Field calculator
Fall clearance calculator (fall arrest)
A personal fall arrest system only protects the worker if there is enough open distance below them to stop the fall before they hit the lower level. This calculator adds the four parts of required fall clearance: the free fall distance before the system starts to slow the worker, the deceleration distance as a shock-absorbing lanyard or self-retracting lifeline pays out and stops the fall, the worker height from the dorsal D-ring down to the feet, and a safety margin. Enter each in feet. A common 6-foot shock-absorbing lanyard tied off at foot level can need roughly 18.5 feet of clearance below the anchor, which is why short falls into a floor below are a real hazard and why a self-retracting lifeline or a higher anchor is often the answer. Treat the result as a planning number, confirm it against the specific equipment manufacturer instructions and OSHA and ANSI Z359, and account for the anchor height and any swing-fall, which change the real clearance needed.
Result
Required fall clearance for a personal fall arrest system is the sum of the free fall distance, the deceleration distance (lanyard or SRL stopping distance), the worker height from the D-ring to the feet, and a safety margin. A 6 ft shock-absorbing lanyard commonly needs roughly 18.5 ft of clearance below the anchor; a self-retracting lifeline needs far less. Enter each value in feet. These are planning numbers; confirm against the specific equipment instructions and OSHA / ANSI Z359, and remember the anchor height and swing-fall change the real result.
anvilfield.com/calculators/fall-clearance-calculator · Free field calculators and FieldOS. A planning estimate, verify against the code, the manufacturer, and the engineer of record.
More Roofing calculators
Fall clearance FAQ
At what height is fall protection required on a roof?
Fall protection is required at 6 feet above a lower level for construction work under OSHA 1926.501, and at 4 feet for general industry maintenance under 1910.28. Holes, skylights, and dangerous equipment can trigger protection at any height. The competent person and the adopted standard govern which applies.
What is a personal fall arrest system?
A personal fall arrest system, or PFAS, catches a worker after a fall and limits the forces on the body. It is the ABCD: anchorage rated at 5,000 pounds or engineered, a full-body harness, connectors, and a deceleration device. It only works if it is worn right and the fall clearance is checked.
What is a warning line system?
A warning line system is a roped perimeter set back from the edge for low-slope roofing work, marking where the protected zone ends. The construction setback is at least 6 feet from the edge without mechanical equipment. Anyone between the line and the edge needs conventional fall protection. Confirm the setbacks against the standard.
Do skylights need fall protection?
Yes. OSHA treats a skylight as a hole, so workers must be protected from falling through it by a guardrail, a load-rated cover, a screen, or a personal fall arrest system. People die stepping onto skylights they took for solid roof. A cover must support at least twice the intended load and be secured.
How strong does a fall arrest anchor have to be?
Under OSHA 1926.502, an anchorage for a personal fall arrest system must support at least 5,000 pounds per attached worker, or be designed and used as part of a complete system engineered to a safety factor of at least two. A vent, conduit, or small parapet is not a 5,000 pound anchor.