How to Pour a Concrete Floor for an Existing Garage

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Many garages in homes constructed before 1950 may still have dirt or sand floors since commercial concrete delivery wasn’t available then in all areas. The wall frame on these garages rested upon a concrete or stone stem wall to keep the wood off the ground. Alternately, perhaps you’ve removed a damaged concrete floor from inside your existing garage and now want to replace it. By following some general concrete guidelines, you can pour a new floor in your garage.

Things You'll Need

  • 2-inch by 4-inch dimensional lumber
  • Circular saw
  • Carpenter’s level
  • Concrete tools
  • Concrete
  • Power concrete stud vibrator
  • Assistants

Prepare to Pour

  • Plan to pour the floor in two sections in order to allow room to smooth and work the concrete. If the garage is wider than a normal two-car garage, you may opt to pour it in three sections.

  • Determine the grade of your new garage floor. Using a carpenter’s level and the ratio of a ¾-inch drop for every 4-feet, begin at the back of the garage and hold a string line along the wall at the height of the proposed floor and slope it downwards towards the front of the garage. For proper drainage, use this same grade when you pour the approach in front of the garage.

  • Frame one-half of the garage floor, using 1-inch by 2-inch stakes to support 2-inch by 4-inch dimensional lumber cut to fit. The top of the board will reflect the same grade as determined in Step 2 on both sides of the pour.

  • Fill in the low places of the dirt floor with sand to reduce the risk of settling and cracking. The final soil or sand level should fall at least 3 ½-inches below the top of the proposed floor level.

  • Construct a screed guide by attaching a 2-inch by 4-inch board to the wall of the garage, parallel to the form below and exactly 5 ½-inches higher. This guide will serve to balance the screed, allowing one person to guide the screed from the other side of the pour.

  • Make a custom screed, cut to just a few inches longer than the width of your formed area by using a 2-inch by 6-inch piece of dimensional lumber to which you attach a “dog ear,” a cleat made of 1-inch by 2-inch board, positioned along the top of the screed with 2-inches extending over one edge.

  • Attach the screed vibrator to the top middle of the screed, with the cord extending towards the open side of the formed area. Reinforce the formed area with rebar on a minimum of 16-inch centers. This strengthens the overall concrete. Use rebar chairs to hold the rebar up and off the sand.

  • Measure and order a little more concrete than you think you will need. It’s better to have some left over than too little. Concrete trucks often charge a delivery charge for each trip and they may charge you for a “short load” if you need to order additional concrete. Use a handy online concrete calculator to determine how much concrete to order. (See Resources)

  • Spread the concrete into the corners of the pour when the truck arrives and begins dumping. This is hard work and having assistants with strong backs is imperative. Once you begin pouring concrete, you are working against the clock to smooth it before it hardens. Work quickly.

Ready for Concrete

  • Measure and order a little more concrete than you think you will need. It’s better to have some left over than too little. Concrete trucks often charge a delivery charge for each trip and they may charge you for a “short load” if you need to order additional concrete. Use a handy online concrete calculator to determine how much concrete to order. (See Resources)

  • Spread the concrete into the corners of the pour when the truck arrives and begins dumping. This is hard work and having assistants with strong backs is imperative. Once you begin pouring concrete, you are working against the clock to smooth it before it hardens. Work quickly.

  • Screed off the surface of the wet concrete, positioning the screed at the back of the pour, with the dog-ear cleat resting upon the screed guide and working your way forward. Turn on the power vibrator and guide the screed towards the front of the garage. Repeat this process at least one more time.

  • Float the wet floor with a concrete bull float until it is very smooth. The bull float will bring the finer particles in the wet concrete to the surface. When the concrete sets up slightly, use a corner trowel around the edges for a nice finish. Pour the other section in the same manner.

Tips & Warnings

  • While it is better to pour concrete only against a stone or concrete stem wall, some older garages feature wood low on the walls. When you have to pour concrete against wood, use metal flashing to separate the two.
  • Use proper safety precautions when using power saws to cut boards.
  • Protect your skin and eyes from contact with wet concrete by wearing gloves, safety goggles and rubber boots that pull on over your shoes.
  • Photo Credit Photo, curtesy of Stock.xchng, author images
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