Architectural Flashing Supply
Start a Quote
Trade mathematics and unit conversions · Lesson 1 of 4

Trade mathematics and unit conversions: principles and terminology

Why units travel with every number, which factors are exact by definition, what a rounded factor costs, how an inch divides to sixty-fourths, how pitch, slope and the right triangle relate, and the safety rules that rest on slope and setback.

You will be able to: Define exact and rounded conversion factors, read fractions of an inch as decimals and millimetres, state pitch in its four forms, and name the roof-slope and ladder rules that depend on the arithmetic.

11 min · 10 slides · 10 questions

All lessons in this module
Slide 1 of 10

Why a flashing shop needs exact math

AFS writes its fabrication limits in inches, and drawings arrive in feet and inches or in millimetres. A wrong unit or a rounded factor gives a short piece, a blank too wide for the sheet, or the wrong quantity on a quote.

Sources

Videos for this lesson

What this video shows: A Khan Academy introduction to turning a fraction into a decimal by division, with rounding. Checked against the video's description, not its transcript.

Video: Fraction to decimal with rounding | Decimals | Pre-Algebra | Khan Academy by Khan Academy, on YouTube. Chosen from the video's published description.

What this video shows: A Khan Academy word problem converting yards to inches, done two ways, using U.S. customary units only. Checked against the video's description, not its transcript.

Video: Unit conversion word problem: yards to inches | Introduction to algebra | Algebra I | Khan Academy by Khan Academy, on YouTube. Chosen from the video's published description.

What this video shows: A Khan Academy introduction to slope as rise over run, in general algebra rather than roofing. Checked against the video's title and description, not its transcript.

Video: Introduction to slope | Algebra I | Khan Academy by Khan Academy, on YouTube. Chosen from the video's published description.

Glossary

Exact conversion factor
A conversion factor fixed by definition rather than by measurement, such as 1 in = 25.4 mm, 1 ft = 0.3048 m and 1 lb = 0.45359237 kg. No digits are ever lost when it is used.
Rounded factor
A shortened conversion factor such as 25 mm per inch. It carries its error into every answer, so a trade calculation uses the exact factor and rounds only the final answer.
Pitch
Roof slope written as rise per 12 of run (x:12). A 6:12 pitch rises 6 in for every 12 in of horizontal run, which is also 50 % or 26.57 degrees.
Hypotenuse
The longest side of a right triangle, opposite the right angle. For a sloped edge it is the real, sloped length, and a² + b² = c² gives it from the run and the rise.
Low-slope roof
Under OSHA 29 CFR 1926.500(b), a roof with a slope of 4 in 12 or less (vertical to horizontal). A steep roof is greater than 4 in 12.