Scaffold tie and height-to-base calculator

The rule in plain words

The tipping rule is 29 CFR 1926.451(c)(1), for supported scaffolds. Height is measured against the base width, counting outrigger supports if used. We give the rule in our own words; read the paragraph itself for the exact text.

• Ratio = height ÷ base width. Restraint by guying, tying, bracing or an equivalent means is required when the ratio is more than 4 to 1, (c)(1). Exactly 4 to 1 does not trigger it.
• First tie at 4 × base width. That is the 4:1 height. The tie goes at the closest horizontal member to it, or where the manufacturer says, (c)(1)(ii).
• Repeat every 20 ft or less for a base 3 ft wide or less, every 26 ft or less for a wider base. Each repeat is at a horizontal member, (c)(1)(ii).
• Top tie no further than the 4:1 height from the top. So the highest tie is at or above height minus 4 × base width, (c)(1)(ii).
• Ties at each end and every 30 ft or less along the length. The 30 ft is measured from one end toward the other, (c)(1)(ii). Ties per level = length ÷ 30, rounded up, plus 1.
• Placement. Ties go where horizontal members support both the inner and outer legs, (c)(1)(i).
• Capacity. In general, each scaffold and component must support its own weight and at least 4 times the maximum intended load, 1926.451(a)(1); exceptions are in (a)(2) to (a)(5) and (g). The calculator multiplies your load by 4, so enter the load in the unit you want back.

Ties are also required, whatever the ratio, wherever an eccentric load such as a cantilevered work platform acts on a supported scaffold, (c)(1)(iii).

Worked example 1: 40 ft high on a 5 ft base

Ratio = 40 ÷ 5 = 8:1, which is more than 4:1, so restraint is required. The 4:1 height is 4 × 5 = 20 ft, so the first tie is at 20 ft. The top tie must be at or above 40 − 20 = 20 ft, and the first tie already is, so one level is enough. On a 30 ft long scaffold that is 30 ÷ 30 = 1, plus 1, so 2 ties.

Worked example 2: 60 ft high on a 5 ft base, 70 ft long

Ratio = 60 ÷ 5 = 12:1. First tie at 4 × 5 = 20 ft. The base is wider than 3 ft, so the next level is within 26 ft of the first, at 46 ft or below. The top tie must be at or above 60 − 20 = 40 ft, and 46 ft is, so two levels do it. Along 70 ft: 70 ÷ 30 = 2.33, rounded up to 3, plus 1 is 4 ties per level, so 8 in all. With a 500 lb intended load, the capacity needed is 4 × 500 = 2,000 lb.

Worked example 3: why 3 ft and 3.1 ft differ

The spacing switches at a 3 ft base. A 3 ft wide scaffold 80 ft high has its first tie at 12 ft, then every 20 ft or less: 32, 52 and 72 ft or below, and the top tie must be at or above 80 − 12 = 68 ft. A 3.1 ft wide scaffold of the same height has its first tie at 12.4 ft, then every 26 ft or less: 38.4 ft, 64.4 ft, and then the top. The sizes are invented. These are original arithmetic examples, not exam questions.

How this shows up in CHST study

The CHST blueprint covers working at heights in its first domain, so it is worth knowing which number applies. Be able to say that 4:1 is the trigger for restraint, that the first tie goes at 4 times the base width, that 20 and 26 feet depend on whether the base is 3 feet wide or less, and that capacity is a factor of 4 on the intended load. See the scaffolds (Subpart L) guide for the rest of Subpart L, the ladder angle calculator for the other 4 to 1 rule, and the fall protection (Subpart M) guide.

Scaffold tie questions

For study and calculation checks. The rule was read on the eCFR on 25 Oct 2026 (current to the 22 Oct 2026 issue). This is a study aid, not a site safety determination. On site, the current 29 CFR 1926.451, the manufacturer's tie instructions and a competent person govern, and a qualified person designs scaffolds the rule says need one.