one will have to be inspected to see if nails exist. Simply pry up the shingle with your hand and take a peek. Any nails present will have to be removed or driven all the way in. Either method is acceptable. A flat pry-bar can be used to pry the nails loose or together with a hammer to drive the nails. The exposed part of the flatbar nearest the edge of the shingle above should be tapped with a hammer to drive the nails. Normally, when one shingle is being replaced there will be two or three nails to be pulled or driven flush.

Have someone hold the shingle just above the replacement shingle up and out of the way as much as possible. This should be done without exerting too much pressure in order to avoid damage to the shingle being pried back. Finally, slip the replacement shingle into the same spot that the old one occupied, adding a dab of asphalt cement to secure it. On a moderately warm day a one-and-a-half-inch roofing nail can be pressed into the surface of the new shingle by hand just enough so that it will stand in the shingle on its own. Then, holding the curved end of the pry-bar in one hand, lay the other end on the nail head and gently strike the center of the bar with a hammer. Repeat the process until three nails hold the new shingle in place. Have your helper release the shingle above. Then press it back in place (the sun will flatten what you can't) and the job is done.

This kind of cost-effective home improvement gets to be a lot of work when the replacement of many shingles is required. In those circumstances, we recommend that you call a roofer.

Q. Currently, the finish wall covering in my bathroom is drywall. I have been saving old magazine covers and old sheet music for many years. I would like to paper my wall with these covers.

How must the existing wall be treated so that we will be able to remove everything later without damaging the drywall. I have spoken to several people and they all say that the wall will be damaged. Do you have any suggestions? Mrs. Jean M., Valley View, Ohio.

A. First of all, we strongly recommend that you check with an antique dealer before you paste any of that music onto the wall. Here's how you can prepare the wall so that the wallpaper can be removed.

All you need to do is create a somewhat waterproof barrier between the wallpaper paste and the paper surface of the wall board. This isn't complicated, but we do offer a caveat: Just because the wall has paint on it doesn't mean that it is water-resistant. Interior latex paints, flat or enamel, are very porous.

The barrier you will need is oil-base or alkyd paint. Once a coat of oil-base primer has been applied to the wall, it will prevent the wallpaper adhesive from penetrating into the paper surface of the wallboard.

It is important to remember that wallpaper pastes are all starch-based and are dissolved with enzymes or even plain water, neither of which affect oil paint. On the other hand both water and enzymes can dissolve and-or penetrate water-base paint. For these reasons oil base is the answer.

Don't use a finish coat. Doing so will result in a smooth surface that the adhesive will have trouble attaching itself to.

And most important, don't let anyone sell you wallpaper sizing in lieu of an oil-base primer. Wallpaper sizing is basically thinned wallpaper adhesive.

Separate the wallpaper adhesive from the paper surface of the wallboard with one coat of a good oil-base primer any brand will do and removal later will be simple.

You may want to cover your newly papered wall with a clear latex sealer. It will help to slow deterioration and make the walls easier to clean. Caution: Don't use oil or lacquers or you'll have a mess. And make sure to apply the sealer sparingly. Depending on the age and type of paper and ink, you may experience some bleeding.