Understanding your result
The headline is the extension ladder size to buy, with the length in use and the base distance beneath it. The stats repeat the length in use, the base distance and the angle. The breakdown shows the working length at the 4-to-1 angle, the 3 feet added above the eave (or kept above your feet), the total, the base distance and the ladder that reaches it after the section overlap.
The number to remember is the ladder length in use: it is what the ladder must actually measure, extended, from the ground to its top rung at the right angle, and it is always more than the eave height. Compare it with the reach figure in the last row, not with the ladder’s nominal size.
The eave height itself comes from the ridge height calculator (the fascia height row) when the house is on paper, or from a tape, a laser or a count of the siding courses when it exists. The DIY versus pro calculator is the place to decide whether to be on the roof at all.
How we calculate this
The geometry is a right triangle: the ladder is the hypotenuse, the wall the vertical side and the base distance the horizontal. At 4-to-1 the base is a quarter of the height, so the ladder is √(1 + 1/16) = 1.031 times the height to the point it touches. That 3 percent is small; the 3-foot extension and the section overlap are what move the answer up a size.
The reach table is the industry’s: a 16-foot extension ladder reaches 13 feet, a 20 reaches 17, a 24 reaches 21, a 28 reaches 25, a 32 reaches 29, a 36 reaches 32 and a 40 reaches 35, because the sections must overlap 3 feet up to 36 feet and 4 to 5 feet beyond. The guide to ladders and roof safety covers setting the ladder, tying it off, stand-offs for gutters and the pitches where a ladder should be the only part of the job you do yourself.
The assumptions behind the numbers
| Assumption | Default | Where it comes from |
|---|---|---|
| Angle rule | 4:1 (75.5°) | OSHA 29 CFR 1926.1053(b)(5)(i); ANSI A14.2 |
| Extension above the landing | 3 ft | OSHA 1926.1053(b)(1) |
| Rungs kept above the feet when working | 3 ft (three rungs) | Ladder manufacturers’ and ANSI guidance: never stand on the top three rungs of an extension ladder |
| Extension ladder reach | 16/13, 20/17, 24/21, 28/25, 32/29, 36/32, 40/35 ft | ANSI A14 minimum section overlap (3 ft to 36 ft, 4 ft to 48 ft) as published by ladder makers |
| Working reach from a rung | about 4 ft above the feet | Ladder makers’ guidance for a person of average height |
| Clearance from power lines | 10 ft | OSHA 1926.1053(b)(12) and 1926.416 |
Assumptions last reviewed October 7, 2026.
The calculator does not decide whether a roof is safe to be on, does not size a roof ladder or a ladder hook for a steep pitch, and does not account for soft or sloping ground, which needs levellers. OSHA treats any roof over 4/12 as steep for fall protection, and the pitch calculator says where yours falls; the safety guide says what that means in practice. The ladder sizes sold, with their reach after the section overlap, are also in the standard sizes table.
Two worked examples
Getting onto a 20-foot eave
Eave height 20 feet, stepping onto the roof, 3 feet above the eave, 4-to-1.
- Working length: 20 × 1.031 = 20 ft 7 in
- In use: 20.62 + 3 = 23 ft 7 in
- Base distance: 20 ÷ 4 = 5 ft
- A 24-ft ladder reaches 21 ft; a 28-ft ladder reaches 25 ft and is the one to buy
The 24-footer that the eave height suggests is two and a half feet short once the angle, the extension and the overlap are counted.
Cleaning gutters at 12 feet from the ladder
Work height 12 feet, working from the ladder, 3 feet of ladder kept above the feet.
- Working length: 12 × 1.031 = 12 ft 4 in
- In use: 12.37 + 3 = 15 ft 4 in
- Base distance: 3 ft
- A 16-ft ladder reaches 13 ft; a 20-ft ladder reaches 17 ft
From the fourth rung down, a person can comfortably work about 16 feet above the ground, which covers the gutter and the drip edge. A stand-off bracket keeps the ladder off the gutter.
Where to find your inputs
Height to reach. Ground to the gutter or drip edge, with a laser measure from below, by counting siding courses (lap siding is usually 4 to 8 inches to the weather), or from the fascia height row of the ridge height calculator. Measure from the point where the ladder’s feet will stand, which may be lower than the house floor.
Use. Stepping off onto the roof needs 3 feet above the eave; working from the ladder needs 3 feet of ladder above your feet. Both add the same 3 feet, for different reasons.
Angle rule. Four-to-one unless a manufacturer’s label says otherwise; the calculator warns if you enter a flatter or steeper ratio.
Common mistakes
- Buying the eave height. A 20-foot eave needs a 28-foot ladder. The angle, the 3-foot extension and the overlap add up.
- Too flat. A ladder set further than a quarter of its length from the wall can kick out at the base. Toes at the feet, arms out, palms on the rung.
- Too steep. Set closer than 4-to-1 it can tip backward, especially while stepping off.
- Stepping off sideways. Step straight through the rails onto the roof, holding the rails, and never from a rung above the eave.
- An untied top. On a roof job the ladder is used dozens of times; tie the top to a fixed point or use a stand-off that rests on the roof, not on the gutter.
- Near power lines. Ten feet from any overhead line, and an aluminum ladder nowhere near one. Fiberglass for anything near a service drop.
Questions people ask
- What size ladder do I need for a 20-foot eave?
- A 28-foot extension ladder. At the 4-to-1 angle the working length to 20 feet is 20 × 1.031 = 20.6 feet, plus the 3 feet the ladder must extend above the eave to step off makes 23.6 feet in use. A 24-foot ladder reaches only about 21 feet after its sections overlap; a 28-foot ladder reaches 25. The base sits 5 feet from the wall.
- What is the 4-to-1 ladder rule?
- OSHA (29 CFR 1926.1053(b)(5)(i)) requires a non-self-supporting ladder to be set so the horizontal distance from the wall to the base is one quarter of the working length, which is an angle of about 75½ degrees. The quick check: stand with your toes at the ladder's feet and your arms straight out; your palms should touch the rung at shoulder height.
- How far above the roof should the ladder extend?
- At least 3 feet (three or four rungs) above the landing surface when the ladder is used to get onto a roof (OSHA 1926.1053(b)(1)), or the top must be secured and a grab rail provided. The extension gives a handhold while stepping off and on, which is where most ladder falls happen.
- How high does a 24-foot extension ladder reach?
- About 21 feet of working length: the two 12-foot sections must overlap by 3 feet (ANSI A14 requires 3 feet for ladders up to 36 feet, more for longer ones), and set at 4-to-1 the top touches the wall at about 20 feet. Allowing the 3-foot extension above an eave, a 24-foot ladder is right for eaves up to about 17 feet, which is a tall single storey.
- What duty rating does a roofing ladder need?
- Type I (250 pounds) or Type IA (300 pounds) for a person carrying tools and materials; Type IAA is 375 pounds. The rating covers the user plus the load, and a bundle of shingles is 70 to 80 pounds. Household Type III ladders rated at 200 pounds are not for roofing.
- Can I work from the ladder instead of going on the roof?
- For gutters, drip edge, fascia and the first course, yes, keeping both feet on a rung, your belt buckle between the rails and never standing on the top three rungs. For that use enter the work height and choose "work from the ladder"; the calculator keeps 3 feet of ladder above your feet. For anything on the roof surface itself, the ladder is the way up, not the workplace.