Construction · Lesson 8 of 13Beginner8 min read
Reading Plans and Measuring
Builders turn drawings into buildings with a tape measure, a level and a little math. This lesson shows how to read a set of plans and the measuring tricks every carpenter uses, with calculators you can try.
A set of plans
Following blueprints and building plans is the first duty the Bureau of Labor Statistics lists for carpenters. A set of plans shows the same house several ways:
- Plan views look straight down on each floor, as if the house were sliced through partway up the walls. They show walls, doors, windows, stairs and room sizes, with dimensions.
- Elevations show each outside face of the house: heights, roof slope, windows and siding.
- Sections show the house sliced open, so you can see how the foundation, floors, walls and roof go together.
- Details zoom in on one spot, such as a stair or a window, at a larger scale.
Plans also go to the building department, where a plans examiner checks them against the adopted codes before a permit is issued.
Scale
Drawings are made to scale. At a scale of ¼ inch = 1 foot, every quarter inch on the paper stands for a foot of the house, so a 24-foot wall is drawn 6 inches long. When a dimension is written on the plans, use the number rather than measuring the paper, since copies aren’t always printed at true size.
real length = drawn length × scale factor at ¼″ = 1′, the factor is 48 (1 foot ÷ ¼ inch = 12 ÷ 0.25)
Feet, inches and sixteenths
Builders in the United States measure in feet and inches, and they split inches into halves, quarters, eighths and sixteenths. On plans, 12′-6″ means 12 feet 6 inches. Adding fractions comes up all the time: a stud 1½ inches thick plus a 3⅝-inch gap is 5⅛ inches. Converting everything to sixteenths (1½ = 24/16, 3⅝ = 58/16, total 82/16 = 5 2/16 = 5⅛) is one reliable way to do it in your head.
Squaring a corner with 3-4-5
Foundation forms, walls and decks need square (90-degree) corners. The trick is the Pythagorean theorem: in a right triangle, the square of the long side equals the sum of the squares of the other two. Since 3² + 4² = 9 + 16 = 25 = 5², a triangle with sides 3, 4 and 5 always has a right angle.
So measure 3 feet along one line from the corner and mark it, 4 feet along the other and mark it, and measure between the marks. If the diagonal is exactly 5 feet, the corner is square. If it’s long, the corner is more than 90 degrees; if it’s short, less. Any multiple works too, such as 6-8-10 or 9-12-15, and a bigger triangle makes a small error easier to see on the tape.
Press Measure, then knock the corner out of square and measure again. The law of cosines gives the diagonal when the angle isn’t 90°: c² = a² + b² − 2ab·cos θ.
Checking a whole rectangle
For a rectangle, such as a slab form or a deck, measure both diagonals from corner to corner. If the opposite sides are equal and the two diagonals are equal, the rectangle is square. If one diagonal is longer, nudge the frame toward the shorter one.
Level and plumb
Level means perfectly horizontal and plumb means perfectly vertical. A spirit level has a curved vial of liquid with an air bubble; the bubble rises to the highest point, so it sits between the lines only when the level is flat (Being an Adult 101 explains why). A plumb bob is just a weight on a string: gravity pulls it straight down, toward the center of the Earth, so the string hangs exactly vertical. Lasers do the same jobs over longer distances.
Estimating materials
Estimating is mostly multiplication and division. Concrete is ordered by volume, in cubic yards: one cubic yard is 3 feet × 3 feet × 3 feet, or 27 cubic feet. Drywall is ordered by the sheet: the area to cover divided by the area of one sheet. Studs follow from the wall length and the spacing. Builders add a margin for cuts and mistakes, and suppliers sell in their own sizes, so check before you order.
cubic yards = length (ft) × width (ft) × thickness (ft) ÷ 27 a 4-inch slab is 4/12 = 0.333 ft thick
Change the sizes and the margin. The pictures show the quantity: one block per cubic yard, one panel per sheet, one stick per stud.
NoteMeasure twice, cut once
The old saying is still the best advice. Write measurements down, mark which side of the line to cut on, and check a cut piece against the space before you nail it in.
Check yourself
Tap an answer to check it, or open “Show the answer.”
At a scale of ¼ inch = 1 foot, how long is a wall drawn 6 inches long?
Show the answer
24 feet. Six inches is 24 quarter-inches, and each quarter-inch is a foot.
You measure 6 feet and 8 feet along two lines from a corner. If the corner is square, the diagonal between the marks is…
Show the answer
10 feet. 6-8-10 is just 3-4-5 doubled: 36 + 64 = 100 = 10².
The diagonal of your 3-4-5 check comes out longer than 5 feet. The corner is…
Show the answer
more than 90°. A wider angle opens the triangle, so the diagonal gets longer.
About how many cubic yards of concrete is a 20 × 12 foot slab, 4 inches thick, before any extra?
Show the answer
About 3. 20 × 12 × 0.333 = 80 cubic feet, and 80 ÷ 27 ≈ 2.96 cubic yards.
Which drawing shows how the foundation, floors, walls and roof go together?
Show the answer
A section. A section slices the house open to show the layers.
Go deeper
- Being an Adult 101: Find a Stud and Hang a Picture Studs on center, hanging height and how a bubble level works.
- Science Lab: Forces at Work: Free-Body Diagrams Gravity and the other forces, drawn as free-body diagrams.
- Lesson 4: Framing Lesson 4’s wall builder counts studs and plates for you.
- U.S. Bureau of Labor Statistics, Occupational Outlook Handbook: Carpenters Carpenters read plans every day; the BLS describes the job.
Sources
A free course from Teddy Rook Book. It explains how houses are built in general and follows U.S. sources; the local building code, the building department and the manufacturer’s instructions are the final word.