How to join an old cement foundation to a new slab

Updated July 11, 2018

When building an addition to a house, the first item of construction is the new foundation and slab. The new construction must be on a solid base, it must be joined to the old foundation and the joint must be waterproof. To ensure proper compatibility of the new construction with the existing building, the soil must be prepared properly, footings must be poured, the slabs must be sealed and joined and the walls poured on to the footings. When the work is completed in this way, the new slab will not move away from the old foundation and it will not leak.

Prepare the soil under the new slab and footings. The soil must be compacted and must be similar in structure to the soil under the existing foundation. If there is doubt about soil stability, deeper footings or piers reaching down to rock or to stable soil levels may be necessary. Once the soil is stable, pour the footings around the edge of the new construction.

Prepare to fasten the new slab to the old foundation. If rebar or structural steel is to be used to strengthen the new slab, the old foundation must be drilled and rebar driven into the holes to make the connection. Use the same spacing for the new rebar slab. If no rebar is to be used, steel pins can be inserted into holes in the old foundation and epoxied into place every sixteen inches. The new concrete will grip the rebar or steel pins and remain fastened to the old foundation.

Waterproof the joint. Before pouring the new slab, insert a weatherstrip-type waterproofing material which will adhere to the new concrete and seal the joint. Apply a layer of gravel inside the footings and under the new slab to prevent water from accumulating next to the new concrete. With rebar or steel pins and the waterproof strip in place, pour the new slab and cure.

Drill holes into the old foundation every sixteen inches where the new walls will meet it and drive rebar into the holes or insert steel pins and epoxy into place. This will add lateral strength to the new walls. It will also help prevent their moving inward during backfilling operations and if the earth shifts at any time in the future. Pour the walls and cure. The new slab is securely connected to the old foundation and will not result in leaking should water infiltrate the ground under the new construction.

Things You'll Need

  • Concrete
  • Gravel
  • Rebar or steel pins
  • Weatherstrip waterproofing
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About the Author

Bert Markgraf is a freelance writer with a strong science and engineering background. He started writing technical papers while working as an engineer in the 1980s. More recently, after starting his own business in IT, he helped organize an online community for which he wrote and edited articles as managing editor, business and economics. He holds a Bachelor of Science degree from McGill University.