A rotten sill plate is a common occurrence in old houses and a big problem. The sill is the piece of wood closest to the ground, either on a foundation or piers, and is usually a “timber-sized” board: 3×6, 3×8, 4×6, 4×8, and so on. The studs often rest directly on the sill and are toe-nailed in with no bottom plate as you’d find in a modern house.