Unlike main contracts, which usually have a start and completion date (or dates), subcontracts usually have a period (or periods) for completion. Which prompts the question, if a subcontractor is entitled to an extension of time, where (in the nicest possible way) do you put it? If the subcontract was in delay when the event occurred, do you tack it on to the end of the subcontract period(s) or does it become a stand-alone period separate from the original subcontract period(s)? If the latter, any claim by the main contractor against the subcontractor for loss and expense in respect of the delay period would then be in respect of the period when the subcontract was actually in delay.
In Carillion Construction Ltd v Woods Bagot Europe Ltd, where the subcontract form used was DOM/2, the court ruled that any extension of time under the relevant clause should be added to the end of the current period for completion. This interpretation was practicable and workable and what a reasonable person with all the parties' background knowledge would have thought the clause meant when the contract was entered into.
There were factual scenarios where this interpretation could relieve a subcontractor of liability to an extent that did not truly reflect the consequences of its breach in failing to complete on time but this did not affect what the court considered to be the obvious interpretation of the clause. In any event, these were only potential factual scenarios that would not necessarily arise and the extent to which they could influence the interpretation must consequently be limited. The difficulty with these arguments was that they created a distinction not made in the subcontract between responsibility for delay and contractual liability. The subcontractor was only contractually liable for delay if it failed to complete the subcontract works within the period or periods for completion, so the key issue was what the contract provided in respect of those period(s).