The Frequently Asked Question about covered bridges has to be: why were they covered? There is a short answer. Wooden bridges with exposed superstructures are vulnerable to rot. Covering and roofing them protects them from the weather, and so they last longer.
In one sense, that just puts off the question. Why so many wooden bridges? And why especially in Pennsylvania and the U. S. Northeast?
In eighteen hundred, the northeastern United States was a country in need of bridges. It is a fairly narrow coastal plain cut by many short rivers and creeks. In the "tidewater" region, these little streams and the great estuaries such as the Chesapeake and Delaware Bays had been highways and lifelines. But now the population was surging beyond the tidewater region, drawn both by the growth of agriculture and the call of water-powered industrialization. Inland farmers needed overland transport, and that meant fords or bridges. But the water-powered mills sought out the very places where the streams could not be forded -- the falls and rapids -- and they too needed transportation.
So bridges were needed. The American northeast was a forest country: wood was a plentiful building material, especially in the remote areas where the smaller bridges were needed. And the climate favored wooden construction. The climate of the region is harsh, by European standards -- hot in the summer and icey in the winter, with a freeze-thaw cycle that would overturn stone pavings. But this sort of climate is less destructive of wood than the mild, moist climate of Britain (or Oregon). So wooden bridges there would be.
The young United States had one other necessary ingredient in plenty: ingenuity. Lewis Wernwag, Theodore Burr, Menander Wood and the rest were just as essential as the material and the need. Without them, there would be no historic covered bridges.
As we enjoy our heritage, we honor the memory of those agile minds who created it.