Ladder Size
20 ft
Reach
20.7 ft
Setback
3.4 ft
Recommended Ladder
20 ft
Extension Ladder
Safe Configuration
Two sections that overlap, so it never extends to its rated length
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Inputs
Result
Target 13 ft roofline. At 75.5° the climb is 13 / sin(75.5°) = 13.4 ft, plus the 3 ft that must extend above the edge = 16.4 ft of working length. A 16 ft ladder extends to only 13 ft, so the smallest that covers it is a 20 ft ladder, which extends to 17 ft. Setback from wall: 13 / tan(75.5°) = 3.4 ft.
Inputs
Result
Person reach = 5.67 ft + 1.5 ft overhead = 7.17 ft. Standing height needed = 9 − 7.17 = 1.83 ft, but the calculator floors the standing height at 2 ft. Add 2 ft for the top-step rule = 4 ft, and 4 ft is a stock step ladder size.
An extension ladder never extends to its rated length — the two sections overlap by 3 ft — and it must reach 3 ft above a roof edge, so a 1-story roof (13 ft) needs a 20 ft ladder and a 2-story roof (22 ft) needs a 32 ft ladder.
Work back from the rated length, not up from your reach height. A two-section extension ladder loses 3 ft to the overlap between its sections, so a 24 ft ladder extends to 21 ft. The lean angle costs a little more, and OSHA requires another 3 ft above any roof edge you step onto. In practice the rated ladder is 7–10 ft longer than the point it leans on: a 13 ft roofline needs 20 ft, and a 22 ft roofline needs 32 ft.
| Task | Height | Extension Ladder | Step Ladder |
|---|---|---|---|
| Gutter cleaning | 10 ft | 16 ft | 6 ft (if accessible) |
| 1-story roof | 13 ft | 20 ft | N/A |
| Paint 2-story | 20 ft | 24 ft | N/A |
| 2-story roof | 22 ft | 32 ft | N/A |
The 4-to-1 rule means the base sits 1 foot out for every 4 feet of working length — the length the ladder is actually extended to, not the number printed on it. That gives roughly a 75.5° angle, the best balance of stability and strength. A 20 ft ladder extends to 17 ft, so its base belongs about 4 ft from the wall, not 5.
On a step ladder, never stand on the top two steps (or top cap). On an extension ladder, your feet should be at least 3 rungs from the top. Standing too high on any ladder moves your center of gravity above the support point, making it unstable.
Step ladders are self-supporting and best for indoor work, painting, and tasks where you can't lean against a wall. Extension ladders lean against a structure and reach much higher. Use step ladders up to 10–12 feet of reach; extension ladders for anything above that.
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Last Updated: Oct 19, 2026
This calculator is provided for informational and educational purposes only. Results are estimates and should not be considered professional financial, medical, legal, or other advice. Always consult a qualified professional before making important decisions. UseCalcPro is not responsible for any actions taken based on calculator results.