OSHA requirements are set by statute, standards and regulations. Our interpretation letters explain these requirements and how they apply to particular circumstances, but they cannot create additional employer obligations. This letter constitutes OSHA's interpretation of the requirements discussed. Note that our enforcement guidance may be affected by changes to OSHA rules. Also, from time to time we update our guidance in response to new information. To keep apprised of such developments, you can consult OSHA's website at http://www.osha.gov.

March 10, 2004

David A. Beauchamp, President
Industrial Design Associates
40106 Roshani Drive
Temecula, CA 92591

Re: Whether a mobile scaffold outrigger design meets the requirements of 29 CFR 1926.451(c)(1).

Dear Mr. Beauchamp:

This is in response to your December 30, 2002, letter to the Occupational Safety and Health Administration (OSHA), as well as several conversations with members of my staff. Your inquiry concerns the adequacy of outrigger designs for your "lock-pin" mobile scaffolds used in construction. I apologize for the delay in our completing this letter.

Question: Do the designs that I have submitted regarding "lock-pin" mobile scaffolds meet applicable OSHA construction requirements for height-to-base width ratios for supported scaffolds?


I. Background

First, please note that OSHA neither approves nor endorses products. In addition, the variable working conditions at jobsites and possible alterations or misapplications of an otherwise safe product could easily create a hazardous condition beyond the control of the manufacturer. However, where appropriate, we try to give some guidance to employers to help them assess whether products are appropriate to use and provide OSHA-required protection.

Section 1926.451(c)(1) states:

Criteria for supported scaffolds. (1) Supported scaffolds with a height to base width (including outrigger supports, if used) ratio of more than four to one shall be restrained from tipping by guying, tying, bracing, or equivalent means... [Emphasis added.]

In 1983, OSHA explained that, for the purposes of the scaffold height-to-base width ratio, the Agency uses the "total height" in relationship to the smaller base dimension. OSHA continues to use the relationship between total height and least base dimension to calculate height-to-base width ratio. As the Agency stated in a 1983 letter2

. . . a free standing scaffold shall be considered safe when the total height is equal to or less than four times the minimum or least base dimension. [Emphasis added.]

Based on the information you provided, we were unable to calculate the scaffold's total height. However, in light of the width measurements in your materials, we have calculated the maximum permissible scaffold height for use without restraints.

Your materials describe three "lock-pin" scaffold models. They are nominally 4 feet long by 26 inches wide, 6 feet long by 29 inches wide, and 7 feet long by 40 inches wide. We will refer to them respectively as the 4-foot, 6-foot and 7-foot models.

II. Use of the scaffolds when no one is on them while they are moved

Not using outriggers
The table below shows at what height OSHA's restraint provision (§1926.451(c)(1)) would require the indicated scaffold model, when used without outriggers, to be restrained (by guying, tying, bracing, or equivalent means) to prevent tipping.



(not using outriggers)
Total height* at which
restraint is required
4-foot       (26 inches wide) over 8.668 feet
6-foot       (29 inches wide) over 9.668 feet
7-foot       (40 inches wide) over 13.332 feet
* including vertical posts, guardrails, casters, etc.


Using outriggers
If (as one example) the user were to add 2-foot outriggers at each corner and perpendicular to the long side of the scaffold base section, the base width dimension would increase to such an extent that it becomes greater than the nominal length. Since scaffold stability is assessed based on the least dimension and since the nominal length in this scenario becomes the least dimension, maximum unrestrained heights must not exceed four times the nominal length.

Our analysis shows the base geometry changes on all three models. The addition of your outriggers adds 4 feet to the width and effectively creates a new length for each model. The 4-foot scaffold becomes 4 ft. by 6.167 ft.; the 6-foot scaffold becomes 6 ft. by 6.417 ft.; the 7-foot scaffold becomes 7 ft. by 7.333 ft. Selecting the least dimension provides the following widths: 4-foot scaffold is 4 ft.; 6-foot scaffold is 6 ft.; 7-foot scaffold is 7 feet.

The table below shows at what height OSHA's restraint provision (§1926.451(c)(1)) would require the indicated scaffold model, when used with outriggers as described above, to be restrained (by guying, tying, bracing, or equivalent means) to prevent tipping.







(using outriggers as described above)
Total height* at which
restraint is required
4-foot       (4 feet wide) over 16 feet
6-foot       (6 feet wide) over 24 feet
7-foot       (7 feet wide) over 28 feet
* including vertical posts, guardrails, casters, etc.


III. Use of the scaffolds when an employee is on them while they are moved

Mobile scaffolds must meet a stricter height to base width ratio if workers remain on the scaffold during movement. Section 1926.452(w)(6) prohibits movement of mobile scaffolds with employees riding on them unless certain requirements are met. One of those requirements is §1926.452(w)(6)(ii), which states:





The height to base width ratio of the scaffold during movement is two to one or less, unless the scaffold is designed and constructed to meet or exceed nationally recognized stability test requirements, such as those listed in paragraph [w] of Appendix A to this subpart (ANSI/SIA A92.5 and A92.6). [Emphasis added.]

The materials you submitted do not indicate that you have designed and constructed the "lock-pin" scaffolds to meet or exceed "nationally recognized stability test requirements." If they in fact have not been so designed, the heights listed in the tables above would have to be lowered to meet the 2:1 ratio for use of the scaffolds when they are moved with workers on them.

Note also that, under §1926.452(w)(6)(iii), when a worker rides on a scaffold,

Outrigger frames, when used, [must be] installed on both sides of the scaffold.3



If you need additional information, please contact us by fax (202-693-1689) at: U.S. Department of Labor, OSHA, Office of Construction Standards and Guidance. You can also contact us by mail at U.S. Department of Labor, OSHA, Office of Construction Standards and Guidance, Room N3468, 200 Constitution Avenue, N.W., Washington, D.C. 20210, although there will be a delay in our receiving correspondence by mail.


Russell B. Swanson, Director
Directorate of Construction





1In your letter, you indicated your question is much broader than this -- i.e., whether your designs meet all applicable OSHA requirements for scaffolds used in construction. Many of these requirements relate to the manner in which the scaffold is used and the loads imposed on them. Therefore, it is not possible to comment on the adequacy of a scaffold design without reference to specific use information. In short, in light of this and in view of the limited amount of information submitted, we are able to comment only on the applicable requirements for the base width-to-height ratio. [ back to text ]





2Agency's October 13, 1983, letter from John B. Miles, Director -- Directorate of Field Operations, to Mr. Budd. [ back to text ]





3We note that in the submitted drawings, the outrigger frame is pictured on only one side of the scaffold. However, our calculations in the tables were based on using outriggers on both sides. [ back to text ]