The floor is the last portion of the basement you will pour, after completing the footings and the concrete walls. Usually poured in sections, the basement floor must be level and care taken to prevent future cracking. Unlike pouring an open slab of concrete, the basement floor has the unique aspect of sitting within vertical concrete walls so some special techniques will allow you to smooth the concrete from only one side.
Install plumbing stubs for future drainage and sewer before pouring the floor. Most new basements include these stubs even if there are currently no plans to finish the basement. Cover these pipes where they extend out of the ground to keep them free from debris.
Cover the dirt with sand at least 3-inches thick to reduce the likelihood of the floor shifting and cracking. Not all cracking is preventable, especially in soils that move but sand is the best choice for creating a compact base.
Install rebar reinforcement in the area you are pouring by spacing the bars a minimum of 2-feet apart. Use rebar chairs underneath the bars where they intersect to raise the rebar into the center of the proposed pour.
Establish the height of the concrete floor using a transit or a laser level. This is the most accurate way to make sure the floor will be level but if the top of your basement walls are level, you can measure down to a distance at least 3 ½-inches above your compacted sand and make a mark on the wall.
Install a screed guide, constructed of 2-inch by 4-inch dimensional lumber that is a long as the wall beside which you will pour. Measure upwards exactly 5 ½-inches from the earlier line you made indicating the floor level. Use concrete nails to attach this board to the inside of the concrete wall. The top of the board will be level with the line.
Construct a screed, using a 2-inch by 6-inch dimensional board. Cut this board 1-foot longer than the width of the pour. This board is actually 5 ½ inches wide, not 6-inches, although it is labeled and sold as such. Attach the screed vibrator to the middle of the screed and extend the switch cord to the open end of the pour.
Fashion a cleat, made from 1-inch by 2-inch lumber by cutting a board 2-foot long and fastening it to the top of the screed, leaving a 2-inch overhang. The overhang will rest upon the screed guide you attached to the inside of the wall, allowing you to level the concrete from one side.
Set a form on the other end of the pour for the opposite end of the screed to rest upon. The top of this form will be level with the proposed floor height and will hold in the concrete until it sets. Form an area only large enough that you and your assistants can comfortably pour at one time.
Pour the concrete in one continuous pour – spreading quickly to the corners. Use shovels or concrete drags to push and pull the concrete quickly into the corners. Once you begin pouring the concrete, work quickly to make sure it all sets at the same time.
Pull off the surface of the wet concrete with the screed, placing the cleat on the screed guide and holding the free end by hand. Turn on the screed vibrator and keep the screed upright as you pull it from one end of the pour to the other. Repeat at least one more time.
Use a bull float to smooth the surface of the wet concrete, starting at the open side of the pour and pushing it and pulling it back and forth, moving from one end to the other until the concrete is smooth. The float encourages the finer particles in the concrete to rise to the surface, creating a smoother floor.
Pour the opposite end of the basement floor in the same manner and pour the middle section last. The middle section is the easiest to pour since you need build no forms. The previous two sections provide the form and your screed will fit on top.