Preparing for concrete flooring

Check the criteria for floor design and construction in your town’s building code. Compact the base dirt and put the floor directly on it if it is well-drained. If the soil is soft or loose, you’ll need to dig it out and put a 4 inch crushed stone and 1-inch sand subbase; compact it carefully. Drainage should be provided by sloping concrete floors; basement floors should slope toward drains. The garage floor slopes upwards toward the garage door. By sloping the subgrade beneath the floor, you can achieve a uniform floor thickness. Spread 1 inch of sand over the top of the footing before putting the concrete to prevent the basement or garage floor from bonding with the footing.

Strike-off board support

Install ducting beneath the slab for pipes and cables. Drive 2 x 2s approximately 8 inches into the ground to hold the side forms, making sure their tops are totally flush with the surface. After the stakes have been driven in, lay a vapor barrier of strong polyethylene sheeting over the base, overlapping the sheeting joints by 4 inches.

Vapor barriers

Run the vapor barrier up the wall to the full height of the floor; in a masonry wall, run it up to the first course above grade. For the side forms, use 6-foot lengths of 2 x 4. Attach the side forms after installing the vapor barrier, driving the nails only ½ inches into the stakes. Side forms in this instance are used only as supporting surfaces for the strike-off board and are therefore merely tacked to the stakes so that they can be removed without very much difficulty after doing the concrete flooring.

A ½-inch-thick asphalt-impregnated isolation joint separates the floor from the walls. Around steel drains and Lally columns, isolation joints of the same thickness should be installed. Using a 1:2½:3½  concrete mix, create a 4-inch thick garage or basement floor. As a strike-off board, use a 2 x 4 that’s somewhat smaller than the room’s width. The control joints should be spaced about 10 feet apart to avoid cracking. Cut the joints ⅕ to ¼ inches deep and all the way across the slab with a groover. Finish the surface, then cover it with polyethylene sheeting and wait 6 days for it to cure. For a smooth surface that can be painted or to which tiles are to be applied, finish the concrete with a steel trowel.

Information sponsored by M3 Industries of St. George, UT.