Bristol roofer Gordon Shaw told police he lashed out with his work knife in self defence when his colleague went for him, slashing his mate's arm, neck and ear, a court heard.

Shaw, who was staying at rented digs among a gang of four workers on a construction job in central Bristol, told a jury Russell Parker came at him after they argued over cannabis and he picked up the blade in panic.

The 57-year-old, who was living in Fishponds Road at the time, denies wounding with intent to cause grievous bodily harm as well as an alternative charge of unlawful wounding in February.

Mr Parker told the court that, having gone to the pub and returning home with a kebab, he disturbed Shaw sleeping in his makeshift bedroom in the dining area and Shaw attacked him. He said Shaw was shouting and aggressive, smashed a dining room chair over the back of his head and tried to punch him, leading to a tussle.

He told the jury: "He broke free from me. I was saying 'Look, Gordon, calm down, and he swung at me with his right hand.

"I felt a trickle from my arm and neck and realised he was holding a knife."

The jury heard that, having been cut, Mr Parker was taken to hospital and received 14 stitches to his wounds.

The jury was read a transcript of an interview Shaw gave to police in which he said Mr Parker came back to the digs, ranting and raving, and he told him to keep the noise down.

Shaw described how Mr Parker then slapped him in the face with the palm of his hand and "rammed" his head against a wall.

He told police he picked up a dining chair and smashed it before another workmate intervened and Mr Parker went to his own bedroom.

Shaw said Mr Parker then threatened he was going to "do him" and he told Mr Parker: "If you want to do it, come and do it."

He told officers: "He came flying in like a loony. I picked up a knife, he dived at me; I panicked and lashed out.

"He (Parker) was ranting and raving, saying 'He's cut me'."

Karen Morgan, a forensic scientist, analysed a sample of Shaw's blood taken some six hours after the incident and tested it to have 75 milligrammes of alcohol in 100 millilitres of blood.

The limit for driving is 80. Ms Morgan said that an approximate back-calculation of his inebriation at the time of the incident could be in the range of 130 to 215 milligrammes of alcohol.

Joseph Thompson, who was the workers' boss, told the court he heard Mr Parker "shouting and balling" at Shaw.

Mr Thompson said he saw Shaw smash a chair on the floor, but said larger-built Mr Parker was "100 per cent the driving force of aggression".

The case continues at Bristol Crown Court.