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Standard Interpretations - Table of Contents
• Standard Number: 1926.451(c)(1)(iii); 1926.451(c)(2); 1926.451(c)(2)(i); 1926.451(f)(3); 1926.451(f)(7); 1926.450

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

August 1, 2000

Mr. Donald J. Kemp
Hardy Construction Company
P.O. Box 1203
Billings, MT 59103

Subject:  Scaffold Foundation and Competent Person Knowledge, Training and Inspecting, 1926.450, 1926.451(c)(1)(iii), 1926.451(c)(2), 1926.451(c)(2)(i), 1926.451(f)(3), 1926.451(f)(7)

Dear Mr. Kemp:

This is in response to your letter dated January 28, 2000, addressed to the Occupational Safety and Health Administration requesting an interpretation of the scaffold standard, 29 CFR 1926 Subpart L. Specifically, your questions deal with the following issues: what is considered a "firm foundation" for a scaffold, and who is responsible for determining the adequacy of the foundation?

Question 1: Section 1926.451(c)(2) states: "supported scaffold poles, legs, posts, frames, and uprights shall bear on base plates and mud sills or other adequate foundation." OSHA's Directive Number CPL 2-1.23 states: "A concrete slab would be considered a firm foundation, and therefore mud sills would not be necessary." What is the definition of "other adequate foundation?" What are some of the other options for providing an adequate firm foundation?

Answer: An adequate foundation is one that, like base plates on mud sills, will prevent the scaffold from settling into the ground. The purpose of the mud sill under the scaffold base plate is to uniformly distribute the scaffold load over a larger area than that distributed by the base plate alone, thereby reducing the loading on the ground beneath the base plates. The scaffold baseplates and mudsills increase the area where the downward forces of the scaffold are transmitted. The scaffold load is transferred from the uprights to the base plates and then to the mud sills or other adequate foundation from the scaffold legs, which must show minimal or negligible settlement under a full scaffold loading to be adequate.

The standard recognizes that certain foundations may be of adequate firmness to support a scaffold without the use of mud sills. Generally, the adequacy of a foundation to support a scaffold cannot be determined in the abstract because of the numerous factors which may exist at a work site. Concrete is the lone exception to this rule, which, because of its structural properties and strength, OSHA has determined constitutes a firm foundation.

Section 1926.451(c)(2)(i) goes on to require that: "footings shall be level, sound, rigid, and capable of supporting the loaded scaffold without settling or displacement." The preamble to Subpart L (volume 61 of the Federal Register at page 46043, August 30, 1996) states: "Every scaffold must stand on a firm footing if it is to withstand the load that employees, equipment and materials place on it." In short, an adequate foundation is one that supports the load of the supported scaffold, materials and workers without collapse or shifting of one component or the entire scaffold.

Daily site conditions of scaffold footing can change with the temperature, rain, snow, etc. It is important to plan the erection, use and dismantling of the scaffold to address all possible conditions including the site's soil conditions.

Question 2: Would soil compacted to 96 - 99.9% density, then frozen in the cold of winter be considered adequate? What about cold asphalt paving (in winter)? Hot asphalt paving (in summer)? Wood decking? Compacted gravel roads?

Answer: As mentioned previously a site's conditions and soil types have varying degrees of load bearing ability. Also, the load bearing ability of the materials you mention varies. Therefore, it is impossible to provide exact answers under the conditions you propose because of the multitude of other variables that may exist. We discuss some of the problems in each of your examples below. In all cases a competent person must inspect and approve the foundation (daily).

  1. Compacted Soils & Then Frozen - Compacted soils, as with all soils, swell and heave due to moisture content or water contained within the soil. This creates a dangerous situation during the freeze/thaw cycle (which can occur in the course of a day) causing the soil to swell or settle and may not result in a level rigid footing. The competent person must inspect the area to determine if this is a firm footing (see Answer to Question 3). Then this must be inspected when conditions change, such as sunny conditions, warming temperatures, etc.

  2. Cold asphalt paving (winter) - Asphalt has minimal compressive strength, even when cold, and especially when applied. By itself it is not normally a useable foundation material.

  3. Hot asphalt paving (summer) - This type of asphalt (top coat) is soft when placed and then compacted with rollers. It is generally applied as a thin coat over a base coat and takes several days to achieve its designed strength. Only the base coat gives the asphalt compressive strength.1

  4. Wood decking - This may lend itself for use as a foundation. However, an evaluation of the deck for its maximum allowable loading would be required, since the decking or deck structure could fail under a full scaffold load. The competent person would need to know the individual scaffold base plate loading and if each one is below the maximum allowable deck loading.

    A light weight supported scaffold could be used on wood decking when the competent person has determined the structure would support the scaffold and its entire load.

  5. Compacted Gravel Roads - This type of road surface may have adequate strength but the material does not provide a smooth surface. Additionally, there is no material cohesion; during a storm the material can wash away under the scaffold base plates.

In each of the examples cited, some type of settlement or displacement could occur when used without mud sills. The foundation where the scaffold would be erected must be evaluated in the pre-job planning. This determination is extremely important to the erection, use and dismantling of the scaffold. The determination must be made by a competent person with the knowledge of the type of scaffold, the expected live and dead loads, and the weather conditions for the area.

Question 3: Who is responsible for determining whether a foundation is adequate? What qualifications must that person have?

Answer: Under §1926.451(c)(1)(iii), the employer must have a designated competent person make this determination. A competent person is defined in §1926.450 as one who is capable of identifying existing and predictable hazards in the surroundings or work in the surroundings or working conditions which are unsanitary, hazardous, or dangerous to employees, and who has authorization to take prompt corrective measures to eliminate them. With respect to the adequacy of foundations, the competent person must be knowledgeable about foundation surfaces and soils, and able to recognize conditions such as a soil wash under a scaffold leg.

Question 4: How do employees get to be competent to determine an adequate scaffold foundation?

Answer: The competent person is designated by the employer and has the authorization to take prompt corrective measures to eliminate hazards. In making that determination, the employer should inquire into the individual's work credentials and experience. The preamble to Subpart L (volume 61 of the Federal Register at page 46,059, Aug. 30, 1996) gives the following example of what is required to become a "competent person" on scaffolding.
A 'competent person' for the purpose of this provision must have had specific training in and be knowledgable about the structural integrity of scaffolds as well as the degree of maintenance needed to maintain them. The competent person must also be able to evaluate the effects of occurrences such as, a dropped load, or a truck backing into a support leg that could damage a scaffold. In addition, the competent person must be knowledgeable about the requirements of this standard. A competent person must have training or knowledge in these areas in order to identify and correct hazards encountered in scaffold work.
In the situation you have described, the competent person must be able to assess the load of employees, equipment and materials placed on the scaffold's foundation. While an engineering degree is not a prerequisite, that assessment normally requires some technical skills, and the competent person making that determination would have to have those skills.

If you need additional information, please contact us by fax at: U.S. Department of Labor, OSHA, Directorate of Construction, Office of Construction Standards and Guidance, fax # 202-693-1689. You can also contact us by mail at the above office, 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

[Corrected 6/2/2005]

Standard Interpretations - Table of Contents

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