Extension Ladder Overlap and True Working Height
An extension ladder never reaches its label. Sections must overlap - 3 ft up to 36 ft nominal, 4 ft at 40 ft and longer - so a 24-ft ladder gives 21 ft of working length, and that length is the HYPOTENUSE at the 4:1 setup, putting the top support at 20.4 ft with the base 5.1 ft out. Subtract the 3-ft landing extension and the highest floor it serves is 17.4 ft. Three-section ladders lose overlap at two joints. The ladder-angle tile gives the setup; this gives what you can actually reach.
Formula and source
overlap = 3 ft up to 36 ft nominal, 4 ft at 40 ft and longer; working length = nominal - overlap x (sections - 1); at a 4:1 setup the working length is the hypotenuse, so top support = L / sqrt(1 + 1/16) and base offset = height / 4; highest landing served = top support - 3 ft.
Extension-ladder section overlap and the 3-ft-above-a-landing rule as stated in the US Office of Congressional Workplace Rights extension-ladder fast facts, restating the OSHA/ANSI requirement, by name.
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