Some of the pros and cons of a "sister" foundation versus a complete foundation replacement:
Pros:
· Typically, a sister foundation is less expensive.
· The exterior of the house is not affected. This is especially important with historic structures or homes with beautiful river rock or brick foundations.
Cons: · The weight of the exterior walls of the house will still be supported mostly by the original foundation.
· A sister foundation is not always practical. Certain clearances are needed.
SISTER FOUNDATION RETROFIT STEPS:
CLICK ON PHOTOS FOR LARGER IMAGES.
A house full of character with river rock as part of its architectural features: to strengthen the foundation, a sister foundation, rather than a new foundation, may be constructed in order to preserve the architectural features. The new sister foundation will not be visible from the exterior. (Photo A)
1. A trench is dug in the crawl space adjacent to the old foundation to accommodate the new sister foundation. (Photo B)
2. Pockets are formed in the old foundation in order to join the two foundations (old and new).
3. Steel reinforcements (rebars) are then installed in the trench and are elevated above the soil with concrete spacers. Foundation bolts with large square washers are installed as well. Wooden forms are built to contain the concrete.(Photo C)
A. House with River Rock Foundation
B. Trench for New Sister Foundation
C. Steel Reinforcement and Forms In Place
4. Ready-mix concrete, delivered to the job site by concrete trucks, is pumped into the forms. (Photo D)
5. After a day or two the wooden forms are removed. (Photo E)
D. Pumping Concrete for Sister Foundation
E. Sister Foundation Is Complete
6. Sometimes solid wood blocking is added in between the floor joists above the new foundation.
7. Framing anchors are installed to attach the new blocking to the new mudsill.
