Chapter 24 Duty to warn of defects in design

A contract for the performance of work includes an implied warranty to warn of any design defects which a contractor believes to exist, or any defects the existence of which an ordinarily competent contractor of similar skill ought to have been aware. Failure to meet the duty will result in a contractor being held liable for a design not being fit for purpose if the contractor should have known of the existence of the defect, even if the contractor completed the design exactly as instructed. The duty to warn of defects in design arises from the implied obligation to act with due skill and care.  It effectively implies a warranty of fitness for purpose.

The extent of the duty to warn, and thus the scope for liability of the contractor for unsuitable design, depends in large part on the degree of reliance placed on the skill and judgement of the contractor in executing the design.


Plant Constructions plc v Clive Adams Associates  [2000] BLR 137

  • Plant Constructions was engaged by Ford to construct two pits for engine mount rigs.
  • Plant Constructions engaged a subcontractor to carry out substructure works.
  • The subcontractor was required to comply with all instructions given by Ford’s senior engineer.
  • The subcontractor complied with an instruction from Ford’s senior engineer in relation to propping to provide support following the removal of an existing steel column.
  • Excavation works were carried out following that instruction, and the roof then collapsed.
  • The English Court of Appeal held that the subcontractor was liable for breaching its duty to warn of defects for two reasons:
    • the works were dangerous and imposed a threat to safety; and
    • the subcontractor had knowledge concerning the defective design, as the evidence showed that representatives of the subcontractor had raised concerns regarding the adequacy of the propping, which were rejected by the representatives of Plant Constructions and Ford.
  • The subcontractor’s knowledge of the danger of the design was a burden greatly increased by the magnitude of the threat to the workers on the site, a burden the relief of which was held to have required formal warnings or even refusal to complete the design to discharge.