mudsill

/ˈmʌdˌsɪl/
noun
  1. A horizontal timber or beam placed at the bottom of a foundation or wall, often in contact with the ground, to support a structure.
    • Before building the shed, we laid a pressure-treated mudsill on the concrete slab.
    • The carpenter replaced the rotted mudsill under the old barn.
    • A mudsill helps distribute the weight of a building evenly across the foundation.