Home Improvement Questions? Ask a Handyman for Answers
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Now..a yard of concrete is a cube 36" x 36" x 36".
So, you have (in inches) a slab that is 120" x 240" x 3.5"
Therefore you need (120x240x3.5) divided by 36x36x36 which is just over 2 yards (2.16 yards). If you order 2.5 yards, you can get almost 4" of thickness.
Let me know if you need further information.
Putting new sheetrock in bathroom. old house had to