How to Join a New Concrete Slab to an Old Concrete Slab

Written by erin moseley
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Whether you want to add a concrete extension for a room addition or for a patio, you can join a new concrete slab to an existing one. It's just a matter of preparing the site, mixing the concrete and taking care with your measurements. If you can make accurate measurements and can follow packaging instructions, you can do the job. And by using some basic concrete construction skills, you can take charge and make that extension look great.

Skill level:

Things you need

  • Cement mix
  • Sand
  • Cement mixer
  • Bubble level
  • Wood
  • Levels
  • 2X4's
  • Rake
  • Float
  • Mallet

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  1. 1

    Prepare the work site for the new slab. Remove any vegetation and then dig the area to a depth of about 4 inches.

  2. 2

    Make a wood frame for the three open sides. Match the frame height to the height of the existing slab height. Attach wood to stakes inside the depression and along the perimeter on the three open sides. The existing slab will serve as the fourth side. Check to see that the frame is level by placing a bubble level on top of the wood frame. Check about every 3 to 4 feet and adjust the frame accordingly.

  3. 3

    Mix concrete. Prepare the concrete in the cement mixer. Pour the concrete into the depression all the way to the top of the frame. Spread concrete with a cement rake, and then press it out using a 2X4 length of wood. Use a float to smooth out the concrete. Allow concrete to dry for at least 24 hours or according to cement package directions.

  4. 4

    Remove the wood surrounding the slab. Using a mallet, pound the wood away from the concrete. Use the claw end of a hammer to pry the wood loose if you need extra help. The new slab will now be connected to the old concrete slab.

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