Reclaimed barn wood is tough, but in certain applications it may need additional processing to "season" it, or dry it out so it reaches a particular moisture content. When wood is first cut, it's called green wood, not because it's environmentally friendly but because it's fresh or new wood (similar to a "green" employee on his first day on the job). When the green wood is dried so that it's moisture content reaches a particular level, it's called seasoned wood. When you buy reclaimed wood, however, you may hear a lot about how it was kiln-dried. This is a feature you want to look for if you're going to use the wood for certain building purposes.
Green Wood Was Good for Barn Building
Green wood tends to be easier to work with because the moisture content makes it softer. It's not soft like cloth, of course, but it's easier for someone to drive a nail through green wood than it is to drive a nail through seasoned wood, which is very hard.
Older buildings like wood barns were often built with green wood for this reason. While the wood would be exposed to heat and dry weather repeatedly over the years, there's no way to monitor the seasoning of the wood as it sits on a barn side year after year. When that wood is reclaimed, it has to be evaluated and kiln-dried if necessary to ensure it's reached a particular moisture content level.
Green Wood Is Not Good for Flooring
So if seasoned wood is so hard to work with, why would you want to use it? Why not use green wood instead? You can do a lot of building with green wood, but in some applications, such as subfloors that aren't exposed to the air, the moisture content could actually contribute to fungal growth. One would hope that all the moisture would have gone out of the wood as it sat outside as part of the barn, but rainstorms and snow can certainly keep setting that back. Even if the internal moisture is long gone, a recent rainstorm could have added a little back, and the wood could have been reclaimed before it dried out again. Kiln drying ensures all the unnecessary moisture is gone.
If you want to use reclaimed wood to build something, it's best to speak to a representative from a lumber company that uses reclaimed wood to find out what moisture levels they have available, and which types of wood as well. They can help you find the best type of wood for your particular project.