His wife Janet, who denied knowing anything about the cannabis farm, walked from court after being found guilty of allowing the drug to be grown in her house.
Teesside Crown Court heard how the granddad-of-two turned his hand to cannabis growing so he could continue to live the lifestyle he was used to and pay off debts.
He made £2,000 from the operation and stood to pocket an estimated £38,000 before police swooped on his home in Haswell Avenue, in the Foggy Furze area of Hartlepool.
A judge said the crop, complete with its own automatic watering and feeding system, was one of the most sophisticated cannabis farms he had seen.
Johnson, who had to give up work nine years ago, admitted producing and supplying cannabis.
Rachel Masters, prosecuting, said: “In interview, Mr Johnson told police he had been growing cannabis for financial gain because he was struggling to maintain the lifestyle he had become used to.
“He himself did not use cannabis.”
The loft had been converted into two separate growing areas with reflective sheeting covering the walls and floor while the electricity meter had been tampered with to aid the growth.
Johnson’s wife Janet, also 59, received a 12-months prison sentence suspended for two years after she was convicted of permitting her premises to be used for producing cannabis.
The former property sales executive, who was made redundant around three years ago, claimed she did not know it was there.
Christine Egerton, mitigating for David Johnson, said: “The reason he became involved in this and he knows it’s not an excuse, is because he had debts.
“His biggest regret is that his wife became caught up in all this.
“He has also brought shame on himself and his family by what he has done.”
Recorder Jonathan Sandiford said: “It is one of the most sophisticated farms I have ever encountered.
“This was a commercial operation where the only motive was to make profit.
“It involved a substantial degree of planning and a very deliberate calculating flouting of the law.”
Detective Sergeant Daryll Tomlinson, of Hartlepool Drugs Unit, said: “We welcome the sentences issued today.
“It sends a clear message to anyone prepared to produce cannabis in their homes, that if you take the risk of producing cannabis to fund your lifestyle then you run the risk of being sent to prison.
“We will continue to act on community intelligence and to target those making healthy profits from producing and selling cannabis.”