At the High Court in London, Deputy Judge James Stewart QC, said Martyn Ginder had been asked at least 10 times over a three-month period to mend the storeroom window through which his 21- month-old son, Daniel, had climbed out.
His wife, Jean, 36, is now confined to a wheelchair, and her claim - against her husband's household insurance - is estimated at pounds 500,000.
She was also ruled negligent in not locking the storeroom door on the morning of the accident, in March 1991, which she had left open briefly to sort out the laundry.
The judge did not accept that Mrs Ginder, of the Old Bakery, Bricket Wood, near Watford, Hertfordshire, was at fault in climbing out on to the roof after her son. 'In the heat and anxiety of the moment she behaved no differently from the way any other mother in like circumstances might have behaved,' he said.
Daniel was rescued unharmed from the roof after workmen found him dangling by his clothes caught on a nail.
The damages will be assessed at later date.
Counsel for Mr Ginder, a 36- year-old undertaker, said an appeal would be considered.
Mr Ginder, who has two other sons, Adam and Russell, said afterwards: 'I have never lost a case to such a wonderful plaintiff.'
He said that the division of duty in the household was that he was in charge of 'man's work' and his wife was in charge of 'woman's work'. 'We had an agreement long before the children were born that she would deal with the nappies and I would deal with the sink.'
Referring to his wife's evidence that she had to find the right moment to ask him to do a household task so as not to put him in a bad temper, he said: 'I should think every woman in the country has the same problems as my wife.
'I don't think I am difficult or more difficult than any other husband. But I do have the knack of being able to just sit down and switch off completely.'.
Mr Ginder said neither he nor his wife, whom he married in 1983 after meeting on a blind date, blamed each other for the incident. 'It was an unfortunate accident. Our relationship survived because we were very much in love before it happened and we still are. Whatever is thrown at you, you cope with it,' he said.
Mrs Ginder said after the hearing she felt 'relief' at the ruling. 'It means we can go forward now to claim against the insurance policy so we can have something for the future.'
She did not bring the case to lay the blame on her husband, she said. 'It was the only way to get the insurance.'
She said her lawyers took the view that it should not have been necessary to bring such a traumatic court action to gain access to their insurance cover.
'At the end of the day, we obviously don't hold each other responsible for what happened and we are still very happy together. An injury of this kind has split up many couples, but we have a very strong marriage. This case certainly has not harmed us.'