The New Zealander is one of more than a dozen teachers who have moved into the site to ward off squatters. Mr Grennel, 25, said the attraction was that he only had to spend pound;35 a week on bills.
The music teacher had originally been invited to guard a mansion house an hour's drive from London but chose the bakery in Dalston, east London, because he does not own a car.
"If you don't mind living somewhere a bit shabby, it's sweet," he said.
"The holes in the roof don't bother me. The only pain is that there is no letter box, so the postman leaves letters at the door and if it's raining they get soggy."
Camelot Property Management, which organises the scheme, has now signed a deal with Capita to offer accommodation to the agency's supply teachers.
Buildings available include an observatory and lighthouse near Liverpool and offices in Wirral.
John Mills left his job as a PE supply teacher last year to work as an account manager for Camelot after moving into an empty factory the company was protecting in Hoxton, London.
He said that teachers made excellent "guardians" because they received criminal records checks as part of their jobs.
They also spent long hours in the properties doing marking and other work.