Supported scaffold safety field guide: OSHA Subpart L frame and mobile

What a supported scaffold is, and why a bad one fails all at once

A supported scaffold is a work platform held up from below by its own legs, frames, or poles standing on the ground or a structure. Frame scaffold, the welded end frames with cross braces that most roofers and masons know, is one type. System or modular scaffold, the kind with ring or cup connections, is another. A mobile or rolling scaffold is the same idea on locking casters. All three put a crew at height on a platform you build, and all three are covered by OSHA's scaffold rules in 29 CFR 1926 Subpart L.

The thing that makes a scaffold dangerous is that it fails as a system, not as a part. A footing that settles, a missing brace, one plank short, a tie that nobody installed: none of those feels like much on its own. Put them together under a loaded platform and the failure is sudden and total. People die three ways on scaffolds. The whole thing collapses, from bad footing or overload or missing bracing. Someone falls, from a gap in the planking or no guardrail. Or something drops on the crew below. The rules exist because all three are preventable.

The short version of those rules is the spine of this guide: a competent person designs and inspects it, the footing is firm and on base plates, the platform is fully planked, guardrails go up above the trigger height, the access is real, and the scaffold is tied to the structure once it gets tall. Fall protection while you are on the platform and while you erect it ties into the roof fall-protection guide, and getting on and off ties into the ladder guide. Confirm every number here against the current OSHA edition and the AHJ, because the standard controls, not the rule of thumb.

Why scaffold safety is not optional

Scaffolding sits near the top of OSHA's most-cited construction standards year after year, and it is not because inspectors are bored. A large share of construction workers spend time on scaffolds, and scaffold collapses and falls kill and maim a meaningful number of them every year. The hazard is built into the work. You are standing on something temporary, at height, that somebody assembled fast under schedule pressure.

What makes it worse than most height work is the all-at-once failure. A ladder drops one person. A scaffold collapse can take the whole crew on the platform plus whoever is underneath. The energy is already stored the moment the platform is loaded wrong, and it releases without warning. There is no slow leak that gives you time to react.

So the standard front-loads the safety into the build and the inspection, before anyone trusts the platform. Get the footing, the planking, the guardrails, the tie-ins, and the access right at assembly, and confirm them before each shift, and the scaffold does its job quietly for the length of the project. Skip the pre-shift look because it went up fine yesterday, and you are betting the crew on the assumption that nothing changed overnight. Weather, other trades, and a forklift bump all change things overnight.

What does the competent person own?

The competent person is the named authority on the scaffold, and OSHA defines the role by what the person can do, not by a certificate on the wall. A competent person is someone who can identify existing and predictable scaffold hazards and has the authority to take prompt action to fix them, which includes pulling the scaffold out of service. That authority is the part that matters. A safety observer who cannot stop the work is not a competent person.

The role runs the whole life of the scaffold. The competent person selects and oversees the design within the manufacturer's limits, supervises and directs erection and dismantling by trained workers, decides whether fall protection is feasible for the erectors, and inspects the scaffold and its components for visible defects before each work shift and after any event that could affect its integrity. On many jobs a qualified person, meaning someone with the engineering or technical background, designs the unusual cases while the competent person runs the field.

Treat the competent person and the pre-shift inspection as the two things that cannot be delegated to habit. Everything else on this list is a rule the competent person enforces. If your job does not have a named, qualified competent person with real authority to red-tag a scaffold, you do not have a scaffold program, you have a pile of frames and some luck. Confirm the qualifications and the role assignment against OSHA's definitions and your site's requirements.

Firm footing on base plates and mud sills

Everything above ground rides on the footing, and the footing is where lazy collapses start. OSHA requires supported scaffold poles, legs, posts, frames, and uprights to bear on base plates and mud sills or another adequate firm foundation. The footing has to be level, sound, rigid, and able to carry the loaded scaffold without settling or moving. That is the whole game at the bottom.

What you do not do is stack scaffold legs on unstable objects. Blocks, bricks, loose pieces of lumber scraps, and buckets are all called out by the prohibition on using unstable objects to support scaffolds or platform units. They crush, tip, or squirt out under load, and when one leg drops an inch the whole frame racks. Use a metal base plate under each leg, sitting on a mud sill, usually a length of plank, that spreads the load across soft or uneven ground.

On a slope or uneven grade, screw jacks (adjustable base plates) bring each leg to level so the scaffold stands plumb and the frames sit square. A scaffold out of plumb loads its legs and braces in directions they were not built for. Set the base, check it level both ways, and check it plumb as you stack. The competent person confirms the foundation before the first frame goes up and looks at it again after rain, because saturated ground that held yesterday can settle under load today.

What is the scaffold capacity rule?

Each scaffold and each component has to support its own weight plus at least four times the maximum intended load, without failure. That 4 to 1 safety factor is an OSHA requirement, and the maximum intended load means everyone and everything on the platform: the workers, their tools, the material staged up there, and any force transmitted to the scaffold. The factor is there because field loads are never exactly what the plan assumed.

The trap is the material pile. A crew loads a pallet of block, a stack of bundles, or a drum of coating onto a deck rated for light or medium duty and quietly blows past the rating. Scaffolds carry a duty rating, light, medium, or heavy, tied to a load per square foot, and the manufacturer's tabulated data gives the real numbers for that system. Know the rating before you stage material, not after the deck is sagging.

Do not overload, and do not assume the 4 to 1 factor is your working margin to spend. The safety factor covers the unknowns, the impact of a dropped load, the worn component, the wind gust. It is not extra capacity for one more pallet. When a load is unusual, heavy, or concentrated, get the system's load tables or an engineer, and confirm the duty rating against the manufacturer's data and OSHA before you load it.

Full planking with no gaps

The platform has to be fully planked or decked between the front uprights and the guardrail supports. A partially planked deck is a hole waiting for a boot. OSHA limits the space between adjacent plank units and between the platform and the uprights to about 1 inch, with a narrow allowance for a wider gap only where the work genuinely requires it. The point is simple: a worker should not be able to step into a gap that drops them through the deck.

Use scaffold-grade plank or manufactured platform units, not whatever lumber is on the truck. Solid-sawn scaffold plank is graded and marked for the job, and a 2x10 of construction lumber is not the same thing. Wood plank has to be supported and not overloaded, and the manufacturer's or grading rules govern the allowable span. Plank that overhangs its support gets cleated or restrained, and overlapped plank laps only over a support, commonly by at least 12 inches unless the planks are nailed or otherwise secured against movement.

The other gap that matters is the one to the wall. OSHA keeps the front edge of the platform within about 14 inches of the face of the work, with a wider allowance for specific tasks like plastering and lathing, so a worker is not leaning out over an open space. If the platform has to sit farther off the face, a guardrail or personal fall arrest system fills the gap on the inboard side. Confirm the plank grade, the span, the overlap, and the face distance against the manufacturer's data and OSHA Subpart L.

Guardrails above the trigger height

Each open side and end of a platform more than about 10 feet above a lower level needs guardrails, fall arrest, or both, and 10 feet is the scaffold trigger height under Subpart L. That is different from the 6-foot trigger for most other construction fall hazards, which the roof fall-protection guide covers, so do not carry the 6-foot number onto a scaffold by reflex. On a scaffold, the platform-edge number is about 10 feet.

A complete guardrail system is a top rail, a mid rail, and a toeboard. For scaffolds built and put in service in recent decades, OSHA sets the top rail between about 38 and 45 inches above the platform, with older scaffolds allowed a slightly lower minimum. The mid rail sits roughly halfway between the top rail and the deck. Cross bracing can stand in for a mid rail or a top rail only when the brace crossing point falls in the height band OSHA allows, and it cannot do both jobs at once on the same bay.

Guardrails also have to take a load, not just look like a rail. OSHA requires the top rail to withstand a force of at least 200 pounds in any outward or downward direction for most scaffolds, and the mid rail to take at least 150 pounds, so a rail you can push over with one hand does not meet the standard. The toeboard does double duty, stopping a worker's foot and keeping tools and material from sliding off onto people below. Confirm the exact heights, the force values, and the cross-brace allowance against the current OSHA edition, because the dimensions have shifted between cycles.

When do scaffolds need fall protection?

Fall protection is required for workers on a supported scaffold platform more than about 10 feet above a lower level, and the protection is guardrails, a personal fall arrest system (PFAS), or both depending on the scaffold and the task. For most supported scaffolds, a proper guardrail system is the protection. The two systems are not interchangeable for every type. A suspended scaffold, the kind that hangs on ropes, requires a personal fall arrest system, because the platform itself can fail in a way a guardrail will not catch.

The dangerous phase is erection and dismantling. While the scaffold is going up or coming down, the guardrails and the planking are not all in place yet, so the people building it are the most exposed workers on the job. OSHA requires the competent person to determine whether providing fall protection for erectors and dismantlers is feasible and does not itself create a greater hazard, and to provide it where it is feasible. That is a judgment call the standard hands to a qualified person on purpose, and it is not an excuse to skip protection because it is inconvenient.

Anchorage is the catch with PFAS on a scaffold. A personal fall arrest system needs an anchor that can hold the arrest forces, and a scaffold member is not automatically a qualified anchor. The anchorage, the harness, the lanyard, and the rescue plan all have to meet the fall-protection rules, which the roof fall-protection guide covers in detail. Confirm the fall-protection method, the anchorage, and the erection-phase plan with the competent person and against OSHA before anyone goes up.

Cross bracing, pins, and connections

A frame scaffold is only as good as its bracing and its connections, and both have to be complete and tight before the frame is loaded. Cross braces hold the frames square and stop them from racking sideways. Pins, coupling pins, and locking devices hold the stacked frames together so an uplift or a sideways load does not separate them. Every brace and every connection the manufacturer's design calls for goes in, every level, every bay.

The failure here is the missing or removed brace. A crew pulls a cross brace to pass material or to clear a doorway and forgets to put it back, or skips a coupling pin because the holes did not line up easily. The scaffold stands fine until a load or a gust finds the weak bay, and then it racks and folds. Do not remove bracing from a standing, loaded scaffold, and if a brace has to come out for access, the competent person decides how to maintain stability first.

On system and tube-and-coupler scaffolds, the couplers carry the loads between members and have to be the right type, oriented right, and tightened to the manufacturer's spec. A loose or wrong coupler is the same hazard as a missing brace. Confirm the bracing pattern, the connection hardware, and the coupler torque against the manufacturer's tabulated data and OSHA before the platform carries weight.

Tie the scaffold to the structure

A free-standing scaffold tips once it gets tall relative to its base, so OSHA requires a supported scaffold with a height-to-base-width ratio greater than 4 to 1 to be restrained from tipping by guying, tying, bracing, or equivalent means. The first tie goes in at or near the point where the scaffold reaches that 4 to 1 ratio, and ties repeat up the height on a vertical spacing the standard sets. A tall scaffold that is not tied is a tip-over waiting for a load shift, a wind gust, or a worker leaning out.

The vertical spacing depends on the scaffold width and the manufacturer's recommendation. As a working picture, narrow scaffolds get ties more often up the height than wide ones, and the manufacturer's data or the closest horizontal member governs where each tie lands. The ties have to resist both inward and outward movement, because a scaffold can pull away from a building as easily as it leans into it. A tie that only stops it from falling toward the wall is half a tie.

Counting bays and eyeballing it is how scaffolds end up under-tied. Use the manufacturer's tie schedule, mark the tie locations, and have the competent person confirm each tie is installed and anchored to something that can hold it, not to a gutter or a piece of flashing. Confirm the 4 to 1 trigger, the vertical and horizontal tie spacing, and the anchorage against the manufacturer's data and OSHA, because the exact spacing numbers vary with the scaffold.

Proper access, not the cross braces

Getting on and off a scaffold needs a real means of access when the platform is more than about 2 feet above or below the point of access, and that means a built-in scaffold ladder, an attached ladder, a stair tower, a ramp, or an equivalent. OSHA prohibits climbing the cross braces. The braces are not spaced or built for hands and feet, they pitch you outward, and they are the single most common shortcut that puts a worker on the ground.

A stair tower is the right answer when traffic is heavy or the climb is tall, because people carrying tools and material cannot keep three points of contact on a ladder. For shorter access, a frame with built-in ladder rungs or a separate attached ladder works, set up so the worker can transition onto the platform safely. The access is part of the design, not an afterthought you solve by finding a foothold.

The handoff to the ladder guide matters here. Whatever ladder gives access to or off the scaffold follows the portable-ladder rules: the right ladder, the angle, the extension above the landing, three points of contact, and the tie-off. A ladder leaned against a scaffold and not secured slides the same way it slides against a wall. Confirm the access method and its setup against OSHA Subpart L and the ladder rules.

Mobile and rolling scaffolds

A mobile or rolling scaffold, the Baker or narrow-frame tower on casters, is the same platform with the added hazard that it moves. The casters have to be locked with positive wheel locks, or swivel and wheel locks, whenever the scaffold is used as a work platform, and the caster stems have to be pinned or secured into the legs so a wheel cannot drop out. A caster left free is how a tower rolls out from under a worker.

Do not ride a rolling scaffold while it is being moved unless it is specifically designed and rated to be moved with workers on it, and the surface is level and clear. The usual case is no: everyone comes down, the route is checked for holes, debris, and overhead obstructions including power lines, and the scaffold is rolled by applying force near the base, not at the top. Pushing a tall tower from the top tips it. After moving, lock the casters again before anyone climbs back up.

Height-to-base limits are tighter on a rolling scaffold because it is narrow and it moves. Manufacturers and OSHA cap how tall a mobile scaffold can stand relative to its base, with outriggers used to widen the effective base when more height is needed. A narrow tower stacked too high is a tip-over even standing still. Confirm the caster locking, the riding restriction, and the height-to-base limit against the manufacturer's data and OSHA before the tower carries anyone.

Scissor lifts and boom lifts are a different rulebook

A scissor lift or a boom lift is not a scaffold. These are mobile elevating work platforms (MEWPs), powered machines with their own standard and their own operating rules, separate from Subpart L scaffolds. People mix them up because both put a crew at height on a platform, but the controls, the fall-protection requirements, the stability limits, and the training are different.

Do not apply scaffold habits to a lift or lift habits to a scaffold. A boom lift, for example, generally requires a personal fall arrest system tied to the basket, which is not how a guardrailed supported scaffold works. If the job uses lifts, follow the aerial-lift requirements and the manufacturer's manual for that machine, and confirm the operating and fall-protection rules against the standard that actually governs MEWPs.

Keeping objects off the people below

A scaffold puts material and tools above other workers, and a dropped wrench from height is lethal. OSHA requires protection from falling objects for anyone below a scaffold. The first line is the toeboard, which keeps tools and small material from sliding off the platform edge. Where material is stacked higher than the toeboard, you add a panel or screen between the toeboard and the top rail, or a debris net, to catch what the toeboard cannot.

Below the scaffold, barricade the area so workers are not walking under the drop zone, or build a canopy or catch platform where the work below cannot stop. Hard hats are required for everyone in the area and they are the last line, not the plan. A hard hat handles a dropped bolt. It does not handle a dropped bundle, which is what the toeboards, screens, and barricades are for.

The mistake is treating falling-object protection as optional once the platform itself is safe. The crew on the deck is protected by guardrails, but the laborer walking past at grade is protected only by what you put between them and the edge above. Confirm the toeboard, screening, barricade, and overhead-protection requirements against OSHA Subpart L for the material and the layout you actually have.

Clearance from power lines

A metal scaffold is a conductor, and a worker on it is grounded through the frame, so a scaffold that touches or arcs to an energized line electrocutes the crew. OSHA sets minimum clearance distances between scaffolds and power lines, and the distance grows with the voltage. Insulated lines at lower voltages have a smaller required clearance than uninsulated high-voltage transmission lines, which require a wide standoff.

Plan the clearance before the scaffold goes up, and re-check it when a rolling scaffold moves, because the lines do not move but the scaffold does. Where the required clearance cannot be kept, the utility de-energizes, grounds, insulates, or relocates the line before the work proceeds. Do not assume a line is dead, and do not guess the voltage. Confirm the line voltage and the required clearance with the utility and against OSHA before any part of the scaffold or a worker's tool can reach it.

Wind, ice, and re-inspection after weather

Weather changes a scaffold faster than anything else on the job. OSHA prohibits work on or from scaffolds during storms or high winds unless the competent person decides it is safe and the workers are protected, for example by a personal fall arrest system or a wind screen. Wind on a tall scaffold with material and tarps on it becomes a sail, and a gust can rack or tip a scaffold that stood fine in calm air.

Ice and snow turn a planked deck into a slick surface and add load the design never counted. Clear the platform before the crew works on it, and watch for ice on the rungs and the access. A diagonal layer of frost on the plank is a slip-and-fall that the guardrail may or may not catch.

Re-inspect after any storm, high wind, freeze-thaw, or anything that could have moved the scaffold, before anyone trusts it again. This is the after-any-change part of the competent person's inspection duty. The scaffold that held through a calm week is a different scaffold after a night of wind and rain saturated the footing and loosened a tie. Confirm the weather restrictions and the re-inspection trigger against OSHA and the competent person's judgment.

Erection and dismantling, the highest-risk phase

Building and tearing down a scaffold is when the people doing it are most exposed, because the guardrails, the planking, and the ties are not all in place yet. OSHA requires erection and dismantling to be done under the supervision and direction of a competent person and by experienced and trained workers, because the sequence and the partial states are where the judgment lives. This is not general-labor work you hand to whoever is free.

The order matters. You build from a solid, level base up, installing bracing and ties as you climb so the structure is stable at every stage, and you take it down in the reverse order so it never becomes top-heavy or unbraced under load. The competent person determines whether fall protection for the erectors is feasible and provides it where it is, and decides when a section is safe to load. Rushing the build or skipping a tie to save a step at this stage is how scaffolds collapse during assembly.

Trained erectors know the load paths, the brace and tie schedule, and the partial-state hazards. A crew that has only ever stood on a finished scaffold does not. If your erectors are not trained for the specific scaffold system, that is a gap to close before the build, not a thing to learn on the way up. Confirm the supervision, the training, and the erection sequence against OSHA and the manufacturer's instructions.

The scaffold tag system

A scaffold tag tells anyone walking up whether the scaffold is safe to use, and on most jobs it is a three-color system. Green means the scaffold has been inspected and is complete and safe to use as built. Yellow means caution, the scaffold is incomplete or has a condition the user has to work around, such as a missing guardrail section that requires personal fall arrest. Red means do not use, the scaffold is under construction, incomplete, or out of service.

The tag is not the inspection, it is the record of it. The competent person inspects, then tags, and the tag carries the date and who signed it so the next shift knows the inspection happened and when. A scaffold with no tag should be treated as a red scaffold until the competent person inspects it and says otherwise.

The tag system is a practice, not a Subpart L mandate written word for word, so the exact colors and procedure follow the employer's program and the site. The principle holds everywhere: nobody gets on a scaffold that has not been inspected and cleared. Confirm your site's tag procedure and tie it to the competent person's documented pre-shift inspection.

When a scaffold needs an engineer

Most frame and system scaffolds get built within the manufacturer's tabulated data, which is the load tables and configuration limits the maker publishes for that product. Stay inside that envelope and the design is already done. Step outside it and you need a qualified person, and for the unusual cases a registered professional engineer (PE), to design the scaffold.

The clear trigger is height. OSHA requires scaffolds over a defined height, commonly cited around 125 feet for certain types, to be designed by a registered professional engineer and built to that design. Heavy or concentrated loads, odd geometry, scaffolds carrying hoists or bridging gaps, and scaffolds supporting other structures are the other cases where the manufacturer's data runs out and engineered design takes over.

When in doubt about whether a configuration is inside the tabulated data, treat it as outside until a qualified person says it is in. The cost of an engineered design is small next to a collapse. Confirm the height threshold and the cases requiring a registered professional engineer against OSHA Subpart L and the manufacturer's data for the system you are using.

Training users and erectors

OSHA requires anyone who works on a scaffold to be trained by a qualified person to recognize the hazards and understand the procedures to control or minimize them. That covers the nature of the hazards, the correct use of the scaffold, the maximum intended load and the load-carrying capacity, the electrical hazards, the fall hazards, and the falling-object hazards. A worker who does not know the deck's load rating cannot avoid overloading it.

Erectors, dismantlers, repairers, movers, and inspectors get additional training, by a competent person, on the parts of the work they do: the design and the load criteria, the erection and dismantling procedures, and the hazards of those operations. Building a scaffold safely is a different skill set than standing on one, and OSHA treats it that way.

Retraining is required when the work or the equipment changes or when a worker shows they have not retained it. Training is not a one-time orientation that covers a career. Confirm the training content and the retraining triggers against OSHA's scaffold training requirements in 1926.454 and your site's program.

How often must a scaffold be inspected, and what gets recorded

A competent person inspects the scaffold and its components for visible defects before each work shift, and again after any occurrence that could affect the scaffold's structural integrity, such as a storm, an impact, or an alteration. Before each shift is the floor, not the ceiling. The scaffold that passed yesterday is a different scaffold after a night of weather and a day of other trades working around it.

The inspection walks the same spine this guide does: the footing and base plates, the bracing and connections, the planking and gaps, the guardrails and toeboards, the ties, the access, and any sign of overload, damage, or movement. Anything that fails comes out of service, by tag or by physically blocking access, until it is fixed and re-inspected. The competent person's authority to do that is the whole point of the role.

Records turn a one-time check into a defensible history. Capture the date and shift, who inspected, the scaffold location and type, what was checked, any defects found and the action taken, and the tag status. A field tool like FieldOS keeps the pre-shift inspection, the photos, and the sign-off in one place per scaffold, so the next shift and the inspector can see the scaffold was looked at and cleared, by whom, and when. Confirm the inspection frequency and the recordkeeping against OSHA Subpart L and your site's program.

The failures that put scaffolds in the news

Scaffold deaths cluster, and the pattern repeats every year: falls and collapses, not exotic engineering failures. The value for a competent person is knowing where the fatal failures actually originate, so the inspection chases the structural causes instead of the cosmetic ones.

Group them by root cause and four categories carry almost all of it. Foundation: a leg set on blocks or fill instead of base plates and mud sills settles, and the whole frame racks and sheds its load sideways. Capacity: the platform gets loaded past its duty rating, eating into the 4 to 1 structural margin the components were built around. Access and fall protection: people climb the cross braces and work open edges because the stair tower or the guardrail was never built in. Stability: a tall scaffold stands with no tie-in and goes over under a load shift, a lean, or a gust. The pre-shift inspection is the control that catches all four before they stack, which is why a missed inspection is the failure behind the failure rather than a fifth item on the list.

What to document

The record is what proves the scaffold was built and inspected to the rules, and it is what an inspector, a GC, or a lawyer asks for after an incident. Tie each requirement to the rule that drives it and a field note, and verify the specifics against OSHA Subpart L for your scaffold and jurisdiction.

At minimum, document the footing and base, the capacity and duty rating, the planking, the guardrails, the bracing and ties, the access, the competent-person designation, the training, and each pre-shift inspection with the tag status. The table is a starting frame, not a substitute for the standard.

Field checklist

Common mistakes

• Setting scaffold legs on blocks, bricks, or lumber scraps instead of base plates and mud sills.
• Loading the platform past the duty rating and treating the 4 to 1 safety factor as working margin.
• Leaving the deck partially planked, with gaps a worker can step into or material can fall through.
• Skipping guardrails above the trigger height, or relying on a rail that cannot take the required force.
• Climbing the cross braces instead of using a built-in ladder or a stair tower.
• Standing a tall scaffold with no tie-in, so it tips under a load shift, a lean, or a gust.
• Moving a rolling scaffold with the casters not locked, with someone on it, or into a power line.
• Trusting yesterday's scaffold and skipping the pre-shift inspection after weather or other trades worked around it.

Standards and references

OSHA's scaffold rules for construction live in 29 CFR 1926 Subpart L. The general requirements that apply to most scaffolds, including capacity, platform construction, footing, fall protection, falling-object protection, access, and inspection, are in 1926.451. Additional requirements for specific scaffold types, including frame, tube-and-coupler, and mobile scaffolds, are in 1926.452. The training requirements for users and erectors are in 1926.454. The non-mandatory appendices, including Appendix A, give specifications and worked examples that help you build inside the rules.

Alongside OSHA, the scaffold manufacturer's tabulated data is the source for the load tables, the duty ratings, the brace and tie schedule, and the configuration limits for the specific system you are standing on. Build to that data, because OSHA's general rules and the manufacturer's specifics work together. For scaffolds over the defined height, commonly cited around 125 feet for certain types, and for unusual or heavily loaded configurations, a registered professional engineer designs the scaffold.

The threads to pull through all of it: the competent person and the pre-shift inspection, the footing and full planking and guardrails, and the tie-ins and proper access. Section numbers, heights, ratios, capacities, and clearances move between code cycles and can be amended locally, so verify every specific in this guide against the current OSHA edition, the manufacturer's data, a registered professional engineer where required, and the authority having jurisdiction before you rely on it. The standard controls, not the rule of thumb.

Units and terms

Scaffold work uses a handful of terms that show up across the standard, the manufacturer's data, and the jobsite, and they do not always match word for word.

Supported scaffold means a platform held up from below by legs, frames, or poles, as opposed to a suspended scaffold that hangs on ropes. Frame scaffold, system or modular scaffold, and tube-and-coupler scaffold are the common supported types. A mobile or rolling scaffold is a supported scaffold on casters. Maximum intended load is everyone and everything the scaffold carries, and the 4 to 1 capacity factor is applied to it. Duty rating is the load class, light, medium, or heavy, tied to a load per square foot.

Supported scaffold

A platform held up from below by legs, frames, or poles bearing on a firm foundation

Competent person

Someone able to identify scaffold hazards and with authority to take prompt corrective action, including tagging it out

Maximum intended load

The total of workers, tools, material, and any force on the scaffold; capacity is at least 4 times this

Duty rating

The scaffold's load class, light, medium, or heavy, tied to a load per square foot from the manufacturer

Mud sill

A plank or pad under the base plate that spreads the leg load across soft or uneven ground

Screw jack

An adjustable base that levels each leg so the scaffold stands plumb on uneven grade

Tabulated data

The manufacturer's load tables and configuration limits for a specific scaffold system

Related tools

Calculators and readiness checks for this work

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