Why ladder angle matters
Falls from ladders are one of the leading causes of serious injury at home and at work. Many are caused by a ladder set at the wrong angle: too shallow and the base slides out; too steep and the ladder tips backward.
The 4-to-1 rule
Place the base 1 foot away from the wall for every 4 feet of height to the point where the ladder rests. That gives an angle of about 75.5°, the angle OSHA requires for extension and other non-self-supporting ladders.
A quick check: stand with your toes at the ladder’s feet and stretch your arms straight out. Your palms should just reach the rung at shoulder height.
The math
For a ladder of length L at an angle θ with the ground:
- height reached = L × sin θ
- base distance = L × cos θ
- length needed to reach a height h and extend a distance x above it = (h + x) ÷ sin θ
Worked example
A 20 ft extension ladder set at 75.5°:
| Result | Value |
|---|---|
| Distance from the wall to the base | 5.01 ft |
| Height reached on the wall | 19.36 ft |
| Base distance by the 4-to-1 rule | 4.84 ft |
To reach a roof edge 16 ft high and extend 3 ft above it, as OSHA requires for access to an upper level, you need about 19.6 ft of working length.
Choosing a ladder length
Extension ladders are sold by their total section length, but the overlap between sections reduces how far they reach. A “24 ft” ladder typically extends to about 21 ft. Check the maximum working length on the label, and remember that you should not stand on the top three rungs of an extension ladder.
| Roof edge height | Working length needed (with 3 ft extension) |
|---|---|
| 10 ft | about 13.4 ft |
| 13 ft | about 16.5 ft |
| 16 ft | about 19.6 ft |
| 20 ft | about 23.8 ft |
Safety tips
- Set the feet on firm, level ground, and secure the top or have someone hold the base.
- Keep three points of contact: two hands and a foot, or two feet and a hand.
- Do not overreach: keep your belt buckle between the rails.
- Check the duty rating: it must cover your weight plus tools and materials.
- Keep metal ladders well away from power lines.