Page 57 - 2018 National Home Improvement

2018 National Home Improvement Estimator, All Rights Reserved Page 42

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. It’s 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, won’t 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 that’s expensive to repair.

Concrete forms. Most concrete form lumber can be used more than once, either several times for
concrete forming or once for forming and then on some other part of the job when forming is done. The
figures that follow include options for multiple use of form lumber. These options assume that forms can
be removed, partially disassembled and cleaned. Then 75 percent of the form material will be reused.
Costs include assembly, oiling, setting, stripping and cleaning. Adjust material costs in this section to
reflect your actual cost of lumber. Here’s how: Divide your actual lumber cost per MBF by the assumed
lumber cost. Then multiply the cost in the material column by this adjustment factor.