Historically, new building materials came into use not for any esthetic reason but because they solved a particular problem.

One of the most pressing needs has always been for fireproof building materials. Before the introduction of electric lighting and central heating there were open fireplaces burning candles, kerosene lamps, and unlined chimneys in all types of buildings. They presented a constant fire hazard.

Wooden houses with wooden roofs were especially vulnerable. While wood shingle (or shake) roofs are very popular with today's homeowner, in the 18th or 19th century wood shakes were replaced whenever possible by roofs that would not burn; roofs made of metal or slate.

The first metal roofing materials, copper and lead, came into use in the 18th century. Sparks from the chimney could fall on such a roof and do no damage. The roofs were made of sheets of the material, 18 to 24 inches wide, joined by a ridge (s a standing seam) 1/2 to 1 inch high. The copper gradually acquired its green patina while the lead retained its gray color.

There were also tin-plated iron roofs. They were first used in Canada in the 18th century. Thomas Jfferson put a tin-plated iron roof on Monticello. The tin was painted with a red oxide or green pant; the later to imitate copper's patina.

It was not until the 19th century that the fire protection offered by metal roofs was available to the middle classes. Two discoveries, both in Europe, made that possible. In 1829, a process was developed for corrugating metal, stiffening the sheets and allowing a greater span over a lighter framework. Eight years later, in 1837, a process was developed to protect the metal from rust. Galvanizing, as the process is called, coats the iron with zinc. Galvanized, corrugated metal roofs became a practical, inexpensive, if not attractive, alternative to wood shakes.

Metal roofs were replaced in popularity in the 20th century with the advent of asphalt shingles. Colonial Williamsburg, for example, uses asphalt shingles on its buildings because of their resistance to fire.

While other roofs may have to be replaced every 10 yers, a good metal roof will last from 20 to 30 years. The roof must be painted to protect the metal from the weather. There should be semi-annual inspections of the flashing (sheet metal) used to join the roof to the chimney. Breaks in the flashing will let moisture under the roof, damaging the rafters.

The 20th century has added new dangers for metal roofs. Airborne pollutants, acid rainwater and tannic acids from nearby cedar shakes produce a chemical reaction with any unpainted metal and will eventually dissolve the roof. Cast iron ornaments on a metal roof can set up the same type of chemical reaction, with rainwater acting as the electrolyte. Painting will protect the metal from the airborne dangers. A rubber gasket separating the two materials will prevent that danger.

Incompetent repairman present a special danger. Do not let a repairman cover the roof with tar. The tar may initially seal any hole but it eventually separates from the metal. Water will seep under the tar, rusting the metal and again exposing the interior to moisture damage. The original problem quickly recurs but is hidden by the tar patch.

Insist on knowing exactly what the roof repairer will be doing and check the work as it progresses.