How to Install Concrete Floors

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One of the first steps in a home refurbishing project is to determine the type of sub-flooring you will need. If you are enclosing the garage in an older home, it may very well have a dirt floor. Also, if you are adding a room to your house, you will need to decide what type of flooring you need there. For either area, the most stable flooring is concrete. The steps that follow will accomplish either type of addition.

Things You'll Need

  • Rebar
  • Vapor barrier material
  • Straight edge (usually aluminum or a long 2 by 4)
  • Rubber boots
  • Steel trowels
  • Edging tool
  • Concrete vibrator
  • Bags of sakrete
  • Type 1 cement

Prepare the Foundation

  • Lay out the dimensions of the concrete slab by setting stakes at the corners of the proposed structure. These stakes offset to the outside of the exterior walls of the slab by three feet and run parallel to the formwork. The stakes will have notched grade marks needed to set the height of your forms.

  • Make your forms from wooden 2-by-4's and half-inch plywood. Put the forms together with double-headed nails. Use stakes to secure the forms. The forms are set to grade by following a string line pulled from point A to B. The corners will be set where two string lines cross. From here, you can insure that the formwork is level.

  • Know how deep you need to go with the slab. The depth of a typical slab is four inches, with the exterior walls going down below the natural grade of landscaping by at least six inches. However, if you live in an area with heavy freeze and thaw cycles, go down to below the normal frost line. Bring in sand for fill to bring up the interior grade to four inches. Use a string line stretched across and measured down to the top of sub-grade for control.

  • Attach string line to the top of the wooden forms about every 12 inches. The string line should cross over to the parallel board. Check the level on each line using your leveling tool to insure accuracy. This process will level the dirt floor to an even grade.

  • Spread plastic sheeting across the interior of your sub-grade, going down into all areas on the inside of the form work. This makes a vapor barrier to protect the moisture from the concrete from penetrating the formwork.

  • Place reinforcing steel (typically #3 rebar) horizontally around the exterior grade beam and tie it together using tie-wire. At the corners, use a #3 L bar to make the corners and #3 L bars to tie the steel in exterior grade beams to the steel used in the slab itself (either a wire mat or #3 bars on 24" centers each way). Keep slab steel off the ground by using pieces of brick so the concrete can get around the steel.

  • Pull string line from side to side of the form. Take short pieces of rebar and drive them into the ground (down the center and at the quarter points) to just below the string line.

Prepare the Floor

  • Contact a concrete company for delivery of 3,000 PSI concrete (approximately 4 to 5 sacks per cubic yard of concrete). To mix the concrete yourself, buy Sakrete at your local home improvement store. Invite your friends to help. You can bribe them with some great barbecue.

  • Pour the concrete in the beam at the outer edges. Rent a concrete vibrator to use in consolidating the concrete. Use the probe of the vibrator along the outer edges of the beam every 12 inches. Once vibrated, do not go back. Pour it as dry as possible. Start at one corner and work toward your exit.

  • Begin leveling the grade of the floor using the straight edge. One person will make sure there is concrete in front of the straight edge as you pull it towards you.

  • Control the depth of the floor by using the pins that you set earlier. Take a metal trowel and strike off the concrete at the top of one of the pins. Take a hammer, and drive the pin down so it will not become a problem in the future. Repeat with all other pins down to half of the floor. Allow the concrete to partially set.

Prepare the Finish

  • Take a smooth steel trowel and a plywood pad (precut to 12 to 24 inches). Ease the pad onto the concrete and kneel down upon it. Try not to dig anything else into the concrete. The plywood should spread your weight.

  • Smooth in a circular motion to give a smooth finish. Work your way from first to last poured. Place bolts on the edge of the concrete of the outer walls to hold the framing down.

  • Add water using a misting spray bottle (as needed) during the finishing. This will help to smooth the surface. You may need to add some dry cement (typically Type 1 Cement). Sprinkle on the surface (as needed) to help you put on a nice slick finish.

Tips & Warnings

  • All of the above tools are available by rental or at a home improvement store.
  • You can rent a concrete mixer large enough to hold 2 to 3 sacks of Sakrete at a time if pouring the concrete yourself.
  • You should rent a concrete vibrator to use in consolidating the concrete.
  • If you know how to operate a concrete finishing machine, you should rent one as well.
  • This should be a one to two day project for the amateur renovator.
  • Prior to pouring your floor, buy several bags of Sakrete and use one bag at a time to practice your steel trowel finishing.
  • This is a big job--take it on with caution. Have an expert in easy reach for advice.
  • Be sure to check to see if you need a building permit for this work.
  • Remember that concrete can still crack in a floor just as it does in any other project.

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