DEAR TIM: My older home has a slate roof. Some of the pieces of slate have cracked and fallen. Entire pieces have come loose. The slate is very brittle. Is it time for my roof to be replaced? If not, why is the slate breaking and falling? In my attic you can see the bottom of the slate. Half of my roof boards are missing. Is this the cause of my problems? D. D.

DEAR D. D.: Brittle is good! If you had told me that your slate was soft and flaky like my favorite dinner rolls, you would have a major job ahead. Slate is one of the finest roofing materials available. It is fireproof, resists hail damage, possesses unquestionable beauty, and often has a service life of 100 years or more.

Prior to the development of asphalt roofing products, slate was a major player in the roofing products industry. Slate peaked in popularity at the turn of the century. The move away from steep roof slopes and less expensive roofing products have caused the slate roofing industry to shrink to one twentieth of its original size. Cost is a factor as well. New slate roofs can cost as much as $900 per square (100 square feet) installed! Compare that figure to say $60 per square for a standard asphalt shingle roof.

There are several possibilities why your slate is cracking and falling. Slate is a rigid natural stone product. Think of it as a piece of glass. If you stress a piece of glass, it usually cracks. Slate is no different.

Stress can be introduced to slate in several ways. The nails used to attach the individual pieces of slate are possibly to blame. They need to be fastened so that the slate merely hangs on the nail much like a picture on your wall. If the nail is pounded too tightly, it will pinch the slate. Should the nail stick slightly above the slate, the overlapping piece above may crack from this hidden pressure point. There is very little room for error.

The wood decking may be to blame. Wide swings in temperature and humidity cause your wood roof sheathing to expand and contract. This creates stresses on the slate. If the nails are in a bind, a corner may pop off and the slate will crack or fall.

The nails themselves may have failed. If a galvanized nail was used, it simply may have reached the end of its life. Nails, flashings, etc. used in conjunction with slate roofing should have the same service life as the slate itself. Copper nails and heavy gauge copper flashings are a must.

Slate can succumb to the forces of nature. Not all slate is created equal; quality can vary significantly. Acid rain and freezing temperatures can damage slate and shorten its life. It is important that slate dries as rapidly as possible after a rain. Your missing roof boards allow the slate to dry rapidly. Your roof was built this way on purpose.