Why? What’s the point of covering a bridge, when there’s no need to cover the roads?
The deck surface or other lower parts of those bridges are usually wood, too. The roof and sides protect that wood from the elements.
Yep. To quote Wiki : “Uncovered wooden bridges typically have a lifespan of only 20 years because of the effects of rain and sun, but a covered bridge could last 100 years”
That, and in winter the most common road vehicle was a sleigh, not a wagon. The roof kept the sun off the snow that would be shoveled onto the deck. The walls helped keep horses from shying when they crossed the bridge.
And the covering is also subject to the weather, and so also rots out after a while and needs to be replaced. But the covering is a lot cheaper and easier to replace than the trusses and roadway.
They’re found outside of New England, too, by the way. There are plenty in Ohio.
Another reminder that this is hardly a NE phenomenon. Google “covered bridges in <a state>” to get list. Here’s one for Oregon. 50 are given including one in Multnomah county that was built way back … in 1982. (Although the purists say it’s not actually a covered bridge, despite appearances.) I’ve been across that one recentishly.
Of course states with a lot of timber tend to have more than those that don’t.
I Imagine it helps with freezing weathers as well. I’d imagine a covered bridge is less likely to freeze over.