When sub-contractors take on design responsibilities

Bob Richardson is head of technical at the NFRC
Bob Richardson is head of technical at the NFRC

There are a range of different contracts used in the building and construction industry and the responsibility for some sections of contracts are increasingly being delegated from the architect/main contractor to the sub-contractor.

This produces unclear sections within the contract, which can result in confusion for the sub-contractors who are directly responsible and have liability for the design and any future mechanical or structural failures.

Care is therefore needed to understand where design responsibility lies, and designers must ensure they consider all aspects, and that they have the correct PI insurance to cover the design. Where a system is fully specified, it should be followed in detail; if a sub-contractor alters a specification, they are taking design responsibility. The same applies when there is a performance specification and the sub-contractor designs the cladding assembly to meet this (or the attachment of).

Wind load calculation
Calculation of wind loading will depend on a number of factors, which take into account the above, and include the type of contract that the works to be undertaken are let under, where the requirement is either supplied by the client’s structural engineer or is supplied via the client to the roofing contractor.

This responsibility should include the design of all structural members or components to ensure the stability and the safe transfer of loads and will consider factors such as a building’s overall dimensions, roof pitch, roof configuration, wind speeds, topography and orientation.

The specified/used product system
The manufacturer of the product/system specified by either the architect or client, should provide the data so that those responsible for design and installation can establish how the specified product or system will satisfactorily perform under the calculated wind and live/dead loadings.

This data may take the form of:

  • The manufacturer providing fastener data to enable the fixing calculation to be carried out
  • The calculation would also need to advise on the wind load zone size in line with BS EN 1991-1-4:2005+A1:2010
  • Tabulated format showing design loads/spans for the specified roof/cladding systems
  • Maximum design/loadings for products/systems which are fixed to the manufacturer’s recommendations.

Responsibility and liability
If the above information has not been provided by the designer, the sub-contractor – by suggesting or recommending any component, member, fixing or fastener, Code of Practice, British Standard or Eurocode – has contractually assumed the design responsibility and liability. In such circumstances it is important that they to obtain some form of guidance on performance or design criteria/warranty from the supplier of any mechanical fixings, components, particularly if the supplier has been involved in either designing or advising upon the use of the fixings.

It is also advisable that the sub-contractor should clearly define the limits of his responsibility in writing with the main contractor to avoid any future disputes, for example, where the sub-contractor is supplying fixings to be used with purlins to be supplied by the others.

PI Insurance
An important fact that must also be established concerns indemnity insurance. Increasingly, sub-contractors are being asked to obtain Pl insurance where they undertake or take part in the design process. There may also be problems that arise from the requirement to take on collateral warranties, which should also be brought to the attention of the Pl insurer to ensure there is no conflict.

If a building or construction falls within the terms of the Defect Premises Act 1979 concerning any failure to the roof/cladding system and any of its component parts, then the responsibility is established by statute and is automatic.

If the mechanical fixing and design requirements are not supplied, the roofing contractor should not assume the responsibility to interpret or part-design any aspect of the structure roof/cladding, Code of Practise, British Standard or Eurocode. Any problems or uncertainties which exist concerning interpretation, specification or recommendations must be referred back for clarification to the following:

  • Architect/client for the: building design; shape; orientation; topography; roof/cladding system to be used
  • Structural engineer/client for the: wind loads; live/dead loads; structural calculations, including main structure, rail/purlin types and spacings.

This information is based on the revised NFRC Technical Bulletin (TB10) ‘Wind loadings & Mechanical Fixings and Fasteners for Industrial Roofs’, which is freely available to members or to purchase from www.nfrc.co.uk.