You've been warned

It underpinned the flank wall of 70 Winnington Road, London N2, so that a new basement could be constructed at number 68. Once that was done and dusted, the builder, Avonforce, began its excavations smack bang next to the flank wall. It had paid the subcontracted underpinning man and all was well. But it wasn't long before there was another smack bang.

The central section of the excavation collapsed as a result of rotational failure of the underpinning. The toes of the mass concrete underpinning shifted horizontally into the excavation because, of course, there was no passive soil resistance, nor had the builder installed any temporary supports or propping to prevent failure of the underpinning. The bill was £400,000. And so to litigation. The case is called Aurum Investments Ltd vs Avonforce and others.

Can you see what is coming? The builder turned to the subcontractor and asked, presumably in no uncertain terms: "Why didn't you warn me of the need to provide lateral supports to the underpinning while excavating my basement?" The reply was something to the effect that: "The scope and duty of a subcontractor to warn the main contractor of risks and dangers is limited in scope." In short, no chance.

You might have thought that the law regarding duty to warn is by now well and truly explained by many cases. It isn't. I must say that this judgment, by Mr Justice Dyson, is very clear, helpful and mercifully short – seven pages.

He really does make plain how far we have come, and how cautious the courts are when invited to import a silent promise into a contract, such as an implied duty to warn others about something they are doing or about to do. Take as an example a disastrous turn of events at a Ford factory. Ford's engineers told the contractor what to do when excavating beneath a roof column. The instructions were wrong and the roof collapsed. The Court of Appeal found that in the circumstances of this case, despite other people being responsible, the agreement contained a silent promise that the contractor would speak up if asked to do something dangerous.

You might have thought that the law regarding duty to warn is by now well and truly explained by many cases. It isn't

Past cases show that a duty to warn, if in the contract at all, will refer to warning your customer or client when it is proposing an unsafe course of action.

So, what of 68 and 70 Winnington Road? Should Advanced have warned the main contractor to prop the underpinning when excavating? There was after all a risk and danger. Moreover, it is fair to say that a reasonably competent piling contractor would know that temporary lateral support was necessary.

The court refused to import a term into the contract giving a duty to warn. The essential point was that the underpinning contractor was being asked to warn about something not in its contract but about something to be done by the main contractor or another subcontractor. Advanced had a contract for a specific task and did it. It undertook no design responsibility. Moreover, said its barrister, it could assume that Avonforce as a competent main contractor would take steps to install temporary supports.

The judge explained that the duty to warn plainly ran to an instruction to carry out a dangerous task in that person's work. So, if Advanced was asked to do something dangerous, it must speak up.

Not yet decided by the courts is whether a duty to warn arises when the contractor did not know of the dangers of the engineer's design but should know. Nor is it yet decided whether the duty arises where there is a design defect that does not amount to something dangerous. The law is not fully developed, which is a pity because I can foresee these questions landing in the lap of a lucky adjudicator.