Preparing The Sub-Floor

Published on Wednesday, June 12th, 2013

One of the most popular flooring materials is concrete. It’s strong, versatile, and durable. It is also used as a base for all other types of flooring except wood. Thus, it’s important that the base layer of concrete is flat, smooth and level.

Before ever pouring concrete, prepare the soil underneath the building. Concrete essentially “floats” on the dirt underneath it, and so the soil under it must be firm and well-drained. In areas with a high percentage of clay, mix sand into the top few inches of soil to facilitate drainage and proper compaction. Moisten the dirt as you work with it, but avoid creating mud. Excavate the dirt to 12 inches or more, depending on soil type and height of the building, and replace the backfill a few inches at a time, compacting with each new addition of fill.

Measure out the dimensions of the slab, and drive wooden stakes around the perimeter. In curved areas, you’ll use bender board, and in straight areas 2x4s or 2x6s, depending on how thick the concrete will be. The stakes will be closer together on curves, since bender board is thinner and naturally flexible than lumber. Run a string across the tops of the forms from one side of the slab to the other, and use a string level to determine the proper slant. It should be level for all interior areas, and should feature a drop of one inch per twelve feet in areas that should drain water.

When pouring the concrete itself, the driest possible mixture is best. While a wet mix will spread more easily, a dry mixture is stronger when cured. If there are a lot of rounded corners, a drier mix will be easier to get to the edge of all the forms.

Wait until the water begins to seep out of the top of the mix before troweling a flat, finished surface. Add any texture, and allow to cure. Contrary to what might seem intuitive, concrete cures best when it is kept damp. Concrete that dries out too fast will crack and be weaker in general than concrete that is given a week or so to fully harden. Apply plastic sheeting to the concrete in hot weather, and hose down the concrete to keep a high level of moisture in contact with it during the first week.

Don’t let anyone walk on it, and don’t attempt to put down any other flooring such as tile or carpet for at least two weeks after pouring. You might also need to grout cracks and these will may not appear immediately, so it’s good to wait a little and see how the slab settles. If you’ve done your soil preparation right, the settling should be minimal.

For more information on preparing concrete for flooring materials such as carpet or tile in Scottsdale, give us a call!

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