A frequent false perception by many contractors, and even some legal services practitioners, is the idea that a contractor can absolve potential liability by merely performing in accordance to the precise specifications stated within the contract. This is incorrect and potentially a very financially dangerous false perception.
When a contractor is hired to perform a task, the law expects that the contractor will perform the work as is required to meet or exceed the standard usual to that which would be reasonably performed by a similarly knowledgeable and experienced contractor when faced with the same or similar circumstances. Contrary to common belief, performing merely to the workmanship specifications stated within a contracting agreement can, and does, lead to liability, even when the contractor meets, or sometimes even exceeds, the explicit requirements in the agreement.
Quite simply, a contractor is subject to the same childhood 'rule' that just because someone says to jump off a bridge doesn't make it right - even if the person instructing the contractor to jump off the bridge is the customer.
A contractor must avoid shortcomings in work regardless of the expressed instructions (often including the design specifications) as courts have deemed that 'implied terms' within a contract include a warranty that the materials and workmanship will be reasonably durable and reasonably suitable for the intended result; Dirm Inc. v. Bennington Construction Ltd., 2010 ONSC 3298 at 98 and 99; Alliance Roofing & Sheet Metal Ltd. v. Manorcore Group Inc. 2013 CanLII 60850 at 99 and 100. Where the contractor fails to provide a reasonably durable and reasonably suitable work product, the contractor may be liable for the cost of hiring another contractor to complete or correct the shortcomings, even if these costs are well in excess of the original contract value.
Additionally, when personal injury or property damage occurs as a result of a contractor's workmanship, whether during the work or as a result of the work, the law disallows the contractor from arguing that the failure was due to deficiencies within the specifications to the contract. Instead, the law expects that the contractor will perform to the reasonable standards of the trade so as to properly ensure the safety and security of both the property owner or persons having hired the contractor as well as the general public.
Duty of Care
The expectations are such that a contractor may be responsible for harm to various persons, including unknown third parties who may be harmed by the completed work even if at a time well into the future. In Winnipeg Condominium Corporation No. 36 v. Bird Construction Co. Ltd.,  1 S.C.R. 85, the Supreme Court ruled that a contractor,
"... owes a duty of care to those who will use his product, so as to render [the contractor] accountable for negligent workmanship."