In my previous column, I began a discussion about how I create a "unit" for a unit price estimate. I used framing 20 linear feet of a typical 8-foot-high exterior wall as an example and described how to do the materials take-off using a sketch.

Labor is more difficult to estimate. The time it takes to do a task is subject to a multitude of issues that have had estimators pulling out their hair for decades. While there is no "silver bullet" formula for estimating labor that will work for everybody, there are some procedures that I am fairly sure will not work for anybody. The most common is the "guesstimate." This seat-of-the-pants method depends solely on the experience and ability of the estimator. It may work for an individual estimating his own labor on a limited number of projects, but beyond that, this method is the most risky and least accurate.

Other contractors rely on books or websites that publish labor rates for construction tasks. Some specify costs; others provide man-hours to which contractors apply their own labor rates. Similarly, estimating software often comes with cost databases that include labor rates. These resources can be reasonably accurate, as long as you adjust for geographic variations and understand the assumptions they make about working conditions, crew size and ability, and other factors.

The best way to get labor numbers that truly reflect your operation is to keep good job-cost histories and feed that data back into your estimating system (see "Using a Simple Timecard to Collect Labor History," Jun/13). If you haven't yet implemented that system, you can jump start it by choosing a single task, like wall framing, and keeping close track of man hours used over the course of several similar jobs. Then use that data to calculate average labor hours required for the task based on the actual abilities of your crews and the actual conditions on your jobsites.

To use your own data to find hrs/BF or hrs/SF, divide average hours used for each framing task by total SF or BF of materials used. (BF = thickness x width [in inches] x length [in feet] ÷ 12). This table shows figures for tasks associated with framing and sheathing a typical exterior wall.
To use your own data to find hrs/BF or hrs/SF, divide average hours used for each framing task by total SF or BF of materials used. (BF = thickness x width [in inches] x length [in feet] ÷ 12). This table shows figures for tasks associated with framing and sheathing a typical exterior wall.
This table shows how total labor hours are calculated for a typical 20-foot-long exterior wall by multiplying hrs/BF or hrs/SF by total material quantities for each of three framing components.
This table shows how total labor hours are calculated for a typical 20-foot-long exterior wall by multiplying hrs/BF or hrs/SF by total material quantities for each of three framing components.

Whether you use published cost data or your own records, here's a method that applies a unit of time to the material quantities used—the table (top) shows how it applies to our 20-foot wall example. Published cost data usually provides labor in terms of hours per board-foot (hrs/BF) and hours per square-foot (hrs/SF). To convert your own labor data to these units, add together all of the framing labor on all of the jobs for which you've been keeping records—the more data, the better—then divide the total by board feet of lumber and square feet of sheathing used on all of those jobs. This will give you hrs/BF or hrs/SF to plug into the "Hours" column.

The bottom table shows how to find total labor to frame our 20-foot wall by multiplying BF and SF of material times hrs/BF and hrs/SF. Board feet equals thickness x width (in inches) x length (in feet) ÷ by 12. (For example, an 8-foot 2x4 contains 5.33 BF — (2x4x8) ÷ 12). "Total Hours" (6.64) would be plugged into the labor section of the "Item or Task Worksheet" (Aug/13) and multiplied by the hourly crew cost to arrive at the cost of framing labor.

Next time, I will show how to compile all we have discussed into a complete estimate using "assemblies."

—George Weissgerber, a senior vice president atCase Design/Remodeling, in Bethesda, Md., developed the company's estimating system and handyman division.