Creating a New Foundation for an Old House

The house was built on a lot that sloped immediately down from street level, which set the front door sill about 9 inches lower than the sidewalk.

At the back of the original building was an extension that had been built in the 1920s on a full foundation of hand-mixed concrete interlaced with bricks and rubble.

Under the original structure, the builders had expanded the basement area by excavating to within a few feet of the older brick foundation, shoring it with small-diameter logs driven vertically, stockade-style.

Before we jacked anything up or worked on the sills, we stabilized these joints by running a heavy chain from a central interior timber through the tops of the first-floor gable-end windows to a pair of 6x6s running the full width of the building.

To create room to stack cribbing, we hand-dug pocket holes about 4 feet deep, undercutting the foundation near the corners and at the center. After stacking the cribbing, we used bottle jacks to raise the building just enough to slide in short lengths of 6x6s directly under the existing sill.

With the front wall timbers in plane, we bolted a 6x6 ledger to the face of the frame along the entire length of the building, pre-drilling the four 8x8 posts to receive 1/2-by-12-inch lag screws.

We placed four needles parallel to the joists, shimming as needed to pick up the central timber and to make sure that all three sills in the addition were fully supported.

In the front of the house, the joists ran the full width of the building, supported by two intermediate carrying beams that divided the floor system into three sections. To support this part of the house, we inserted two crossing beams, which would eventually rest on top of the two main 50-footers. We then hung two additional beams at a right angles to the crossing beams, aligned directly under the two floor timbers. To hang these beams, we used special heavy-duty Wilton C-clamps capable of supporting 15,000 pounds each.

To pick up the chimney load, we punched a hole through the brick and flue tile and slid a scrap of tube steel through it (10). The standard alternating brick courses transferred the stresses and effectively held the chimney intact with no further reinforcement.

With all the needles placed, we brought in the nine-point hydraulic jacking system, which we had recently purchased used and had completely refurbished. A central manifold controls all nine jacks, which are driven by a single main piston.

The maximum throw of the jacks is about 14 inches, so we had to reset them three or four times. Each reset involved blocking solid between the cribbing and the steel beams, resetting the jack, lifting, then blocking again.

To locate the footings, we used a vertical laser to pinpoint the building corners, then offset the forms by 4 inches. We added 8-inch-thick pads for intermediate Lally columns and for the chimney. For all but the front wall, we poured standard 8-inch walls on 16- by 8-inch footings.

After placing the concrete, we set #5 pins at 18-inch intervals to engage the 10-inch-thick wall. For reinforcement, we wired vertical and horizontal #5 bars in a uniform 18-inch grid and laid a final piece of rebar across the topmost form ties.

After stripping the forms, we gently lowered the building onto its new bearings . We pulled out the beams—including the welded-up 50-footers (which are already in use on another job)—one at a time, and then filled in the beam pockets with concrete block.

Before pulling the beam supporting the decommissioned chimney, we built a concrete-block pier to carry it.

To tie the building to the foundation, we had a steel supplier cut and punch bolt holes in 1/4-inch steel plates, which we prime-painted and lagged to the foundation using a Simpson SET-XP epoxy anchoring system.

There was a considerable amount of re-framing needed to reinforce the old floor system, which we did mostly by sistering in new joists and piggy-backing some others. With a 4-inch concrete slab poured over a moisture barrier, the basement is complete, and we've turned our attention to helping the building face the next 200 years.

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