Find the safe ladder length needed for a given working height.
A ladder is the hypotenuse of a right triangle, so its length follows from the reach and the base offset. The 4:1 rule fixes the offset at a quarter of the reach, which sets the ladder at about 76 degrees — steep enough not to slide out at the feet, shallow enough not to tip backwards — and makes the required length roughly 3% more than the reach itself. If you are stepping off onto a roof rather than working from the ladder, the rails must project three feet past that landing, so the reach grows before the triangle is solved. Then buy longer than the result: an extension ladder loses two to three feet to section overlap, and you should never stand on the top three rungs.
Ladder Height
Reach = support height + 3 ft if stepping onto a roof; base = reach / 4; ladder length = sqrt(reach^2 + base^2); add 3 ft for extension-ladder overlap
Ladder falls cause serious injuries and fatalities. This is a geometry aid, not a safety assessment — follow the manufacturer's duty rating, ANSI/OSHA setup rules, and never use a ladder near overhead power lines or on unstable ground.
About 75-76 degrees, which the 4:1 rule produces automatically. Too upright and it tips backwards when you lean out; too shallow and the feet slide.
Add the section overlap and never plan to use the top three rungs. For a 20 ft gutter, a 24 ft extension ladder is the practical choice even though the geometry says 20.6 ft.