How to Replace a House Foundation Sill Plate

Gather the necessary tools and materials, then excavate down to undisturbed earth near your home's foundation. Make sure not to dig under your foundation at all, as that could create a cracked foundation wall. You will be placing four jacks to a wall so you will need a total of 16 holes dug under the house for this project. To provide the right amount of workspace around the sill plate, place the jacks at least 24 inches back from the sill plate itself on a solid floor joist. This gives a solid foundation for the lift.

Place the the 24-inch square laminated lumber on the ground. A block is then placed on this solid base, followed by the jack, a piece of 4x4 lumber and, finally, a steel plate. The laminated lumber spreads the load of the jack across a larger area, while the 4x4 post provides a solid base for the floor joist to sit on. The steel plate then spreads the load across a larger area on your floor joist, protecting it from damage. You will use this same setup for each jack on each of the four walls.

Slowly begin lifting the house once the jacks are placed. This is the most time-consuming part of the process because you want all of the jacks to lift at the same rate to keep the house level. The ground underneath each jack will also compress somewhat as the jack takes the weight of the house. Sometimes this means that a jack's height will need adjusted to ensure it is always bearing its share of the weight. You can adjust jack height and position by paying meticulous attention to the condition of each jack and building up jack supports when appropriate. Take your time with this part of the process and remember that you only need between 8 to 10 inches of room between the bottom of the house and the foundation wall itself.

Remove the old, damaged sill plate and replace it with a new one. Because there is not a solid piece of lumber that can run the entire 30- to 40-foot length of one side of a house, splice two shorter pieces together. Your best option is to find two 22-foot-long pieces of 8-inch square oak lumber, connecting them together via a 24-inch lap joint and lag screws. The sill plate is then cut to length and secured in place between the foundation wall and the frame of the house. This is easier said than done, as cut-outs for each floor joist will need to be made so the sill plate fits snugly and securely on the bottom of the house.

Slowly and meticulously lower the house back into position once the sill plate is securely fitted to the bottom of the house and has been lagged bolted into place. Replace any siding that was removed, and remove the jacks themselves. If all goes well, your new sill plates will have leveled up your historic home and will provide years of protection from a sagging floor.