Expert: Roger Zona - 9/12/2009

Question
We're building a 22x36' garage in Omaha, Ne.  We have a 42" deep block foundation.  The block wall continues above grade, so the slab is just on grade. We're pouring a 4" slab for the garage floor - the soil is very well compacted and has a layer of crushed limestone.  My question is how much rebar is needed for the slab?  We were told to buy 20' sticks of what looks like 1/2" rebar - is this enough?   thanks.

Answer
Hello Jenny,

Sounds like a good project.  Since garages are seldom heated the frost will extend into the garage - therefore it is recommended that you do provide some type of reinforcing.  It also depends on the load that may be imposed on the slab.  If you will be parking cars only that is not an excessive load.  

Typically a wire mesh (6" x 6" x #10)is used.  This mesh comes in 6' wide rolls and can be cut with a small wire cutter.  The strips should be lapped and wired together.  Unroll the wire on an open area and bend it to eliminate the curling effect.  Hold it 2" short of going to the wall since your floor slab is "floating".  That way the concrete encapsules all the metal.  

If you wish to use reinforcing rod, a #4 re-rod (1/2" dia.) is adequate and you should create a "mesh" by laying the first layer at 6" to 8" apart and the second layer in the opposite direction and wired to the 1st layer.  Because your slab is larger than the 20' rebars, you should extend the rod by lapping another piece of rebar at least 12".  Wire the lap in 2 places.  These rods should also be 2" short of the slab edge.  Generally your concrete supplier (ready mix plant) also sells rebar and they can cut the bars to actual length. 22' less 1'4" for the masonry, another 1" for expansion material and 2" for concrete coverage - 20'-4" rod.  (You could get by with a 20' rod.)

Now, for a very important feature.  I would recommend that you provide a fibre expansion joint around the entire slab that abuts the walls.  Further, you should cut expansion joints in both directions of the slab (one approximately every 10').  Wherever there is a door, run the slab through the opening and put a slight pitch to the opening.  Also, where overhead doors are installed you should provide a recess for the door and track.  Usually a 2" x 4" layed flat and expending beyond the door opening approx. 8" will be adequate.  Check with your door supplier for actual dimensions.

Good luck.  

Roger
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