Ladder Length and Safe Angle Calculator
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A ladder that is too short gets pushed to a steep angle, and a steep ladder is one that slides out from the wall. This works out how long the ladder needs to be for the height of your roof edge, how far to set its feet from the wall, and which extension ladder size to buy.
Ladder length for a roof edge
The ladder length to reach a roof edge, how far from the wall to set its feet, and the extension ladder size to buy.
This calculator needs JavaScript to work. The measurements below are the ones you would need to do the sum by hand.
Ladder length needed, fully extended The length from foot to top, with the extension above the roof edge.
- Set the feet this far from the wall One quarter of the ladder length measured up to the roof edge, on firm and level ground.
- Angle of the ladder
- Ladder length up to the roof edge
- Extension ladder size to buy The size printed on the ladder. An extension ladder loses about 3 ft to the overlap of its sections, so a 24 ft ladder extends to about 21 ft.
A roof edge above about 20 ft, roughly two stories, is a poor place for a single ladder. Wind, a soft footing or a small slip has a long way to go. Scaffolding, a lift or a professional with fall protection is the safer way to work at this height.
No standard extension ladder reaches this high at a safe angle. Do not lean a shorter ladder more steeply to make it fit: that is how ladders slide out. Use scaffolding or hire someone equipped for the height.
The 4 to 1 rule: set the feet one quarter of the working length from the wall, which puts the ladder at about 75.5 degrees. Check the ladder duty rating covers your weight plus tools, tie the top off if you can, and keep three points of contact. A ladder gets you to the roof edge; it is not fall protection for walking on the roof.
The 4 to 1 rule
OSHA guidance sets the feet of a leaning ladder one quarter of the working length away from the wall, where the working length is the distance along the ladder from its foot to the point where it rests. That puts the ladder at about 75.5 degrees, steep enough to climb comfortably and shallow enough not to tip back. A ladder against a roof edge 10 ft up has a working length of about 10.3 ft, so its feet go about 2.6 ft out from the wall.
Why the ladder has to go above the roof edge
When a ladder is used to get onto a roof, its side rails should stand at least 3 ft above the edge. That gives you something to hold while you step across, rather than reaching for a gutter or a shingle. The calculator adds that extension to the working length, so the 10 ft roof edge needs a ladder about 13.3 ft long when it is set up.
Nominal size is not working length
The number printed on an extension ladder is its nominal length, with the sections fully extended and counted end to end. The two sections overlap, which uses up about 3 ft, so a 20 ft ladder extends to about 17 ft. In the example above, a ladder needing 13.3 ft works out to a 20 ft extension ladder, because a 16 footer only reaches about 13 ft. For a roof edge at 20 ft, the calculator asks for a 28 ft ladder.
When not to use a ladder
Above roughly two stories, a single ladder becomes a poor choice. A small shift in the footing has a long way to go, and wind has more ladder to push on. Scaffolding or a lift is safer there, and so is hiring someone with fall protection. Set the feet on firm, level ground, keep three points of contact, and never stand on the top rungs. A ladder gets you to the roof edge; it does not make walking on a roof safe.
Frequently asked questions
What angle should a ladder be at?
About 75.5 degrees, which is the 4 to 1 rule: for every 4 ft of ladder length up to the support, set the feet 1 ft out from the wall.
How far should a ladder extend above the roof?
At least 3 ft above the roof edge, so you have something solid to hold when you step on or off. This is also what OSHA requires for ladders used to reach a landing.
What size ladder do I need for a one-story house?
Most one-story roof edges are 9 to 11 ft high. With the 3 ft extension that needs about 12 to 14 ft of working ladder, so a 20 ft extension ladder is the usual choice, because a 16 ft ladder only extends to about 13 ft.