Imagine you’re a builder carrying out a small domestic project, and you spot a mistake in the architect’s design. Would you save everyone’s time and trouble by working out an ad hoc solution to it?

Mind the gap, says the voice at Piccadilly Circus Tube station.

It’s an ideal message for some of you builders. Mind the gap in the specification, mind the gap in the drawing. There are gaps galore. What I mean is this: it is not so unusual for the architect’s drawings and specification to contain errors, leave certain things silent and be ambiguous or vague.

Take the case of CGA Brown Ltd vs Carr. The architect in the case produced for the homeowner a set of drawings for obtaining Building Regulation approval. I guess those same drawings are what the builder, CGA Brown, priced on. Certainly, when it did the job in 2003, it obeyed the flat roof instruction on the drawing, “to align through with the existing arrangement”.

But then the builder realised that the falls to the new and the existing roofs looked a bit iffy. Truth is, the roof slope on the existing roof was different from the slope shown on the drawing. There was a gap in the architect’s thinking. So the builder, generous chap, filled the gap. On his own initiative, he made a cold joint between the existing roof and the felt he had laid to cover the new roof.

Come on, a decent builder won’t go whinging to his customer every time he stumbles across a problem. He shrugs and finds a solution. But the cold joint solution didn’t work. The roof leaked; water poured in.

By now the Rochdale builder and his Rochdale customer were at odds for all sorts of reasons. Eventually they ended up in Oldham County Court. On the list of claims and counterclaims, the customer came out on top – just. But there was one item that the builder’s lawyers decided to bring to the Court of Appeal. They said the first judge was wrong to hit the builder for the remedy for the wrong falls on the architect’s drawings.

Look, there is a basic starting point for architects and builders. It is this: the architect contractually promises the client to prepare and supply plans for a house that will be fit for living in. The contractor on the other hand contractually promises the client to perform the work on the house in a good and workmanlike manner. The builder’s job is to comply with the drawings and specifications. Builder Brown did comply. And when it was found that the drawings were wrong, it volunteered a remedy. I guess it was arguing that the cause of the roof leak was the wrong specification on the drawings.

The builder, CGA Brown, had a duty to warn at this domestic consumer level. He didn’t. He pressed on. So then he became responsible for the choice

But the judge heard evidence from an expert witness who said that “any reasonably competent builder would have reported the drawing problem to his client”. The judge said the builder had a duty to warn his client and take instructions. It was a mistake not to stop and warn; it was a mistake to crack on and find a remedy.

"Your drawings were a bit vague so I added my own bits ..." Illustration: Simone Lia

So failing to warn was a breach of contract and, worse, finding a remedy that turned out to be duff was another breach. The expense involved in putting the leak right was down to Builder Brown.

Duty to warn is a tricky area. All the more so if the architect is not involved in the building process. The builder supervises his own work and has a duty to scrutinise the architect’s errors and omissions. That applies to small homeowner works, especially when an architect was only wheeled out to get planning and Building Regs approval. It is not safe to assume that the builder is blindly to comply with plans and specifications; he has to keep his eyes peeled for errors.

Those circumstances will not arise in a more “commercial” building contract where the architect plays a full role from start to finish. In fact, there is a strict promise by the builder to comply with the plans and specification. There is nothing in those types of contract that imposes a duty on the contractor to detect faults in plans prepared by the employer’s architect and report them.

But beware. Any building contractor that finds a faux pas in the specification must speak up. Second, any building contractor that half-suspects that what is proposed is dangerous or even mildly risky must speak up and, if told to press on, should go to an even higher authority to press the point. Third, if the building contractor or subcontractor decides for itself to fill the gap in a specification, choose a product and/or choose an idea, remember it then becomes responsible for the product or idea working.

Brown had a duty to warn at this domestic consumer level. He didn’t. He pressed on. So, he then became responsible for the choice. The Tube doesn’t run as far as Rochdale but, even there, you need to mind the gap.