Many reasons have been offered for the covering of old bridges.  Most often heard is
that the roof kept the snow off the bridge.  One interesting tale is about bridges being
built to resemble barns to make the horses comfortable as they approached it.  There was
even a story that the builders covered the bridges to keep the taxpayers from seeing the
shoddy workmanship under the wooden shell!  Some felt that the bridges were designed
to protect travelers caught in storms.  But, the truth of the matter is that the covering
wasn't added to protect people or horses, rather to protect the bridge itself, specifically,
the wooden trusses that held the bridge together.  If the trusses were left to the
elements, a bridge's life expectancy was about ten years.....when bridges were covered,
their life span increased to one hundred years.   -- Nancy