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The short answer: One shift. One of the requirements in 1926 Subpart H - Materials Handling, Storage, Use, and Disposal, is 1926.250(b)(5), which states:
“Materials shall not be stored on scaffolds or runways in excess of supplies needed for immediate operations.”
Storing materials, such as masonry units on scaffolds several days in advance of work activities, as well as storing materials on scaffolds overnight in preparation for the next day, is prohibited. For purposes of 1926.250(b)(5), “immediate operations” means work that will be done in the shift.
Also note that some provisions in the actual scaffold standard would be difficult to meet if materials other than those needed for one shift are stored on a scaffold.
For example, 1926.451(f)(3) states:
“Scaffolds and scaffold components shall be inspected for visible defects by a competent person before each work shift, and after any occurrence which could affect a scaffold’s structural integrity.”