Dry Rot
I was called out to one of my job sites a couple days ago because one of the beams we were strengthening had some dry rot. It turned out to be some of the most extensive dry rot that I've seen in a structural beam. You can see in the picture where they carved all of it out for a length of over three feet and then halfway down the depth of a 6x12 beam. The beam had a span of nearly twenty feet and had deflected over three inches. It probably wouldn't have been much longer before the beam failed completely. Luckily they were doing some work for something else and discovered this.
It's not obvious from the name, but dry rot is the result of the intrusion of water. You can see in the photo the culprit that allowed the water in. The steel angle was used as a brace for some reason and was attached to the beam. This allowed the water to channel straight down through the roofing material and into the beam. The steel angle had also been eaten through completely. In this case, water=very bad.



