When I bought the house in 1994, the front concrete stoop had sunk several inches on the house side. Originally is was poured flush with the door threshold. Rain water was running toward the house into the box sill. I jack-hammered off the concrete stoop and replaced it with a wooden deck, which included a rubber liner around the outside of the box sill. And I dropped the height of the deck by one step (about 6.5").

When I went to install squash blocks to support the post on the main floor, I discovered that the bottom of the box sill had rotted when the stoop sunk and water had run in. So ... I cut open the floor above the box sill to expose the channel and Andrew and I added 2 more box sill boards against the partially rotted one, attaching them with a lot of construction screws. Now, everything is solid.

These pictures show the rot, exposed floor, the added box sill boards, and the squash blocks.