FEW homeowners would argue with the premise that a slate roof adds beauty and value to a house. But while such roofs are often thought to be virtually indestructible -- some have been around for a couple of hundred years -- even the best slate roofs age, leak and eventually have to be replaced.

''The average homeowner will look at an 80-year-old slate roof that's leaking and think, what in the world am I going to do with this?'' said Joseph C. Jenkins, a slate-roof contractor in Grove City, Pa., who is author of ''The Slate Roof Bible'' ($35, Jenkins Publishing, Grove City, 1997). ''And I would say that the most important thing to find out before answering that question is what kind of slate you have.''

Mr. Jenkins explained that while there are a variety of sizes, shapes, colors and styles of slate used on roofs, most slate roofs can be divided into two basic categories: hard and soft.

''Hard slate will last anywhere from 75 to 200 years, while soft slate will last only 50 to 125 years,'' he said, adding that as a result, a homeowner trying to decide whether to repair, restore or replace a slate roof should first determine whether the existing slate is hard or soft.

Generally speaking, Mr. Jenkins said, colored slates are almost always hard slate and black slates are almost always soft. (There are some black slates that are hard, and there are some colored slates that are soft because they're really black slates that have been faded by the sun.) In any case, he said, a slate expert -- known as a ''slater'' -- will be able to tell hard slate from soft simply by tapping on it.

And since most slate roofs on homes in the Northeast were installed in the early 1900's, Mr. Jenkins said, homeowners who have them generally have a decision to make when the roofs start leaking.

''If it's hard slate, you probably should restore it,'' he said, explaining that even a 100-year-old hard slate roof could have another 100 years of useful life. ''But if it's soft, it may not be worth fixing.''

Mr. Jenkins said that in most cases, leaks in slate roofs are caused either by broken or missing tiles or faulty flashing. If the problem is with the tiles themselves, he said, repairs can easily be made by an experienced slater. ''All you have to do is pull out the bad slates and replace them with good ones,'' he said, explaining that a mistake many homeowners make is to wait until there are several broken or missing slate tiles before calling in a contractor.

''That's like having a cavity in your tooth and waiting to get more cavities before getting the first one fixed,'' Mr. Jenkins said, adding that even one cracked or missing tile can allow water penetration that can damage the roof deck.

Leaks caused by faulty flashing, he said, may require a full restoration.

''Restoration means replacing all the metal parts on the roof,'' he said, referring to the sheets of metal flashing used to seal areas where chimneys, vents and dormers protrude through the roof surface. In most cases, he said, the flashing will deteriorate and fail long before the slate itself.

''Restoration is a routine job for an experienced slater,'' Mr. Jenkins said, adding that since every job is different, the price can run from several hundred to several thousand dollars.

In cases where a slate roof is beyond repair, replacement may be necessary. And for preservationists and aesthetic purists, the only appropriate replacement for an old slate roof is a new one.

''A slate roof is regarded as the best roof you can buy,'' said Brian Stearns, president of Vermont Slate and Copper, a slate roofing company based in Stowe. ''It's also one of the most expensive roofs to install.''

In most cases, Mr. Stearns said, a slate roof installation -- including the slate itself and removal of the existing roof -- will cost about $1,500 per square, with each square equal to a 10-by-10-foot area.

He said that the square footage of a roof is usually about the same as the total square footage of the house. So a 2,000-square-foot house would require 20 squares, and a new slate roof would cost about $30,000.

''I would guess that to do the same job with asphalt shingles would cost about $200 a square,'' he said. ''So the question you have to ask yourself is whether it's going to be worth spending that much more money for slate. Then again, if the house had a slate roof to begin with, chances are that the real estate in the area might justify it.''

There are, however, some relatively new high-quality roofing products that look like slate but that cost less than the real thing. One is Celadon Ceramic Slate, a synthetic roof shingle manufactured by CertainTeed Corporation in Valley Forge, Pa.

''Celadon gives you the look of slate without having to pay slate prices,'' said Matt Butler, the owner of Allied Building Products, an East Rutherford, N.J., distributor of CertainTeed products.

Celadon tiles, Mr. Butler said, are lighter than slate -- making it possible to use the product on roofs that would not support the weight of real slate -- and come with a 75-year warranty. And while considerably more expensive than asphalt shingles -- the tiles alone cost about $235 a square, not including installation -- they are considerably less expensive than slate, which costs about $450 a square, not including installation.

Another alternative to slate is Majestic Slate, a product manufactured by EcoStar, a company based in Chicago. ''For lack of a better term, what we're making is imitation slate,'' said Kerston Russell, the company's president. Mr. Russell explained that his product is an injection-molded slate look-alike made mainly of recycled rubber.

''We use car bumpers, radiator hoses, door moldings and even some old tires,'' he said. ''And what we end up with is a material that looks for all the world like real slate without any of slate's disadvantages.''

For example, he said, while slate is brittle -- and can be easily cracked or broken if stepped on during installation or repair -- Majestic Slate is flexible and will not chip or crack. Moreover, he said, the rubber-based composition of the tile makes it possible to simply nail each tile snugly to the roof deck -- a procedure that is less labor-intensive than having to carefully nail through pre-drilled holes in slate without hitting and breaking the slate.

The material cost of Majestic is somewhat less than the material cost of slate -- the price ranges from $285 to $360 per square. And the simplified installation procedures generally result in even greater savings.

Yet another lightweight alternative to slate is StoneCrest, a metal roofing product manufactured by Metalworks, a Pittsburgh company.

''StoneCrest is a galvanized steel shingle that looks like slate and comes in an array of colors,'' said Mark Storti, the company's manager for marketing and technical support.

The product, which carries a lifetime warranty, is so light it can generally be installed right over an existing asphalt shingled roof if the local building code permits. The cost, Mr. Storti said, ranges from $350 to $600 a square, installed.

Information on slate roofs is available on the Web from a publication called The Slate Roof Quarterly at www.slateroofquarterly.com

Drawing (Tom Bloom)