Slate and wood shingles are the only two significant, unadulterated natural materials used for roof coverings in this country today.

The life of cedar roofing depends primarily on its thickness of cut, but not so with slate. Nearly all slate, unless a special order, is cut with a minimum of 3/16-inch thickness. The durability and useful life is determined to a large extent by its geographical origin and the particular veins from which the slate is quarried.

Nearly all of the slate used in this country, certainly east of the Mississippi River, comes from three locations. High-quality gray Buckingham slate is quarried in Virginia. Another high-quality, multi-colored slate, quarried in Vermont and New York, is called Vermont slate.

The less durable Bangor slates are quarried in Pennsylvania. It has a much shorter life than the other two. All Pennsylvania slate, however, is not of the Bangor variety. The Cathedral gray rates high in durability, along with the Vermont and Buckingham slates.

Knowing the age of a house or its roof is paramount in making an analysis of slate. The age of the slate is the best clue to pre-judgement of the expected condition.

It is sometimes startling to be told that the slate roof on a house 20 to 35 years old is about gone. The usual reply is, "Oh, I thought slate lasts a lifetime."

Well, it does, but for whose lifetime? Not the lifetime of the person buying the house! Bangor slate may last as long as 50 years.

Vermont and Buckingham slates are very hard and dense and last up to 100 years. The latter kind covered the original roof of the Ford Theater and later was saved by the roofer and reinstalled on his own house, where it is still holding up.

The first sign of wear with the shorter lived slates is a whitish ring or semi-circular fringe, like a water ring on a piece of clothing. This could indicate that the slate has only 15 years of life left.

The next warning sign is a browning color - not to be confused with the brown slate in the Vermont range of colors. These brown pieces among the gray are softening and decay has gone on a little further, meaning only eight to 12 years of useful life may remain.

The third sign is definite delamination ans peeling, much as plywood left lying in the rain. The same granular structure that allowed the stone to be quarried and cut thin is, in effect, still working. At this point, the slates are wearing thin, becoming softer and exhibiting all three of the aging characteristics.

Many will have been replaced. When all these signs are in evidence, four to seven years of life remains; repair without damaging other slates is more difficult.

Asphalt shingles begin to go after the 14th year and are usually replaced rather than repaired. Slate, however, tends to split, particularly on the south side of a house, where it is exposed to constant freezes and thaws. This accelerates as the shingle softens with age. It does require periodic attention.

Perhaps the most important factor in the life of any roof, slate included, is the pitch of the roof. The steeper the pitch, the longer the life for any roof.

Graphic examples are found on porch roofs tucked in under upstairs windows, where the steeper roof is still intact but the porch roof is either leaking or needs replacing. This is often noticeable on houses built in the 1930s. Slate is very easy to repair. It's a matter of ripping out the old nail with a ripping tool, then nailing down a little ribbon-shaped piece of metal, which should be cut long enough to extend down below the edge of the replacement slate.

Bend the tail end of the metal ribbon back up and over the bottom edge of the new slate. It is very easy to tell how many slates have been repaired by these little pieces of ribbon.

The general condition of a roof may be more important than one or two existing leaks, and a loose, missing or broken slate or two is not necessarily a sign of old age. Any roof can leak, including a brand new one.

More important is the general life expentancy of the entire roof. The cost of a new slate roof may be four to five times that of asphalt shingles. Costs are approximately $250 for a 100-square-foot "square" for the less durable Bangor slate and as much as $350 a square for the Vermont "Bucks" and Cathedral varieties.

Claxton Walker heads a local house inspection and consulting firm.