System design

Horizontal Lifeline System design should only be carried out by competent persons.

When considering a safe system design, the designer must firstly understand the requirements of the user and the need to access the roof. This information can be gathered from many sources but the safest method should be prioritised without the prejudice of a cost saving.

The system designer should always follow the hierarchy of fall protection. Restraint systems should be the preferred option and an arrest system should only be offered as a last resort.

Full considerations to be understood:

  • Reason for access/ purpose of system
  • Access point and method
  • Number of users required per system
  • Full roof plans and elevations
  • Roof substrate and condition
  • Fixing method

Work restraint

Work restraint systems are the safest method of Horizontal Lifeline system design. Keeping a user in restraint removes the possibility of fall occurring. The users path, and what they have access to can be dictated/controlled. Keeping a user in restraint is dictated by the relationship of two key distances

  • Lanyard length (A)
  • Position of the system and distance away from fall hazard (B)

If the achievable distance between position of line and fall hazard varies on the system route, the lesser distance and therefore lanyard length should be preferred before any variable lanyard lengths/multiple lanyards are proposed.

The general spacing from system to roof edge/fall hazard is 2.30m based on a typical lanyard length of 2m. Fall Hazards can be roof edges, roof lights or other fragile roof areas such as windows/glass. Main advantages of Work Restraint systems:

  • No possibility of a fall
  • No need for any rescue plan
  • Adjacent buildings/lower level roofs and fall clearances do not need to be factored
  • Minimal system user training required

Fall Arrest

When a work restraint system cannot be feasibly offered, a fall arrest system can then be considered. Fall arrest systems are designed to ‘limit the consequences of a fall should it occur’.

The Soter™ Horizontal Lifeline system will arrest a users’ fall, only if the required fall clearance has been allowed.

Fall clearances must be factored in when a fall arrest system is designed, considering the following factors;

  • Building height
  • Free fall distance through fragile roof areas/roof lights/canopies etc
  • Free fall distance from roof edges
    • To the ground
    • Lower level roofs/adjacent buildings

Although fall arrest systems are often employed to give a user full roof access, they come with major disadvantages, such as:

  • They do not stop a fall from height occurring as the user has flexibility of PPE lengths.
  • They can only be used on buildings with the required free fall clearance from all fall hazards including through roof lights.
  • A full rescue plan must be in place to return the user to safety, often this is overlooked.
  • The users must be trained so they can use extra PPE equipment safely including extended

Free-fall Clearances

Free fall clearances are often overlooked when a fall arrest system is designed.

It is imperative they are calculated using a calculation programme by the manufacturer, to prove that if a fall was to occur, the distances required for the system to effectively arrest the users fall can be assured.

System deflection and wire elongation is calculated using the Soter calculation package, taking into account system length, post spacing and maximum span length, and the number of users.

This calculation package is available to all recognised Soter installers. SFS can also make design calculations in house on request.

Once system deflection and wire elongation is known, this distance can be added to the following measurements to calculate a free fall clearance distance:

  • Users Height (A)
  • Lanyard Length (B)
  • Deployed shock absorber length (C)
  • Safety Factor (D)

= Minimum free fall clearance required (Z)

Calculation package

Soter comes complete with its own calculation package allowing for calculations to be made to give end loadings, system deflections and other detailed information assisting the system designer to propose a system that is fit for purpose. This calculation package can be used on shock absorbing posts, and components with in-line shock absorbers for systems fitted to rigid/fabricated posts, walls, and rigid structures.


SFS Fall Protection
SFS Group Fastening Technology Ltd.
153 Kirkstall Road
UK-LS4 2AT Leeds
T +44 1924 472251