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A 24-foot ladder works to 21. Know that before you climb.
Ladder falls are the most common jobsite injury, and the setup math is four numbers: base out one foot for every four of height (the 4:1 rule, 75.5°), rail length along the slope, three feet of rail above a roof landing, and the fact that a ladder's nominal size is not its reach — sections overlap. This calculator runs all four against the ANSI A14.2 working-length table.
Roofers and gutter crews use it before the ladder leaves the truck: eave height in, and it tells you whether the ladder on the rack reaches with the required 3 feet above the edge — or whether today needs the 28. Homeowners use it to learn that their 24-footer only works to 21, before finding out at the top.
The sections overlap, so a 24-footer's maximum working length is 21 ft (ANSI A14.2). At the proper 4:1 angle that serves a wall support point up to about 20 ft, or a roof edge at 17–19 ft with the required 3 ft of rail above the landing. A 24 ft ladder never reaches 24 ft.
For every 4 feet of height to the support point, the base sits 1 foot out from the wall — an angle of about 75.5°. Too steep and the ladder tips backward; too shallow and the feet slide out. The field check: stand with your toes at the rails, arms straight out — your palms should land on a rung.
At least 3 feet (OSHA 1926.1053(b)(1)) — and not more than 4. That rail above the edge is your handhold for stepping on and off; stepping around the top of a short ladder is how eave falls happen.