Bridges were covered to protect the wood.  Preservatives for wood were not available; once they did exist it essentially eliminated the need for covered bridges. 

Plank floor, truss bottom chords and floor beams require treatment if it is known that these members will be alternately wet and dry from road traffic.  As in buildings with roof cover, preservative treatment should not be a normal requirement in covered bridges.

Smith Rapids Covered Bridge      Chequamegon National Forest      South Fork Flambeau River      Price County, WI

Covered Bridges Page 2 
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