Lesson 12 of 13
From Geometry to a Material Order
Everything in this class so far produces a number. This lesson turns numbers into what the truck brings. It is the least glamorous arithmetic on the job and the only kind that costs you money directly when it is wrong.
Step one
Read the material
Three ideas, and the first one is a rule with no exceptions.
Concept one
The yard sells whole things
Your calculation produces a decimal. The supplier does not sell decimals. A sheet of plywood is four feet by eight, which is thirty-two square feet, and there is no such thing as two-thirds of one.
So divide the area by the coverage of one unit, and then round up, always. Never round down and never round to the nearest. A room of 384 square feet needs 384 divided by 32, which is exactly 12 sheets — and a room of 390 square feet needs 12.19, which is 13 sheets, not 12.
The same rule governs everything sold in units: bags of mix, boxes of tile, rolls of underlayment, sticks of conduit. Compute, divide, round up. Nobody has ever been sent home for having one sheet left. Everybody remembers the afternoon they were one short.
Concept two
Waste is not pessimism. It is arithmetic.
The number you computed assumes every square foot of material lands on the building. It never does. Sheets get cut around openings, the offcut is too small to use, a board splits, the pattern has to line up, and the last row is three inches wide.
So you add a waste factor before you round. Roughly ten percent on a plain rectangle with square cuts. Fifteen percent when there are many openings, angles, or a diagonal layout. Twenty or more on a complicated hip roof or a herringbone floor, where almost every piece is cut twice.
Add the waste to the area, then divide, then round up. Doing it in the other order — rounding first and adding waste after — quietly produces a different and usually smaller number.
Waste is not padding the estimate. It is admitting that a saw makes offcuts.
Concept three
Never mix linear, square and cubic
Three different kinds of quantity get ordered on the same job and they cannot be added together. Linear feet buy trim, pipe, conduit and framing lumber — length only. Square feet buy sheathing, flooring, drywall and roofing — area. Cubic yards buy concrete, gravel and fill — volume.
Lumber has its own unit that trips people: the board foot, which is thickness in inches times width in inches times length in feet, divided by twelve. A two by eight, twelve feet long, is 2 times 8 times 12 divided by 12, which is 16 board feet. The nominal size is what you use in the formula, even though the board actually measures an inch and a half by seven and a quarter.
Write the unit next to every number on the takeoff. A list of bare figures with no units is how a hundred and twenty square feet becomes a hundred and twenty linear feet on the order form.
Step two
Work it with your hands on it
Set a room and a waste factor. Watch the order change, and watch what happens when you add the waste after rounding instead of before.
Hands on it, off the screen. Take a room you can measure. Work the sheets for the floor, the linear feet of base trim around it, and the cubic yards if you were pouring four inches of slab under it. Three quantities, three different units, one room.
Then price them at your local yard. That number, on paper, in your handwriting, is a takeoff.
Step three
Prove you understood it
One question per idea, new numbers every time.
All three solid.
You can turn an area into whole units, add waste in the right order, and keep your linear, square and cubic quantities from contaminating each other. One lesson left, and it uses everything.
Trades Geometry
Lesson twelve of thirteen. Written by our founder, Dr. Gene A Constant, and donated to the Foundation. Published by Global Sovereign University Press.
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