Scaffold Load Calculations: OSHA Classes, Dead Loads, and Tie Requirements

OSHA Scaffold Loading Classes

OSHA defines three scaffold duty rating classes based on the intended use. Light-duty scaffolds are rated at 25 pounds per square foot (PSF) and are intended for painters, inspectors, and light repair work. They support no more than one 250-lb worker plus 20 lbs of tools per platform section.

Medium-duty scaffolds are rated at 50 PSF and support plastering, lathing, general construction, and moderate material storage. They handle two workers with tools and a reasonable quantity of materials on each platform bay.

Heavy-duty scaffolds are rated at 75 PSF and support bricklaying, stone work, and heavy material storage such as pallets of brick or mortar tubs. Some heavy-duty applications require ratings up to 100 PSF when significant quantities of masonry materials are stored on the platform.

The scaffold manufacturer's rated capacity must meet or exceed the intended loading class. Mixing components from different manufacturers, using damaged components, or exceeding the rated capacity are all violations that can result in citations and, more importantly, collapse.

Estimating Dead Load and Live Load

Dead load is the weight of the scaffold itself: frames, cross-braces, platforms, guardrails, and accessories. A standard 5-foot by 7-foot frame scaffold section with steel planks weighs approximately 75 to 120 pounds per bay per lift. Wood plank platforms add 10 to 15 lbs per plank (2x10, 7-foot span). Aluminum planks weigh 25 to 35 lbs each.

Live load is everything else: workers, tools, materials, equipment, and environmental loads like ice and wind. A worker with tools and personal fall arrest equipment weighs 250 to 300 lbs. A mortar tub weighs about 300 lbs full. A pallet of standard brick (500 bricks) weighs approximately 1,700 lbs.

Calculate the total load on each leg as the sum of dead and live loads above that point divided by the number of legs sharing the load. A 5-lift scaffold with a fully loaded top platform puts different loads on the legs depending on the frame type and bracing arrangement.

Wind load on scaffolds with enclosure or debris netting can be significant. A scaffold wrapped in monofilament netting in a 30 mph wind can see lateral forces of 5 to 10 PSF. Solid enclosures in the same wind see 15 to 25 PSF. Additional ties and bracing are required for enclosed scaffolds.

Mudsill and Base Plate Sizing

Every scaffold leg must bear on a base plate, and every base plate must bear on a firm foundation. On concrete or asphalt, the base plate alone is usually sufficient. On soil, a mudsill (timber pad) distributes the leg load over a larger area to prevent sinking.

Mudsill area = Leg load (lbs) ÷ Allowable soil bearing pressure (PSF). Typical allowable bearing pressures: compacted gravel 3,000 to 4,000 PSF, undisturbed clay 1,500 to 2,000 PSF, soft clay 500 to 1,000 PSF, loose sand 1,000 to 2,000 PSF. A 6,000-lb leg load on soft clay at 750 PSF needs 8 square feet of mudsill area — roughly a 2x10 plank 4 feet long is not enough. You need a 3-foot by 3-foot pad.

Screw jacks provide leveling capability up to 12 inches of adjustment. Never extend screw jacks more than the manufacturer's rated extension or more than one-third of their length. Over-extended jacks buckle under load.

Tie Spacing and Anchorage Requirements

Scaffold ties connect the scaffold to the building or structure to prevent tipping. OSHA 1926.451(c)(1) requires ties at the closest horizontal member to the 4:1 height-to-base ratio, then at 26-foot vertical intervals and 30-foot horizontal intervals thereafter.

The first tie is typically at the second or third lift. As the scaffold grows taller, ties every 26 feet vertically (about every 4 lifts at standard 6.5-foot frame height) and every 30 feet horizontally (about every 4 bays at 7-foot frame width).

Each tie must resist a 300-lb lateral force in both directions (push and pull). Tie-back to window frames, mullions, or decorative features that cannot resist this load is a common violation. Ties must anchor to structural elements: steel beams, concrete floors, masonry walls with adequate anchor capacity.

Competent Person Duties and Fall Protection

OSHA requires a competent person to direct scaffold erection, moving, dismantling, and alteration. The competent person must be able to identify existing and predictable hazards and has the authority to take corrective measures. This is not optional — it is a cited violation with penalties.

The competent person inspects the scaffold before each work shift and after any event that could affect structural integrity (rain, wind, impact, loading changes). They verify footings, ties, bracing, planking, guardrails, access, and load limits.

Fall protection is required at 10 feet above a lower level. For supported scaffolds, this means guardrails (top rail at 38 to 45 inches, mid rail, toeboard). Personal fall arrest systems are an alternative but require an adequate anchorage point on the scaffold rated for 5,000 lbs per worker. Cross-braces are not guardrails and do not meet the guardrail requirement.

Access to scaffold platforms must be by ladder, stair tower, or integral access frame. Climbing cross-braces is a frequently cited OSHA violation and a leading cause of falls from scaffolds.