My wife was watching "The Bridges of Madison County" the other night when she suddenly asked, "Why did they build roofs over bridges?"
In my infinite knowledge of all things mechanical and construction-type stuff, I replied, "I dunno."
I did a web search and found out that thousands of covered bridges were built in the 19th century in Canada and the U.S. Most of the sites showed barn-like structures, complete with windows, interior walkways, and hand rails, spanning a creek or narrow river.
The ends of the structures were open to horse-drawn wagons and such.

The web sites gave two reasons for covering bridges: (1) to keep snow, rain and such from weathering the supporting timbers. Apparently, an uncovered bridge would last about 10 years and a covered one maybe 50 years.
(2) Horses were reluctant and had a propensity to bolt if driven over an open bridge with flowing water beneath. Covered, the bridge looked like a familiar barn to them and they didn't spook.

I don't entirely believe either answer. Timber was plentiful, close by, and labor was cheap. It would be just as cost effective to not build a structure over them.
Horses have been ridden or pulling wagons over open bridges for centuries, so neither answer is particularly compelling.

Do any of you agree/disagree? Do you have a better answer?

I googled some Dutch sites and although some of them corroborated the weathering reason, one of them suggested that it was because the construction is much more solid when there's a roof. I think that makes sense, because if you make a square out of four pieces of wood, it's more solid than a U-shape or just a plain piece of wood. I'd give you the link to the site for a better explanation, but since it's in Dutch I don't think it'll make much sense.

Thanks for the info, Leau78. The only other reason for covering a bridge that I could think of was to keep the road timbers dry enough so that horses wouldn't stumble on a slick surface.

Years ago I was told that these bridges were to protect travellers from rain and thunder storms. However, I googled around and got the same answers as you guys, I don’t find them particularly satisfying but I can find no reference at all to what I was told.

I haven't played it but there is a quiz in Quizzyland called Covered Bridges. That may be able to provide some info. There is also The Covered Bridges of Madison County.

My grandmother was a 'collector' of them and always photographed them wherever she went. There was one up in Oregon that appeared in the film' Once a Great Notion' by Ken Kesey. I got to see that one filmed.
I assumed it was to avoid ice buildup.

I was looking for some Amish sites and found this site about covered bridges by accident. Along with other information it has the following to add to our knowledge of covered bridges.

Quote:

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While covered bridges are sometimes called kissing bridges, the real reason for the covering is to protect the bridge's truss design from the weather. The longest covered bridge in the world was built in Lancaster County in 1814. It crossed the Susquehanna River between Columbia and Wrightsville, a distance of over a mile (5,960 feet). Ice and high water destroyed it in 1832.

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