The architect’s, engineer’s, builder’s etc. liability to the employer for negligence may be reduced to the extent that the employer has also been negligent and, in so doing, has contributed to the damage suffered. This arises under the Law Reform (Contributory Negligence) Act 1945.
Where any person suffers damage as the result partly of his own fault and partly of the fault of any other person or persons, a claim in respect of that damage shall not be defeated by reason of the fault of the person suffering the damage, but the damages recoverable in respect thereof shall be reduced to such extent as the court thinks just and equitable having regard to the claimant's share in the responsibility for the damage 
It is well established that this applies to claims in tort. It may apply to a breach of a general duty of care under a contract, which duty is the same as would exist in common law, but it most probably is not applicable to a claim in respect of a breach of a strict contractual duty: ‘Provided that this subsection shall not operate to defeat any defence arising under a contract’.
It does not alter the effect on any claim of the Statute of Limitations:
Provided that where any contract or enactment providing for the limitation of liability is applicable to the claim, the amount of damages recoverable by the claimant by virtue of this subsection shall not exceed the maximum limit so applicable.