- Written by
- Scaffold Exchange Team
- Published
- September 26, 2026
- Reading time
- 7 min
Capacity: the 4:1 rule
The maximum intended load is the total of all people, tools, materials and equipment that will be on the scaffold at once. The 4:1 factor is the margin between that load and the load at which something breaks. Suspension ropes and their connecting hardware carry a higher margin, 6 times the maximum intended load, and the stall load of a scaffold hoist may not exceed 3 times its rated load.
Scaffolds must be designed by a qualified person and built and loaded in accordance with that design (1926.451(a)(6)). For a supplier's standard frame or system scaffold the design is the manufacturer's load chart, and the qualified person's job is to confirm the configuration matches it. Once the scaffold is up, the rule in 1926.451(f)(1) is simple: no scaffold may be loaded beyond its maximum intended load or rated capacity, whichever is less.
Three duty ratings are defined in 1926.450(b) and used throughout the industry for planning platform loads:
| Rating | Uniform load | Typical use |
|---|---|---|
| Light duty | 25 lb per sq ft | Painting, cleaning, inspection: people and hand tools only |
| Medium duty (bricklayers') | 50 lb per sq ft | Trades with some stored material on the platform |
| Heavy duty (stone setters') | 75 lb per sq ft | Masonry, stone and other heavy material stocked on the platform |
The uniform load is the design figure; the workers and the pallet are the reality. Two masons, a mortar tub and a cube of block on a 5-by-7 platform is a heavy duty load, and if the scaffold was rented as light duty it is overloaded whatever the workers weigh.
Platform construction
Section 1926.451(b) sets out how a platform is built. In the order a competent person checks them:
- Fully planked or decked. Every platform on every working level is fully planked between the front uprights and the guardrail supports. Gaps between planks and between the platform and the uprights may not exceed 1 inch, unless a wider gap is unavoidable around an upright, in which case the gap may not exceed 9.5 inches (1926.451(b)(1)).
- At least 18 inches wide. Platforms and walkways must be at least 18 inches wide. Ladder jack, top plate bracket, roof bracket and pump jack scaffolds may be 12 inches. Where the space is too tight for 18 inches, the platform may be as wide as the space allows, provided guardrails or PFAS protect the workers (1926.451(b)(2)).
- Within 14 inches of the work. The front edge of the platform may be no more than 14 inches from the face of the work unless guardrails are erected along the front edge or PFAS is used. The limit is 3 inches for outrigger scaffolds and 18 inches for plastering and lathing (1926.451(b)(3)).
- Ends extend past supports. Each end of a platform must extend over the centerline of its support by at least 6 inches, unless it is cleated or otherwise restrained by hooks (1926.451(b)(4)).
- But not too far. A platform 10 feet or less in length may not extend more than 12 inches past its support; a longer platform may extend up to 18 inches, unless the overhang is designed and installed so that the cantilever can support workers without tipping, or guardrails block access to it (1926.451(b)(5)).
- Overlaps of at least 12 inches. On a continuous run, abutting planks must rest on separate bearers or overlap by at least 12 inches over a support, unless restrained from movement (1926.451(b)(6), (b)(7)).
- Direction changes. Where a platform turns a corner, the plank resting on a bearer at an angle other than a right angle is laid first, and the square plank rests on top of it (1926.451(b)(8)).
- No opaque finishes. Wood platforms may not be covered with paint or any opaque finish that would hide defects; edges may be marked for identification and preservative treatments are allowed (1926.451(b)(9)).
- No mixing manufacturers unless the components fit together without force and the competent person has determined the assembly is structurally sound (1926.451(b)(10), (b)(11)).
Scaffold-grade planks and spans
The non-mandatory Appendix A to Subpart L is where OSHA describes what it considers a compliant plank. Solid sawn wood planks must be scaffold grade or equivalent, as recognized by a grading rule for the species in use. Scaffold grade is a specific grade with limits on knots, slope of grain, wane and checks; ordinary construction-grade 2x10 from a lumber yard is not scaffold plank, however clean it looks.
Appendix A also gives maximum spans for solid sawn planks at each duty rating. Platform deflection under load may not exceed 1/60 of the span; a 10-foot plank may sag no more than 2 inches at full load.
| Maximum intended load | Full-thickness undressed lumber | Nominal-thickness lumber |
|---|---|---|
| 25 lb per sq ft (light duty) | 10 ft | 8 ft |
| 50 lb per sq ft (medium duty) | 8 ft | 6 ft |
| 75 lb per sq ft (heavy duty) | 6 ft | Not permitted |
Laminated veneer lumber (LVL) planks, aluminum planks, aluminum-framed plywood decks and steel decks are rated by their manufacturers rather than by grading rules. Use the manufacturer's span and load tables, keep the rating stencil legible, and retire any plank whose rating can no longer be read. Suppliers of each type are listed under OSHA wood scaffold planks and aluminum scaffold planks.
When to pull a plank
- Cracks, splits or checks that run with the grain through the depth of the plank
- Saw cuts, notches or drilled holes, however small
- Delamination on LVL, or a bent, dented or cracked aluminum frame
- Rot, soft spots, heavy weathering or a plank that has been wet and dried repeatedly
- Paint or any coating that hides the surface
- Damage from being used as a ramp, a lever or a mud sill
Loading and using the platform
The rules on use in 1926.451(f) round out the platform requirements:
- Never load beyond the maximum intended load or the rated capacity, whichever is lower (1926.451(f)(1))
- Keep debris from accumulating on platforms (1926.451(f)(13))
- Do not use boxes, barrels, ladders or other makeshift devices on top of a platform to gain height, with a narrow exception for ladders on large-area scaffolds under specific conditions (1926.451(f)(14), (f)(15))
- Do not work on platforms covered with snow, ice or other slippery material except to remove it (1926.451(f)(8))
- Land material on the platform gently and spread it out; a pallet dropped from a forklift is an impact load the 4:1 factor was not meant to absorb
- Stock material against the building side, never against the guardrail, and never above the toe board without screening (1926.451(h))
If you are unsure what a rented scaffold is rated for, ask the supplier for the load chart in writing and have the qualified person confirm the configuration. Our consulting practice reviews scaffold plans and load assumptions for contractors and owners.