Calculating slab area. Patio slabs and driveways are often irregular in shape. For example, a driveway may be wider at the garage end and narrower at the street. The easy way to figure the area is to measure the width at both ends and then divide by two. Then multiply the result by the length of the driveway. For example, assume:
  The length of the driveway is 30 feet
  The driveway width at the apron end is 22 feet
  The driveway width at the street tapers to 8 feet
  Add width at the two ends: 20' + 8' = 28'
  Then divide by two: 28 feet divided by 2 is 14 feet
  Then multiply by the average width by the length: 14 feet times 30 feet is 420 square feet.
A cubic yard of concrete will cover the following area, assuming no waste:
  At 2" thick, 1 cubic yard covers 162 square feet
  At 3" thick, 1 cubic yard covers 108 square feet
  At 4" thick, 1 cubic yard covers 81 square feet
  At 5" thick, 1 cubic yard covers 64.8 square feet
  At 6" thick, 1 cubic yard covers 54 square feet
  At 7" thick, 1 cubic yard covers 46.2 square feet
  At 8" thick, 1 cubic yard covers 40.5 square feet.
If the job requires 420 square feet of 4" concrete, divide 420 by 81 to determine that 5.18 cubic yards are needed. Itís good estimating practice to allow 5 percent for waste and over-excavation (removing more soil than needed to maintain slab or footing depth). Add 5 percent to find the order quantity of 5.44 cubic yards.
Garage slabs. Slabs are poured monolithic and should be pitched from rear to the front opening so snow melt drains to the street. Garage slab floor drains are poor practice in colder climates, as melted snow freezes in the drain and tends to crack the slab. Keep the slab surface 4" above grade to provide good drainage. When grade beams are required by code, excavate the additional depth around the perimeter of the slab. No additional forming is needed. The beam is poured as an integral part of the garage floor. Wire mesh or rod reinforcement is good practice, even if not required by code. If the driveway includes a change in grade, be sure the incline, including any swale, wonít be too steep for most cars. Be sure the grade at the street or alley matches the existing grade.
Consider the drainage pattern before quoting concrete work. Water draining around the perimeter of a slab can cause damage thatís expensive to repair.

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