This particular extension was simply built on top of an existing deck. This creates a crawlspace under the deck of soil. You can imagine the humidity problems that can occur from building directly over the ground not to mention the possibility of Radon gas entering the home. This particular problem of rot was mostly due to the fact that the room was only partially covering the deck. Meaning that the exposed portion of the deck allowed water to flow directly under the wall. The combination of water and high humidity caused some major rot in this general area.