How To Prepare A Foundation Slab

Once you have your forms in place for your foundation slab, either wood or block, you must prep the slab for the pour. This entails installing plumbing drains, water supply lines, any electric needs for floor receptacles of islands, backfill, termite treatment, reinforcing wire, plastic sheeting, and screed boards. Refer to the plumbing pages for the plumbing information. Refer to the electrical pages for the electrical information.

STEP 1: Place any back-fill required. The purpose of the back-fill is to reduce the amount of concrete you will pour in the foundation slab. I recommend a coarse sand for this fill. As you place the fill sand, be careful not to disturb the electric and plumbing. Pull strings across the foundation area to maintain the floor height as a reference and place the fill up to four inches below the strings. Use a hard tooth rake to level the sand and even it out. Compact the fill as needed. Compaction requirements will vary depending upon the type fill you use.

You can use power equipment to place the fill, but you must be extremely careful with it. Any tracked vehicle slides when it turns and you can very easily knock your forms over.

STEP 2: When the fill is in place, dig out any grade beams required.

STEP 3: Clean the footing trenches out and lay the rebar in the trenches. We are using three bars of 1/2”. Use a plastic or metal wire stand, called a “three bar rod chair” to hold the rebar in place, up off the trench floor. Space these out every four or five feet and on either side of corners.

Overlap the rebar and tie it together with wire ties. These are 6 or 8 inches long pieces of wire with a loop on either end. Wrap the wire around the rebar and use a tie tool to hook the loops and twirl the wire around several times until it tightens on the rebar. The tie tool is available where you get the wire ties. Sometimes the ties are called “bar ties”. Wrap each junction at least twice. In corners, run the rebar long enough into the corner to reach the opposite rebar on the other side of the corner. This does not have to be precise.

STEP 4: After all the rebar is placed in your foundation slab, cut four foot pieces of rebar, three for each corner. Bend these pieces 90 degrees, as sharp a bend as you can reasonably make. A trailer hitch receiver makes a fairly good holder in a pinch. They also make rebar bending tools that help, but don't do any better job than the trailer receiver.

STEP 5: Place the corner pieces in the corners, one to each run of rebar and tie them in place, at least two ties on each side of the corner. All the rebar should be in the “top of the bottom third” of the height of the trench. So, a 12 inch deep trench means the rebar should be about 4 inches off the bottom. Turn one of the three pieces the opposite direction in the trench end at the front of the wall that divides the garage and the house.

STEP 6: After all the dirt and fill work is done have the termite pre-treatment done for your foundation slab. Be sure to keep the record of this treatment.

STEP 7: As soon as the treatment is completed cover the foundation slab site with 6 mil poly. The poly is to prevent moisture from coming up thru the concrete and affecting your floors. Cut or notch the plastic around all protrusions as close as possible. Lap subsequent sheets at least a couple of feet. Allow the plastic to hang over into the footing trench but do not cover the rebar. A few nails thru the plastic will hold it in place.

STEP 8: Next place the 6x6 concrete reinforcing wire in the foundation slab. This wire helps strengthen the slab. They have a fiber product that can be added to the concrete that supposedly will replace the need for wire. I have never been impressed with it, or had much luck with it, either. I only use fiber instead of wire in walkways and porches. For foundation slabs and driveways, I use steel and wire.

Word to the Wise: Concrete wire is difficult to handle and it can roll up on you and injure you if you are not careful. Wear gloves and be careful!

STEP 9: On a large open area roll the roll of wire out the length of your foundation. The sections are 6 inches so you can walk the length, carrying the bolt cutters with you. Have someone stand on the end of the wire to hold it down.

STEP 10: When you reach the correct length, cut the wire a foot or two shorter than the foundation, while standing on the wire.

STEP 11: When you finish the cut, drop the cutters and pick up the wire with both hands.

STEP 12: After you have a secure grip, step off the wire and turn around facing your helper. Have your helper grab the wire securely and step around to face you, as well.

Word to the Wise: In the event that one or the other person lets the wire go, the other person simply needs to let go, as well. If they don’t, a train wreck is about to happen!

STEP 13: Now flip the wire over.

STEP 14: On each end, lift the wire and walk back down the wire length ten feet or so, bending the wire back as you go. Once this is done, allow the wire to return to laying flat and it won't roll up.

It is easiest to pre-cut most of the wire lengths you will need.

STEP 15: Then, with a helper or two, carry the wire into the slab area and lay it in place. Do not cover the footing trench, but you can allow it to hang over into it. You may need to clip a section or two to allow the wire to lay around your plumbing.

STEP 16: Repeat the process with other lengths of wire until the entire slab area is covered. Lap the wire runs over each other one section, then tie wire the runs together every two or three feet.

When you are finished, the entire bed of wire should effectively be one piece, all tied together. Make sure that the wire is laying flat and will not stick up above the concrete in the foundation slab.

STEP 17: Now you need to place your screed section boards. These are boards placed inside the slab area to insure you maintain the correct thickness of concrete and finished height. Divide the house slab into sections of 14 feet.

STEP 18: Pull a string from one end to the other on your section division marks. Along that string, using surveyor stakes and 1x4x10' boards, set the 1x4 on edge, screwed to the surveyor stakes driven into the ground of the slab. These stakes must be deep and secure enough to maintain the height, but not so deep as to prevent them from pulling out fairly easily. Hammer the stakes in place then screw the 1x4 to it, at the exact height of the layout string. A little weaving in and out of the 1x4 is no problem – what matters is the height.

The 1x4s should butt to the form board and run all the way to the other end, butting end to end, and butt to the far form board as well.

Word to the Wise: Use ten foot long 1x4s because when you go to pull them out, you will be reaching from one end. If you use really long boards, you will be forced to mess up your newly placed concrete to get the screed boards out, causing extra work.

Your foundation slab is now ready to pour.

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