The rule
OSHA says a non-self-supporting ladder goes up at an angle where the base is out about one quarter of the ladder's working length (1926.1053(b)(5)(i)). The working length is the distance along the ladder from its feet to the top support. On the ground that's the familiar 1 foot out for every 4 feet up, at an angle of about 75.5°.
When the ladder is how people get onto a roof, deck or other landing, the side rails have to extend at least 3 feet above it (1926.1053(b)(1)). If that isn't possible, the ladder has to be secured at the top and a grab rail provided.
An example
A landing 16 feet up. The working length is 4 × 16 ÷ √15, about 16 ft 6 in, so the base sits about 4 ft 2 in out from the wall. With the 3-foot extension, the ladder needs at least 19 ft 6 in of usable length.
Before anyone climbs
- On an extension ladder, the length on the label isn't the usable length. Allow for the overlap the manufacturer requires.
- Check the duty rating against the worker plus tools and material.
- Set the feet on a firm, level base, and tie off or secure the top where it could shift.
- Keep metal ladders away from energized lines.