The foundation of every computerized estimating system is its database of unit prices. Unit pricing looks at typical project components, such as an exterior wall or a floor system, and calculates the cost per a particular unit of measure, such as a square foot, a square yard, or a linear foot. To develop a unit price, you need to do a stick estimate, but you need to do it only once for each component in your database. The stick estimate system I discussed in the previous article has all the elements needed to create a unit price.

In this article, I’ll look at the example of calculating a unit price to frame a typical exterior wall. I find that working from a sketch of the wall, such as the one above, makes it easier to visualize all the parts and pieces that need to be included. As with the custom-item worksheet, the unit-price worksheet uses a unit of measure that fits the kinds of materials and dimensions for the task at hand. In this example, I have chosen to build a unit price based on the materials needed to frame an exterior wall that is 20 feet long and 8 feet high. (We’ll look at adding labor to this unit price next time.)

Waste is something to consider when making this initial takeoff. For example, I’ve listed 24 8-foot studs for this wall, which is the actual count. But you may discover over time, as I did, that jobs keep coming up short by a few 2-bys. My solution today is to add one additional stud.

There are other strategies: Some contractors add a waste factor to allow for damaged material or to provide lumber for bracing; others simply round up or add a separate line item. However you treat this issue, in the end it’s cheaper to have figured a little extra than to make a run to the lumberyard for more material.

Because the base wall has a total area of 160 square feet (8x20), all of the costs listed in the worksheet will have to be divided by 160 to yield the price per square foot (see table, facing page). I’ve also included calculations for a price per linear foot, which some contractors prefer. The difference is that a linear-foot price applies to a wall of a particular height, while a square-foot price can be used on shorter or taller walls, provided they are constructed in more or less the same way. A square-foot price could also be applied to a gable wall, although labor might vary with wall height and roof pitch, as well as whether the walls are framed “on the ground” or in place.

This is a fairly simple example, but you get the idea. You could create similar linear-foot and square-foot unit prices for exterior walls constructed using 2x6s at 24 inches on-center. And you could create linear-foot versions for 9- and 10-foot-high walls, and the square-foot unit prices for 2x4 or 2x6 exterior walls could also be used to find the cost of walls taller than 8 feet. All together, that would give you six options for basic exterior wall framing.

You could also expand the materials covered by the unit price to include metal connectors, insulation, drywall, even siding. And you can build these so-called “assemblies” for other parts of the building, including floor and roof framing, siding and roofing, exterior and interior trim, and so on. Assemblies are common in computerized estimating systems, which often come preconfigured with unit measures and pricing. But if you want to customize estimating software to reflect the way you actually build, you’ll want to go through something like the process I have just described.

You can create as many assemblies as you find useful, but it’s possible to overdo it. This process works well for those building elements for which you have a “standard” method of construction. If you include an element that is used only occasionally (rainscreen siding in a wall assembly, for example), that assembly becomes useless to you on jobs that aren’t built to that spec.

Note that this system gives you a rough material quantity, but it won’t deliver a bill of materials that a lumberyard can use to quote current pricing and build a pick list the way a computer-based system can. Estimating software can also introduce rules that improve accuracy, and they can use menu systems that allow you to specify number and size of windows to change the dimensions of the lumber used in a wall or floor.

That said, once labor hours and prices are added, it will give you a reasonably accurate price for direct costs. We’ll look at adding labor and prices, plus menu systems in future columns. But at least with this, unless you sell pre-construction services, a simple, manual unit-pricing system will keep you from investing too much time in projects for which you may never sign a contract.