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
Shape Affects Finishes
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. A
good finish carpenter could run the base in 20 minutes. Plan B
is a bit more complex. The short walls on the left side
introduce four additional inside corners and four outside
corners, plus another 8 linear feet of straight baseboard. The
alcove on the right adds another two outside and inside corners
as well, plus 10 linear feet of additional baseboard. The
carpenter trimming out Room A will take a little longer in Room
B and will use more material.
Room C is even worse. It requires 16 linear feet more
baseboard than Room A, and its eight inside and four outside
corners are going to chew up more of our trim carpenter's
The exact difference in cost to run the baseboard will vary
- you can do the math. But the point is, if you use a straight
square-foot unit price to estimate the base mold for these
three rooms, you'll lose money on Rooms B and C. A linear-foot
unit price would be better, but the outside and inside corners
- which is where the work is in running baseboard - will still
throw off your price. You'll lose even more if the rooms call
for crown or wainscoting, both of which are more
labor-intensive than baseboard.
Tweak Your Formulas
fairly easy to account for these kinds of cost differences if
you estimate stick by stick, whether manually or by computer,
but you purchase accuracy with the time it takes to laboriously
take off the material, item by item. Unit prices are faster,
but to improve their accuracy, you need to tweak your formulas
to account for small cost differences that accumulate into
prices that are either too high or too low.
To solve the problem of running baseboard in oddly-shaped
rooms, for example, you might add a step to your unit-price
formula that takes into account the number of inside and
outside corners. You could either assign a dollar value to
"extra" corners or bump the square footage up by a percentage.
Likewise, for the floor plans in , you need either several
formulas or several different unit prices to account for
varying shapes. You probably do something like this now for
circular structures or off-angle corners, both of which are
more expensive to build than rectangular, square-cornered
As with any unit-price estimating system, the only way to
develop and check the accuracy of these new formulas is to
compare job-cost data from completed projects. Over time, you
should be able to tweak your pricing so that the 5% or 10%
additional cost to build an oddly-shaped building comes out of
your client's budget, not your profit.