Category Archives: Roof Anchor Design Principles

Engineering a Complete Anchor System: Part 1

Fall arrest and tie-back anchors are primarily designed to protect workers from falls while working on or over the roof edge.

While a clear understanding of codes, regulations, and standards is of the utmost importance; the first consideration (after safety of course) is function when designing proper window washing, suspended maintenance, and fall protection systems.  Often buildings will install a system only to meet the needs to comply with building codes, Federal standards, and safety regulations.  It is however essential to consider function to achieve and ensure long term success of any system.


What is Function?

Firstly, you must consider what type of work is being completed while the system is in use?

  • Window washing
  • Exterior building maintenance (caulking, restoration, replacement, etc.)
  • Fixing/servicing/replacing equipment (cooling tower, drains, mechanical units) on the roof

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Secondly, you must consider if the workers will think the system is convenient and easy to use.  This beyond anything else is the first thing that will jeopardize a workers safety.  Like all professions really, a worker will bypass or modify elements of the system if they believe it is inefficient and slowing down their pace.


Technical Audits

It is fortunate to note that building technical audits (for warranty programs) are becoming more concerned with inadequately designed and impractical systems.  The audits are bringing this to the building’s attention as a way of saying that the inconvenient system is just as dangerous as a poorly engineered system because no worker will use it.

The Design Process

If you want to ensure that a system is compliant and efficient; manufacturers like Pro-Bel will work closely with architects, construction manager, engineers, and general contractors to provide design services which encourage and initiate discussions regarding the design (at an early stage of the design process).

This process seeks to:

  • Collect and analyze safe access and egress methods
  • Determine unique building needs
  • Establish functional and common relationship in equipment locations
  • Establish maintenance goals
  • State conventional rigging problems and methods
  • Uncover test methods and inspection practices



This process also looks to balance budget, compliance, and function.  It must be stressed that caution should be used when budget is the main consideration for any design (as functionality is the first factor to go).

The design process discussions have a significant impact on the design of the system (and building even) as there are various perspectives included in the conversation.  It is almost a way of conducting thorough due diligence and quality assurance.

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Roof Anchor Design Principles

The selection of proper roof anchors is critical and requires an in depth knowledge of fall window washing, suspended maintenance, and fall protection codes and rigging methods.  Also, recognizing that each building is different, most architects, developers, and general contractors find the selection of a professional roof anchor company a daunting task.


A standard lifeline is 5/8 inch (1.59 centimeters) in diameter and has a breaking strength of 5,000 pounds (2,267.96 kilograms).  However, tying off to an anchor using a knot in a rope can reduce its strength by 50% (or more) to the cutting action of the lifeline or lanyard.

Therefore, good design must ensure that the anchor eye is greater in diameter than the rope itself.  The proper anchor eye for a possible knot connection is 3/4 inch (or 1.9 centimeters).  This design principle will ensure that the anchor eye will not cut, damage, or weaken the rope.


Flate Plate Anchors

Special caution should be noted when considering a flat thin plate anchor.  These anchor designs are intended to be used with a snap hook or D-ring connection.  However, if a knot is tied to these anchors the rope is more likely to sever and fail.


D-Rings and Snap Hooks

D-rings and snap hooks are designed with minimum tensile strength of 5,000 pounds (2,267.96 kilograms).  D-rings and snap hooks shall be proof-tested to a minimum tensile load of 3,600 pounds (1,632.93 kilograms) without cracking, breaking, or suffering permanent deformation.

Other problematic compatibility issues include the use of non-locking snap hooks or D-rings that are sized incorrectly and are not compatible with the anchor diameter and/or inside dimension of the anchor.  These incompatible dimensions, relative to the snap hook, will cause the connected object to depress the snap hook keeper and release unintentionally.


Forged Eye Bolts

Eye bolts are rated along the axis of the bolt and its strength is greatly reduced if the force is applied at an angle to this axis in the direction of shear.

Generally roll out can occur using small eyebolts of 1/2 inch (1.27 centimeters) and 5/8 inch (1.59 centimeters) diameter.  These eyebolts are also too rigid so when put to a fall arrest test they will damage the securement stud and cannot be designed with redundancy.  They are too small and should not be used as part of a fall arrest system.

Also, it should be stated that forged eyebolts, non-shoulder, and shoulder eye bolts are not designed and approved by the manufacturer as a life safety product.  These anchors are designed for vertical loading not angle loading.   Also, recognizing that forged eye bolts are rigid, the securement bolt is almost always damaged under a test load or cyclical normal use conditions.

A hook must be compatible when the diameter of anchor to which the snap hook is attached is greater than the inside length of the snap hook when measured from the bottom (hinged end) of the keeper to the inside curved top of the snap hook.  Thus, no matter how the D-ring is positioned or moved (rolls) with the snap hook attached, the D-ring cannot touch the outside of the keeper, thus depressing it open.


Why 5,000 pounds?

The design rule is that 5,000 pounds (2,267.96 kilograms) fracture or pull out is sufficient to accept all tie-back and life line loads that are likely to be subjected to the anchor using conventional equipment on a roof top.

The anchor eye, base, or base plate should accept this energy and load not the securement studs to support a worker in the event of a fall.  The codes state that the 5,000 pounds (2,267.96 kilograms) based on the weight of a worker being 250 pounds (113.4 kilograms), experiencing a force of gravity multipled by 10 times a safety factor of 2.

250 x 10= 2,500 x 2= 5,000 pounds

While the energy absorbing lanyards hold in excess of 5,000 pounds (2,267.96 kilograms) when fully absorbed, most limit the load during the fall to under 1,250 pounds (566.99 kilograms).

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