How this calculator works
Safe ladder setup follows the 4 to 1 rule: set the base one foot out from the wall for every four feet of ladder between the foot and the point where it rests. That is an angle of about 75.5 degrees. The ladder length along the slope is the height divided by the sine of that angle, which is 0.968, so a support point at 18 ft needs 18.6 ft of ladder. If you will step onto a roof, the ladder must extend 3 ft above the edge. An extension ladder is sold by its nominal length, but its two sections overlap by 3 ft (up to 36 ft), 4 ft (up to 48 ft) or 5 ft, so the usable length is shorter.
Worked example: to reach a roof edge 18 ft up, you need 21.6 ft of working ladder (18.6 ft to the edge and 3 ft above it), so a 28 ft extension ladder, set 4.6 ft from the wall. A 10 ft window sill with no landing takes a 16 ft ladder, and a 20 ft roof edge a 28 ft ladder. A 28 ft ladder reaches a roof edge 21.3 ft up, a 24 ft ladder reaches 20.3 ft against a wall, and a 40 ft ladder reaches 32.0 ft.
Extension ladder size for common heights (stepping onto a roof)
The ladder to buy for a roof edge or gutter at each height, and the base distance.
| Roof edge height | Ladder | Base distance |
|---|---|---|
| 8 ft | 16 ft | 2.1 ft |
| 10 ft | 20 ft | 2.6 ft |
| 12 ft | 20 ft | 3.1 ft |
| 14 ft | 24 ft | 3.6 ft |
| 16 ft | 24 ft | 4.1 ft |
| 18 ft | 28 ft | 4.6 ft |
| 20 ft | 28 ft | 5.2 ft |
| 22 ft | 32 ft | 5.7 ft |
| 24 ft | 32 ft | 6.2 ft |
| 26 ft | 36 ft | 6.7 ft |
Tips before you buy
- Measure the height of the support point: the eave, gutter or roof edge the top of the ladder will touch. Add 3 ft above it if you will step onto the roof.
- An extension ladder is sold by its nominal length, which is the two sections laid end to end. Because they overlap, it works about 3 ft shorter.
- A 28 ft ladder is the usual size for a two-story house. Check the size on the label, not the box.
- Use stabilizers or a stand-off at the top to keep the ladder off the gutter, and level the feet on uneven ground.
- Choose fiberglass near power lines or electrical wires, and keep at least 10 ft away from overhead lines.
Tools & materials for this project
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Frequently asked questions
What size ladder do I need to reach a roof?
For a roof edge 18 ft up, a 28 ft extension ladder: it needs 21.6 ft of working length, including 3 ft above the edge. A 20 ft edge needs a 28 ft ladder.
How far should a ladder be from the wall?
One foot for every four feet of ladder up to the support point, an angle of about 75.5 degrees. For a ladder reaching 18 ft that is 4.6 ft (56 inches).
How high will a 28 ft extension ladder reach?
About 21.3 ft up to a roof edge that you step onto (with 3 ft above it), or 24.2 ft against a wall.
How high will a 24 ft ladder reach?
About 20.3 ft to the support point against a wall; with a landing it is 17.4 ft.
How much higher must a ladder extend above a roof?
At least 3 feet above the landing, so that you can hold the rails when you step on and off.
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Estimates only. Results assume standard materials and simple shapes. Supplier products vary, and the final call on quantity, structure and safety is yours. Check your local code and your supplier's guidance before ordering.