A specimen of cast-iron plates for roofing of buildings, says the Philadelphia Ledger, has been exhibited at the Exchange, in Philadelphia, by the inventor and patentee, Mr. Wm. Beach. The plates are about a foot square, and are made to fit one into another so as to render the roof perfectly water-tight, with the application of white lead to the joints. In every respect this material for roofing is preferable to any other description now in use. As to its durability, there can be no doubt that it would remain perfectly whole for ages, if covered occasionally with a coat of paint, and even without that preservative, rust would not affect it materially for a period of fifty years at least. As compared with copper, the cost would be nearly one half, as it is expected the iron can be furnished at 16 cents per square foot, while copper would at the most moderate estimate cost 28 cents. As regards the weight of an iron roof, which at first sight would appear an objection, it is far less than one formed of slate, and does not much exceed one of copper. The iron plates weigh three and a half pounds per square foot. A slate roof would cost about eight cents per square foot, but for durability, and the ease with which it can be put on and made water tight, the iron roofing would appear to be far preferable. The plates exhibited were cast at Troy, New York, and are of the very best quality. The patent for the eastern States is now owned by Mr. Hiram Hemmistone, of Troy, in which neighborhood the adaptation of such a durable material for roofing is rapidly attracting public attention there. Starbuck's machine shop and foundry at Troy has been covered on this plan, and it has also been adopted for the roofing of an arsenal at West Point.