Whether you estimate by hand or with a computer, you probably use square-foot unit prices to calculate some costs. In other words, you use square-foot area scaled off the floor plans to come up with the total cost of construction. Even "stick" estimators, who laboriously count joists and rafters, use unit prices for roofing, drywall insulation, flooring, painting, and other parts of an estimate. Unit-price estimating is fast, but it's not always as accurate as it needs to be. The method is least accurate when it's applied to the overall square footage of an entire building, but it still falls short when you break a job down into smaller parts and estimate each one separately. The problem is that unit pricing based on floor area doesn't account for the shape of the structure you're estimating. To illustrate what I mean, let's look at an example. The three simple floor plans shown here all cover 400 square feet (see ). Their shape, however, varies from a perfect square (A) to progressively longer and narrower rectangles (B & C). Estimating by the square foot, the cost to build each of these floor plans would be the same. A closer look reveals, however, that the cost actually varies quite a bit, and the difference is a result of the shape. To see how much the price could vary, let's look at the quick and dirty takeoff, also shown in . As we do so, keep in mind that the three shapes we're looking at could be just a small part of a larger floor plan for a two-story building. In that case, any cost differences between them could be five or ten times higher than the figures we'll come up with. Floor framing. To frame Plan A with conventional joists, we'd need to build a beam at midspan and install columns to hold it up. Since Plans B & C can be framed with 16- and 14-foot joists, respectively, right off the bat Plan A requires extra material and labor to build. (If we flush-frame the beam and use joist hangers, the floor system in Plan A will cost even more.) When it comes to the joists, Plan A is also more expensive. Even though the joists differ in length in the three plans, the total board footage is about the same, so material costs won't differ much (plywood subflooring is the same for all three, too). But it will take more time to handle, cut, and install the 32 10-foot-long joists in Plan A than it does the 20 16-footers in Plan B or the 25 14-footers in Plan C. Wall framing. Because the three floor plans are different shapes, the perimeter dimension varies. This affects the linear footage of wall plates we'll need, as well as the number of studs and sheets of wall sheathing. The material and labor differences are small but remember, these shapes could represent just one of several similarly sized rooms, so the total difference in the cost to frame all of the walls could be much higher. Roof framing. The cost is also different if we estimate simple 12/12 gable roof framing with conventional rafters. Assuming the ridge runs left to right in the illustration, Plan A requires the fewest number of rafters, but the board footage is the highest because each rafter is 16 feet long. Plan B has six additional rafters, which, although only 14 feet long, will require proportionately more labor to install. Plan C has 12 additional rafters that will take half again as long to cut and install as in Plan A. In addition, Plan B requires 25% less plywood roof sheathing than the other two plans. Taking material and labor together, the cost to frame the roof is about the same for Plan A and Plan B, but 8% to 12% higher for Plan C. (Roofing costs are nearly identical - a little over 6 squares for each plan.) Finishes. Insulation, drywall, siding, and painting are also affected by the different perimeter dimensions. There are an additional 15 square feet to cover in Plan B and an extra 72 square feet in Plan C. Compared with Plan A, the installed cost for all of this work is about 11/2% higher for Plan B, and 7% higher for Plan C. It all adds up. Now that we've got our three floor plans framed and dried in, let's see how these incremental differences in cost add up. Plan B comes in low at $8,492; Plan A is just 1% higher at $8,582. Plan C, however, is 6% more expensive, at $9,027, than Plan A. That difference in cost is just about what we'd take home in profit on the job - and we've only estimated part of the work, so there's still more money to lose. Plus, if we consider that these plans may represent only one part of a more complex structure, the differences in cost are even greater. If the building had a second floor, for example, the dollar differences would be almost double.
One final example further illustrates how shape affects cost when using unit prices based on floor area. The three "rooms" in all contain exactly the same square footage, but they differ radically in shape. Looking only at the cost of running baseboard, we again find that shape affects cost. Room A is a simple rectangle with four inside corners....
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