A bored teenager who made a habit of setting fire to the socks on his feet had to hot-foot it when a fuel-splashed garage caught alight.

The 14-year-old said that unlike the other times he had lit his sock, when he did it on April 14 in the garage belonging to a friend's family the flame did not go out by itself so he stomped his foot on the ground to extinguish it.

A fuel-spattered carpet ignited and the fire quickly spread. He and a friend tried to stop the fire but in the end had to flee, with the friend grabbing his Xbox as he went. The garage was destroyed.

The boy admitted his mother's partner had been a fireman and had talked to him "heaps" about fires and fire safety.

He was found to have committed arson by recklessly damaging the garage but an appeal to the High Court at Wellington succeeded and the arson finding has been quashed.

Justice Jill Mallon said it was not proved the teenager appreciated that what he was doing was reckless. "[He] was 14 years old, and although his intelligence appeared to the [Youth Court] judge to be average, average 14-year-old boys may do things without any thought of the risks involved, even when they have been told about the risks on other occasions," she said.

On the evidence given in the Youth Court, it was possible the boy did not appreciate the risk that the garage would be damaged, she said.

Petrol had been spilt in it earlier in the day while the teenager and his friends "hung out".

But the teenager said he did not think of the spilled petrol when he lit his socks, which he had done many times before without apparent damage.

"He explained that he uses black 'business' socks for this," the judge said. "He said that the sock has 'a little mini flame' and 'it looks mean how it runs across the sock'.

The boy's mother confirmed his sock-lighting habit and said she had never seen any damage to the socks or anything else until the garage fire.

"She said she called the boys clowns but it had not concerned her because the flame was tiny and did not last long before it went out," Justice Mallon said.

A senior fire service officer, who oversees a programme for children and teenagers fascinated by fire, said he had not heard of sock-lighting but some young people would experiment with anything.

Peter Wilding, fire investigation and arson reduction national manager, said a free fire awareness and intervention programme had a 98 per cent success rate in stopping fire lighting.

About 700 young people a year go through the course, which is based on teaching the consequences – often unintended – of their actions.